pounce-nl 0.11.0

//! Flat-tape reverse-mode AD for `.nl` expression trees.
//!
//! Replaces the FD-based Hessian path with a port of the tape AD used
//! in `ripopt::nl::autodiff`. The tape is a `Vec<TapeOp>` where each op
//! refers to its operands by tape-slot index; forward evaluation runs
//! through the slice once filling a parallel `Vec<f64>` of values, and
//! reverse-mode adjoints walk the same buffer backwards.
//!
//! Sparse Hessians are computed by forward-over-reverse: for each
//! variable `j` that the tape depends on, run a forward tangent sweep
//! seeded with `e_j`, then a second-order reverse sweep that produces
//! column `j` of the Hessian. The caller supplies a `(row, col) -> nnz
//! position` map (lower triangle, row >= col), and contributions are
//! accumulated in place — the outer loop in `eval_h` calls the same
//! map for the objective and every active constraint, so every
//! Lagrangian term lands in the right slot.
//!
//! Common subexpressions are tape-emitted **once**: when the recursive
//! builder hits `Expr::Cse(rc)` it keys on the `Arc` pointer identity,
//! emitting the body the first time and returning the cached
//! result-slot index on subsequent references. The forward pass then
//! computes each CSE once and the reverse pass folds adjoints from
//! every reference into a single slot — exact chain-rule behaviour.

use std::collections::{BTreeSet, HashMap, HashSet};
use std::sync::Arc;

use super::nl_external::{EvalResult, ExternalArg, ExternalLibrary, ExternalResolver};
use super::nl_reader::{BinOp, CmpOp, Expr, FuncallArg, UnaryOp};

/// One operation in the flattened tape. Operand fields are tape-slot
/// indices into the same tape; `Var(i)` references problem variable
/// index `i` (read from the input `x` slice during forward).
#[derive(Debug, Clone)]
pub enum TapeOp {
    Const(f64),
    Var(usize),
    Add(usize, usize),
    Sub(usize, usize),
    Mul(usize, usize),
    Div(usize, usize),
    Pow(usize, usize),
    Neg(usize),
    Abs(usize),
    Sqrt(usize),
    Exp(usize),
    Log(usize),
    Log10(usize),
    Sin(usize),
    Cos(usize),
    Tan(usize),
    Atan(usize),
    Acos(usize),
    Sinh(usize),
    Cosh(usize),
    Tanh(usize),
    Asin(usize),
    Acosh(usize),
    Asinh(usize),
    Atanh(usize),
    /// Gauss error function `erf(vals[a])` (issue #469). Unlike the other
    /// transcendentals here it has no AMPL `.nl` opcode — AMPL has no `erf`
    /// — so it is reachable only through the in-memory `Expr` path
    /// (`UnaryOp::Erf`), which is exactly the case it was added for:
    /// modeling frontends that build a tape directly instead of round-
    /// tripping through `.nl`. Smooth everywhere, with closed-form
    /// derivatives `erf'(u) = 2/√π·exp(-u²)` and `erf''(u) = -2u·erf'(u)`.
    Erf(usize),
    /// `a·ln(a)` with the `a = 0` limit `0` (GAMS `entropy` up to sign).
    ///
    /// Like [`TapeOp::Erf`] it has no AMPL `.nl` opcode and is reachable only
    /// through the in-memory `Expr` path (`UnaryOp::XLogX`). Unlike `Erf` it is
    /// fused for **correctness**, not convenience: the second derivative is
    /// `1/a`, which is finite for every positive `a` down to `1e-308`, but
    /// *every* chain-rule decomposition routes through `ln''(a) = -1/a²`. At
    /// `a = 1e-299` that intermediate is `-1e598` — past `f64::MAX` — so the
    /// composed Hessian is `inf`/`nan` while the answer it is computing,
    /// `1e299`, is an ordinary number. No amount of care in the `Mul`/`Log`
    /// rules reaches it; only an op that never forms `1/a²` does.
    XLogX(usize),
    /// `a·ln(a/b)` with the `a = 0` limit `0` — GAMS `centropy`. Operands
    /// `(a, b)`; `b > 0` is the caller's responsibility, as for `Log`.
    ///
    /// Fused for [`TapeOp::XLogX`]'s reason (`∂²/∂a² = 1/a` is unreachable
    /// through `ln''`) plus one of its own: `∂²/∂b² = a/b²` squares the
    /// denominator, which overflows for `|b| > 1.3e154` even when `a/b²` is
    /// itself in range. The fused rule computes `q = a/b` once and expresses
    /// every second-order term as a division by `b`, never by `b²`.
    CEntropy(usize, usize),
    /// Two-argument arctangent `atan2(vals[a], vals[b])` (operands are
    /// `(y, x)`, matching AMPL's `atan2(y, x)` / `.nl` opcode o48).
    Atan2(usize, usize),
    /// Pairwise minimum `min(vals[a], vals[b])`. Piecewise linear: the
    /// value/tangent/adjoint route through whichever operand is smaller
    /// (ties pick the first), and the second derivative is identically
    /// zero. n-ary AMPL `min` (opcode o11) folds to a chain of these.
    Min(usize, usize),
    /// Pairwise maximum `max(vals[a], vals[b])` — the `Min` mirror;
    /// n-ary AMPL `max` (opcode o12) folds to a chain of these.
    Max(usize, usize),
    /// Relational comparison `vals[a] OP vals[b]` → `1.0`/`0.0`.
    /// Piecewise constant, so its derivative is identically zero — the
    /// AD passes treat it as a constant w.r.t. its operands.
    Cmp(CmpOp, usize, usize),
    /// Logical AND: `1.0` iff both operands are nonzero. Zero derivative.
    And(usize, usize),
    /// Logical OR: `1.0` iff either operand is nonzero. Zero derivative.
    Or(usize, usize),
    /// Logical NOT: `1.0` iff the operand is zero. Zero derivative.
    Not(usize),
    /// `if-then-else`: operands `(cond, then, else)`. The value is
    /// `vals[then]` when `vals[cond] != 0` else `vals[else]`, and the
    /// value/tangent/adjoint all route through the active branch only.
    /// The condition contributes no derivative (the branch switch is a
    /// non-smooth event the AD ignores).
    Select(usize, usize, usize),
    /// AMPL imported (external) function call. The payload (library
    /// handle, name, and argument list) is boxed so this rare variant
    /// does not inflate `size_of::<TapeOp>()`: without the box the
    /// `Arc`+`String`+`Vec` make every op ~64 bytes, which on a
    /// summand-split objective with millions of tiny tapes (e.g.
    /// `sensors`) costs gigabytes. Boxing drops the common arithmetic
    /// ops back to the size of the next-largest variant.
    Funcall(Box<FuncallData>),
}

/// Boxed payload of [`TapeOp::Funcall`]. The library is kept alive by
/// the `Arc`; `name` is the registered function name; `args` carries
/// positional arguments where real-valued args reference earlier tape
/// slots and string args are inline literals.
#[derive(Debug, Clone)]
pub struct FuncallData {
    pub lib: Arc<ExternalLibrary>,
    pub name: String,
    pub args: Vec<TapeFuncallArg>,
}

/// One argument of a `TapeOp::Funcall`. Real arguments are tape-slot indices
/// (their values come from the running `vals[]` during forward); string
/// arguments are owned literals (AMPL `h<len>:<chars>` tokens).
#[derive(Debug, Clone)]
pub enum TapeFuncallArg {
    Tape(usize),
    Str(String),
}

/// `ln(a/b)`, computed without ever materializing an out-of-range `a/b`.
///
/// Shared by every arm of [`TapeOp::CEntropy`] so the value and the derivatives
/// cannot drift apart. Three regimes:
///
///   * `a ≈ b` — the ratio is near 1, where `ln` loses digits to cancellation.
///     `(a - b)` is exact by Sterbenz whenever `b/2 ≤ a ≤ 2b`, so `ln_1p` of
///     `(a - b)/b` keeps full precision where `q.ln()` would not.
///   * `a/b` finite and positive — plain `q.ln()`.
///   * `a/b` overflowed, underflowed to zero, or went non-finite — fall back to
///     `ln(a) - ln(b)`. At `a = 1e300, b = 1e-300` the ratio is `1e600` (inf)
///     but `ln` of it is a perfectly ordinary `1381.55`; the difference form
///     reaches it. This is the only regime where cancellation is a risk, and
///     it cannot bite here because the ratio being out of range means the two
///     logs are far apart.
#[inline]
pub(crate) fn ln_ratio(a: f64, b: f64) -> f64 {
    let q = a / b;
    if q.is_finite() && q > 0.0 {
        let t = (a - b) / b;
        if t.abs() < 0.5 { t.ln_1p() } else { q.ln() }
    } else {
        a.ln() - b.ln()
    }
}

/// `a·ln(a)`, the value arm of [`TapeOp::XLogX`] — GAMS `entropy`.
///
/// Exists as a FUSED op because the chain rule provably cannot evaluate its
/// second derivative. `(a·ln a)'' = 1/a`, which at `a = 1e-299` is `1e299` and
/// perfectly representable — but any decomposition routes through
/// `ln''(a) = -1/a² = -1e598`, which exceeds `f64::MAX`. The composite is in
/// range while the factor it is built from is not, so no rule for `Mul` and
/// `Log` can recover it, however carefully written. Only a fused op that never
/// forms `1/a²` gets there.
///
/// `0·ln 0` is `0` by the limit, not `NaN`.
#[inline]
pub(crate) fn xlogx(a: f64) -> f64 {
    if a == 0.0 { 0.0 } else { a * a.ln() }
}

/// `a·ln(a/b)`, the value arm of [`TapeOp::CEntropy`] — GAMS `centropy`.
///
/// Fused for the same reason as [`xlogx`], plus one of its own: `∂²/∂b²` is
/// `a/b²`, and `b²` overflows for `|b| > 1.3e154` while `a/b²` itself stays in
/// range. The fused rule computes it as `q/b` and never squares anything.
///
/// `0·ln(0/b)` is `0` by the limit.
#[inline]
pub(crate) fn centropy(a: f64, b: f64) -> f64 {
    if a == 0.0 { 0.0 } else { a * ln_ratio(a, b) }
}

/// `(a·ln a)' = ln(a) + 1`. `-inf` at `a = 0`, which is the true one-sided limit.
#[inline]
pub(crate) fn xlogx_d1(a: f64) -> f64 {
    a.ln() + 1.0
}

/// `(a·ln a)'' = 1/a`.
///
/// The entire reason [`TapeOp::XLogX`] exists: this is finite for every positive
/// `a`, while the `ln''` any decomposition would go through is not.
#[inline]
pub(crate) fn xlogx_d2(a: f64) -> f64 {
    1.0 / a
}

/// `∂/∂a [a·ln(a/b)] = ln(a/b) + 1`.
#[inline]
pub(crate) fn centropy_da(a: f64, b: f64) -> f64 {
    ln_ratio(a, b) + 1.0
}

/// `∂/∂b [a·ln(a/b)] = -a/b`.
#[inline]
pub(crate) fn centropy_db(a: f64, b: f64) -> f64 {
    -(a / b)
}

/// `∂²/∂a² [a·ln(a/b)] = 1/a` — independent of `b`; see [`xlogx_d2`].
#[inline]
pub(crate) fn centropy_daa(a: f64) -> f64 {
    1.0 / a
}

/// `∂²/∂a∂b [a·ln(a/b)] = -1/b` — independent of `a`.
#[inline]
pub(crate) fn centropy_dab(b: f64) -> f64 {
    -1.0 / b
}

/// `∂²/∂b² [a·ln(a/b)] = a/b²`, evaluated as `(a/b)/b`.
///
/// Never as `a/(b*b)`: at `a = 1e300, b = 1e200` the true value is `1e-100`, but
/// `b*b` is `inf` and the squared form returns `0.0`. Dividing twice keeps it.
#[inline]
pub(crate) fn centropy_dbb(a: f64, b: f64) -> f64 {
    (a / b) / b
}

/// Gauss error function, the value arm of [`TapeOp::Erf`].
///
/// Delegates to `libm` (the rust-lang port of musl's libm) rather than a
/// series approximation: `erf` shows up inside residuals, so a 1e-7-accurate
/// Abramowitz–Stegun fit would cap the achievable KKT error well above what
/// the IPM asks for.
#[inline]
pub(crate) fn erf(u: f64) -> f64 {
    libm::erf(u)
}

/// `erf'(u) = 2/√π · exp(-u²)`.
///
/// The AD sweeps all call this (rather than each inlining the constant) so
/// the tangent and adjoint arms of [`TapeOp::Erf`] cannot drift apart.
#[inline]
pub(crate) fn erf_d1(u: f64) -> f64 {
    std::f64::consts::FRAC_2_SQRT_PI * (-u * u).exp()
}

/// `erf''(u) = -2u · erf'(u)`.
///
/// Parenthesized as `-2·(u·erf'(u))`, not `(-2·u)·erf'(u)`. The two differ
/// at the top of the range: `erf_d1` underflows to `0.0` around `|u| > 27`,
/// while `-2.0 * u` overflows to `±inf` for `|u| > f64::MAX/2`, and
/// `inf * 0.0` is `NaN` where the true limit is `0`. Multiplying `u` into
/// the already-underflowed derivative first keeps every finite magnitude
/// finite. (`u = ±inf` is `NaN` either way, correctly — the model has
/// already left the reals by then.) Only the Hessian arms read this; the
/// value and gradient arms are unaffected.
#[inline]
pub(crate) fn erf_d2(u: f64) -> f64 {
    -2.0 * (u * erf_d1(u))
}

/// Evaluate a relational opcode on two scalar values, returning the
/// boolean truth (callers map it to `1.0`/`0.0`).
#[inline]
fn cmp_holds(op: CmpOp, a: f64, b: f64) -> bool {
    match op {
        CmpOp::Lt => a < b,
        CmpOp::Le => a <= b,
        CmpOp::Eq => a == b,
        CmpOp::Ge => a >= b,
        CmpOp::Gt => a > b,
        CmpOp::Ne => a != b,
    }
}

fn funcall_to_ext_args<'a>(args: &'a [TapeFuncallArg], vals: &[f64]) -> Vec<ExternalArg<'a>> {
    args.iter()
        .map(|a| match a {
            TapeFuncallArg::Tape(idx) => ExternalArg::Real(vals[*idx]),
            TapeFuncallArg::Str(s) => ExternalArg::Str(s.as_str()),
        })
        .collect()
}

/// Evaluate an external (AMPL imported) function, poisoning the result with
/// `NaN` instead of panicking when the library reports an error.
///
/// An external eval fails on user-controllable conditions — most commonly an
/// out-of-domain property evaluation (e.g. an IDAES Helmholtz thermo call
/// outside its valid pressure/temperature range). We mirror the tape's own
/// arithmetic domain-error semantics (`log(-1) → NaN`): hand back NaN so the
/// IPM sees a failed evaluation and the line search backs off, rather than
/// raising an uncatchable panic across the pyo3 boundary on the `read_nl`
/// surface. The NaN derivative/Hessian vectors are sized by the full argument
/// count — an upper bound on the real-arg count a successful eval returns — so
/// every downstream index into them stays in range.
fn ext_eval_or_nan(
    lib: &ExternalLibrary,
    name: &str,
    call_args: &[ExternalArg<'_>],
    n_args: usize,
    want_derivs: bool,
    want_hes: bool,
) -> EvalResult {
    lib.eval(name, call_args, want_derivs, want_hes)
        .unwrap_or_else(|_| EvalResult {
            value: f64::NAN,
            derivs: want_derivs.then(|| vec![f64::NAN; n_args]),
            hessian: want_hes.then(|| vec![f64::NAN; n_args * (n_args + 1) / 2]),
        })
}

/// A flattened expression tape. The result of evaluation is the value
/// at slot `ops.len() - 1` (i.e. the last op).
#[derive(Debug, Clone)]
pub struct Tape {
    pub ops: Vec<TapeOp>,
}

impl Tape {
    /// Build a tape from an `Expr` tree (no AMPL external functions). CSE
    /// bodies (`Expr::Cse(rc)`) are cached by `Arc` pointer identity so each
    /// body is emitted once even when referenced many times.
    pub fn build(expr: &Expr) -> Self {
        Self::build_with_externals(expr, &ExternalResolver::default())
    }

    /// Build a tape from an `Expr` tree, resolving any `Expr::Funcall`
    /// nodes through `resolver`. Panics if the expression references a
    /// funcall id that is not in the resolver — `NlProblem::resolve_externals`
    /// must populate the resolver before tape construction.
    pub fn build_with_externals(expr: &Expr, resolver: &ExternalResolver) -> Self {
        let mut ops = Vec::new();
        let mut cache: HashMap<*const Expr, usize> = HashMap::new();
        build_recursive(expr, &mut ops, &mut cache, resolver);
        Tape { ops }
    }

    /// Forward sweep: returns `vals[i] = value of tape slot i`. The
    /// scalar tape result is `vals[ops.len() - 1]`.
    pub fn forward(&self, x: &[f64]) -> Vec<f64> {
        let mut vals: Vec<f64> = Vec::with_capacity(self.ops.len());
        for op in &self.ops {
            let v = match op {
                TapeOp::Const(c) => *c,
                TapeOp::Var(i) => x[*i],
                TapeOp::Add(a, b) => vals[*a] + vals[*b],
                TapeOp::Sub(a, b) => vals[*a] - vals[*b],
                TapeOp::Mul(a, b) => vals[*a] * vals[*b],
                TapeOp::Div(a, b) => vals[*a] / vals[*b],
                TapeOp::Pow(a, b) => vals[*a].powf(vals[*b]),
                TapeOp::Neg(a) => -vals[*a],
                TapeOp::Abs(a) => vals[*a].abs(),
                TapeOp::Sqrt(a) => vals[*a].sqrt(),
                TapeOp::Exp(a) => vals[*a].exp(),
                TapeOp::Log(a) => vals[*a].ln(),
                TapeOp::Log10(a) => vals[*a].log10(),
                TapeOp::Sin(a) => vals[*a].sin(),
                TapeOp::Cos(a) => vals[*a].cos(),
                TapeOp::Tan(a) => vals[*a].tan(),
                TapeOp::Atan(a) => vals[*a].atan(),
                TapeOp::Acos(a) => vals[*a].acos(),
                TapeOp::Sinh(a) => vals[*a].sinh(),
                TapeOp::Cosh(a) => vals[*a].cosh(),
                TapeOp::Tanh(a) => vals[*a].tanh(),
                TapeOp::Asin(a) => vals[*a].asin(),
                TapeOp::Acosh(a) => vals[*a].acosh(),
                TapeOp::Asinh(a) => vals[*a].asinh(),
                TapeOp::Atanh(a) => vals[*a].atanh(),
                TapeOp::Erf(a) => erf(vals[*a]),
                TapeOp::XLogX(a) => xlogx(vals[*a]),
                TapeOp::CEntropy(a, b) => centropy(vals[*a], vals[*b]),
                TapeOp::Atan2(a, b) => vals[*a].atan2(vals[*b]),
                TapeOp::Min(a, b) => vals[*a].min(vals[*b]),
                TapeOp::Max(a, b) => vals[*a].max(vals[*b]),
                TapeOp::Cmp(op, a, b) => f64::from(cmp_holds(*op, vals[*a], vals[*b])),
                TapeOp::And(a, b) => f64::from(vals[*a] != 0.0 && vals[*b] != 0.0),
                TapeOp::Or(a, b) => f64::from(vals[*a] != 0.0 || vals[*b] != 0.0),
                TapeOp::Not(a) => f64::from(vals[*a] == 0.0),
                TapeOp::Select(c, t, e) => {
                    if vals[*c] != 0.0 {
                        vals[*t]
                    } else {
                        vals[*e]
                    }
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, &vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), false, false);
                    res.value
                }
            };
            vals.push(v);
        }
        vals
    }

    pub fn eval(&self, x: &[f64]) -> f64 {
        let vals = self.forward(x);
        *vals.last().unwrap_or(&0.0)
    }

    /// Reverse-mode AD: accumulate `seed * df/dx_i` into `grad[i]` for
    /// every problem variable `i` referenced by the tape. `grad` is
    /// **not** zeroed by this routine — the caller can chain multiple
    /// gradient accumulations into the same buffer.
    pub fn gradient_seed(&self, x: &[f64], seed: f64, grad: &mut [f64]) {
        if seed == 0.0 || self.ops.is_empty() {
            return;
        }
        let vals = self.forward(x);
        self.reverse(&vals, seed, grad);
    }

    /// Reverse-mode AD reusing two caller-supplied scratch buffers
    /// (`vals` from [`forward_into`], and an `adj` arena ≥
    /// `self.ops.len()`) instead of allocating a forward-value vector and
    /// an adjoint vector per call like [`gradient_seed`]. The `.nl` design
    /// emits one tiny tape per summand — ~10⁶ on large models — so a single
    /// `eval_jac_g` / `eval_grad_f` drives this millions of times and the
    /// per-call allocation dominated. `grad` is accumulated into (not
    /// zeroed); `adj` may be passed dirty (it is zeroed at the touched
    /// slots internally).
    ///
    /// [`forward_into`]: Tape::forward_into
    pub fn gradient_seed_into(
        &self,
        x: &[f64],
        seed: f64,
        grad: &mut [f64],
        vals: &mut [f64],
        adj: &mut [f64],
    ) {
        if seed == 0.0 || self.ops.is_empty() {
            return;
        }
        debug_assert!(vals.len() >= self.ops.len());
        self.forward_into(x, vals);
        self.reverse_into(vals, seed, grad, adj);
    }

    fn reverse(&self, vals: &[f64], seed: f64, grad: &mut [f64]) {
        let n = self.ops.len();
        let mut adj = vec![0.0f64; n];
        self.reverse_into(vals, seed, grad, &mut adj);
    }

    /// Reverse adjoint sweep into a caller-supplied `adj` scratch buffer
    /// (length ≥ `self.ops.len()`), the allocation-free core of [`reverse`].
    /// `adj` is zeroed over `[0, n)` internally, so a dirty arena is fine;
    /// `grad` is accumulated into (not zeroed).
    fn reverse_into(&self, vals: &[f64], seed: f64, grad: &mut [f64], adj: &mut [f64]) {
        let n = self.ops.len();
        debug_assert!(adj.len() >= n);
        adj[..n].fill(0.0);
        adj[n - 1] = seed;

        for i in (0..n).rev() {
            let a = adj[i];
            if a == 0.0 {
                continue;
            }
            match &self.ops[i] {
                TapeOp::Const(_) => {}
                TapeOp::Var(j) => {
                    grad[*j] += a;
                }
                TapeOp::Add(l, r) => {
                    adj[*l] += a;
                    adj[*r] += a;
                }
                TapeOp::Sub(l, r) => {
                    adj[*l] += a;
                    adj[*r] -= a;
                }
                TapeOp::Mul(l, r) => {
                    adj[*l] += a * vals[*r];
                    adj[*r] += a * vals[*l];
                }
                TapeOp::Div(l, r) => {
                    // ∂(l/r)/∂r = -q/r, NOT -l/r²: r*r overflows for |r| > 1.3e154
                    // and underflows for |r| < 1.5e-154, zeroing an adjoint that is
                    // itself representable. `vals[i]` is q = l/r, already computed.
                    let rv = vals[*r];
                    adj[*l] += a / rv;
                    adj[*r] -= a * vals[i] / rv;
                }
                TapeOp::Pow(l, r) => {
                    let lv = vals[*l];
                    let rv = vals[*r];
                    if rv != 0.0 {
                        adj[*l] += a * rv * lv.powf(rv - 1.0);
                    }
                    if lv > 0.0 {
                        adj[*r] += a * vals[i] * lv.ln();
                    }
                }
                TapeOp::Neg(j) => {
                    adj[*j] -= a;
                }
                TapeOp::Abs(j) => {
                    if vals[*j] >= 0.0 {
                        adj[*j] += a;
                    } else {
                        adj[*j] -= a;
                    }
                }
                TapeOp::Sqrt(j) => {
                    let sv = vals[i];
                    if sv > 0.0 {
                        adj[*j] += a * 0.5 / sv;
                    }
                }
                TapeOp::Exp(j) => {
                    adj[*j] += a * vals[i];
                }
                TapeOp::Log(j) => {
                    adj[*j] += a / vals[*j];
                }
                TapeOp::Log10(j) => {
                    adj[*j] += a / (vals[*j] * std::f64::consts::LN_10);
                }
                TapeOp::Sin(j) => {
                    adj[*j] += a * vals[*j].cos();
                }
                TapeOp::Cos(j) => {
                    adj[*j] -= a * vals[*j].sin();
                }
                TapeOp::Tan(j) => {
                    let t = vals[i];
                    adj[*j] += a * (1.0 + t * t);
                }
                TapeOp::Atan(j) => {
                    let u = vals[*j];
                    adj[*j] += a / (1.0 + u * u);
                }
                TapeOp::Acos(j) => {
                    let u = vals[*j];
                    adj[*j] -= a / (1.0 - u * u).sqrt();
                }
                TapeOp::Sinh(j) => {
                    adj[*j] += a * vals[*j].cosh();
                }
                TapeOp::Cosh(j) => {
                    adj[*j] += a * vals[*j].sinh();
                }
                TapeOp::Tanh(j) => {
                    let t = vals[i];
                    adj[*j] += a * (1.0 - t * t);
                }
                TapeOp::Asin(j) => {
                    let u = vals[*j];
                    adj[*j] += a / (1.0 - u * u).sqrt();
                }
                TapeOp::Acosh(j) => {
                    let u = vals[*j];
                    adj[*j] += a / (u * u - 1.0).sqrt();
                }
                TapeOp::Asinh(j) => {
                    let u = vals[*j];
                    adj[*j] += a / (u * u + 1.0).sqrt();
                }
                TapeOp::Atanh(j) => {
                    let u = vals[*j];
                    adj[*j] += a / (1.0 - u * u);
                }
                TapeOp::Erf(j) => {
                    adj[*j] += a * erf_d1(vals[*j]);
                }
                TapeOp::XLogX(j) => {
                    adj[*j] += a * xlogx_d1(vals[*j]);
                }
                TapeOp::CEntropy(l, r) => {
                    adj[*l] += a * centropy_da(vals[*l], vals[*r]);
                    adj[*r] += a * centropy_db(vals[*l], vals[*r]);
                }
                TapeOp::Atan2(l, r) => {
                    let y = vals[*l];
                    let x = vals[*r];
                    let d = y * y + x * x;
                    adj[*l] += a * (x / d);
                    adj[*r] += a * (-y / d);
                }
                // min/max are piecewise linear: the adjoint flows to the
                // selected operand only (ties pick the first, a valid
                // subgradient choice).
                TapeOp::Min(l, r) => {
                    if vals[*l] <= vals[*r] {
                        adj[*l] += a;
                    } else {
                        adj[*r] += a;
                    }
                }
                TapeOp::Max(l, r) => {
                    if vals[*l] >= vals[*r] {
                        adj[*l] += a;
                    } else {
                        adj[*r] += a;
                    }
                }
                // Comparisons and logical connectives are piecewise
                // constant: zero derivative, so no adjoint propagates.
                TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => {}
                // if-then-else: the adjoint flows entirely into the
                // active branch; the condition gets none.
                TapeOp::Select(c, t, e) => {
                    if vals[*c] != 0.0 {
                        adj[*t] += a;
                    } else {
                        adj[*e] += a;
                    }
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), true, false);
                    let derivs = res.derivs.expect("want_derivs=true returns derivs");
                    let mut k = 0usize;
                    for arg in args {
                        if let TapeFuncallArg::Tape(idx) = arg {
                            adj[*idx] += a * derivs[k];
                            k += 1;
                        }
                    }
                }
            }
        }
    }

    /// Sorted distinct problem-variable indices that the tape depends on.
    pub fn variables(&self) -> Vec<usize> {
        let mut s: BTreeSet<usize> = BTreeSet::new();
        for op in &self.ops {
            if let TapeOp::Var(j) = op {
                s.insert(*j);
            }
        }
        s.into_iter().collect()
    }

    /// Forward tangent sweep: `dot[i] = d(slot_i) / dx_{seed_var}`.
    /// Caller-supplied `dot` buffer is overwritten in full; no zeroing
    /// needed beforehand because every slot is written before it is
    /// read (the loop walks forward and only reads earlier slots).
    fn forward_tangent(&self, vals: &[f64], seed_var: usize, dot: &mut [f64]) {
        let n = self.ops.len();
        debug_assert_eq!(dot.len(), n);
        for i in 0..n {
            dot[i] = match &self.ops[i] {
                TapeOp::Const(_) => 0.0,
                TapeOp::Var(k) => {
                    if *k == seed_var {
                        1.0
                    } else {
                        0.0
                    }
                }
                TapeOp::Add(a, b) => dot[*a] + dot[*b],
                TapeOp::Sub(a, b) => dot[*a] - dot[*b],
                TapeOp::Mul(a, b) => dot[*a] * vals[*b] + vals[*a] * dot[*b],
                TapeOp::Div(a, b) => {
                    // (ȧ - q·ḃ)/b, NOT (ȧ·b - a·ḃ)/b². Squaring the denominator
                    // overflows for |b| > 1.3e154 and underflows for |b| < 1.5e-154,
                    // destroying a tangent that is itself perfectly representable:
                    // at a = b = 1e300 with ḃ = 1 the true -a/b² is -1e-300, but
                    // b*b is inf so the squared form returns -0.0. Dividing by b
                    // twice instead of by b² once is the standard Kahan form, and
                    // costs nothing -- q is this node's own value, already computed.
                    (dot[*a] - vals[i] * dot[*b]) / vals[*b]
                }
                TapeOp::Pow(a, b) => {
                    let u = vals[*a];
                    let r = vals[*b];
                    let du = dot[*a];
                    let dr = dot[*b];
                    let mut result = 0.0;
                    // Match the reverse-mode gradient's guard (`rv != 0.0` only): at base
                    // u == 0 the slope is still well defined for r >= 1 (and a
                    // genuine ±inf for r < 1), so it must not be silently dropped,
                    // or the forward tangent disagrees with the reverse gradient.
                    if r != 0.0 {
                        result += r * u.powf(r - 1.0) * du;
                    }
                    if u > 0.0 {
                        result += vals[i] * u.ln() * dr;
                    }
                    result
                }
                TapeOp::Neg(a) => -dot[*a],
                TapeOp::Abs(a) => {
                    if vals[*a] >= 0.0 {
                        dot[*a]
                    } else {
                        -dot[*a]
                    }
                }
                TapeOp::Sqrt(a) => {
                    let sv = vals[i];
                    if sv > 0.0 { dot[*a] * 0.5 / sv } else { 0.0 }
                }
                TapeOp::Exp(a) => dot[*a] * vals[i],
                TapeOp::Log(a) => dot[*a] / vals[*a],
                TapeOp::Log10(a) => dot[*a] / (vals[*a] * std::f64::consts::LN_10),
                TapeOp::Sin(a) => dot[*a] * vals[*a].cos(),
                TapeOp::Cos(a) => -dot[*a] * vals[*a].sin(),
                TapeOp::Tan(a) => {
                    let t = vals[i];
                    dot[*a] * (1.0 + t * t)
                }
                TapeOp::Atan(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 + u * u)
                }
                TapeOp::Acos(a) => {
                    let u = vals[*a];
                    -dot[*a] / (1.0 - u * u).sqrt()
                }
                TapeOp::Sinh(a) => dot[*a] * vals[*a].cosh(),
                TapeOp::Cosh(a) => dot[*a] * vals[*a].sinh(),
                TapeOp::Tanh(a) => {
                    let t = vals[i];
                    dot[*a] * (1.0 - t * t)
                }
                TapeOp::Asin(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 - u * u).sqrt()
                }
                TapeOp::Acosh(a) => {
                    let u = vals[*a];
                    dot[*a] / (u * u - 1.0).sqrt()
                }
                TapeOp::Asinh(a) => {
                    let u = vals[*a];
                    dot[*a] / (u * u + 1.0).sqrt()
                }
                TapeOp::Atanh(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 - u * u)
                }
                TapeOp::Erf(a) => erf_d1(vals[*a]) * dot[*a],
                TapeOp::XLogX(a) => xlogx_d1(vals[*a]) * dot[*a],
                TapeOp::CEntropy(a, b) => {
                    centropy_da(vals[*a], vals[*b]) * dot[*a]
                        + centropy_db(vals[*a], vals[*b]) * dot[*b]
                }
                TapeOp::Atan2(a, b) => {
                    let y = vals[*a];
                    let x = vals[*b];
                    let d = y * y + x * x;
                    (x * dot[*a] - y * dot[*b]) / d
                }
                // min/max: the tangent follows the selected operand.
                TapeOp::Min(a, b) => {
                    if vals[*a] <= vals[*b] {
                        dot[*a]
                    } else {
                        dot[*b]
                    }
                }
                TapeOp::Max(a, b) => {
                    if vals[*a] >= vals[*b] {
                        dot[*a]
                    } else {
                        dot[*b]
                    }
                }
                TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => 0.0,
                TapeOp::Select(c, t, e) => {
                    if vals[*c] != 0.0 {
                        dot[*t]
                    } else {
                        dot[*e]
                    }
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), true, false);
                    let derivs = res.derivs.expect("want_derivs=true returns derivs");
                    let mut acc = 0.0;
                    let mut k = 0usize;
                    for arg in args {
                        if let TapeFuncallArg::Tape(idx) = arg {
                            acc += derivs[k] * dot[*idx];
                            k += 1;
                        }
                    }
                    acc
                }
            };
        }
    }

    /// Forward sweep into a caller-supplied buffer. Avoids the
    /// per-call allocation of `forward()` so hot paths can reuse
    /// one scratch arena across many tapes.
    pub fn forward_into(&self, x: &[f64], vals: &mut [f64]) {
        let n = self.ops.len();
        debug_assert!(vals.len() >= n);
        for i in 0..n {
            vals[i] = match &self.ops[i] {
                TapeOp::Const(c) => *c,
                TapeOp::Var(j) => x[*j],
                TapeOp::Add(a, b) => vals[*a] + vals[*b],
                TapeOp::Sub(a, b) => vals[*a] - vals[*b],
                TapeOp::Mul(a, b) => vals[*a] * vals[*b],
                TapeOp::Div(a, b) => vals[*a] / vals[*b],
                TapeOp::Pow(a, b) => vals[*a].powf(vals[*b]),
                TapeOp::Neg(a) => -vals[*a],
                TapeOp::Abs(a) => vals[*a].abs(),
                TapeOp::Sqrt(a) => vals[*a].sqrt(),
                TapeOp::Exp(a) => vals[*a].exp(),
                TapeOp::Log(a) => vals[*a].ln(),
                TapeOp::Log10(a) => vals[*a].log10(),
                TapeOp::Sin(a) => vals[*a].sin(),
                TapeOp::Cos(a) => vals[*a].cos(),
                TapeOp::Tan(a) => vals[*a].tan(),
                TapeOp::Atan(a) => vals[*a].atan(),
                TapeOp::Acos(a) => vals[*a].acos(),
                TapeOp::Sinh(a) => vals[*a].sinh(),
                TapeOp::Cosh(a) => vals[*a].cosh(),
                TapeOp::Tanh(a) => vals[*a].tanh(),
                TapeOp::Asin(a) => vals[*a].asin(),
                TapeOp::Acosh(a) => vals[*a].acosh(),
                TapeOp::Asinh(a) => vals[*a].asinh(),
                TapeOp::Atanh(a) => vals[*a].atanh(),
                TapeOp::Erf(a) => erf(vals[*a]),
                TapeOp::XLogX(a) => xlogx(vals[*a]),
                TapeOp::CEntropy(a, b) => centropy(vals[*a], vals[*b]),
                TapeOp::Atan2(a, b) => vals[*a].atan2(vals[*b]),
                TapeOp::Min(a, b) => vals[*a].min(vals[*b]),
                TapeOp::Max(a, b) => vals[*a].max(vals[*b]),
                TapeOp::Cmp(op, a, b) => f64::from(cmp_holds(*op, vals[*a], vals[*b])),
                TapeOp::And(a, b) => f64::from(vals[*a] != 0.0 && vals[*b] != 0.0),
                TapeOp::Or(a, b) => f64::from(vals[*a] != 0.0 || vals[*b] != 0.0),
                TapeOp::Not(a) => f64::from(vals[*a] == 0.0),
                TapeOp::Select(c, t, e) => {
                    if vals[*c] != 0.0 {
                        vals[*t]
                    } else {
                        vals[*e]
                    }
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, &*vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), false, false);
                    res.value
                }
            };
        }
    }

    /// Scalar tape value, reusing a caller-supplied scratch buffer
    /// (`vals.len() >= self.ops.len()`) instead of allocating a fresh
    /// forward-value `Vec` per call like [`eval`]. The `.nl` design emits
    /// one tiny tape per summand — 10⁵–10⁶ on large models — so a single
    /// `eval_f` / `eval_g` drives this once per summand and the per-call
    /// allocation dominated the sweep itself (same motivation as
    /// [`gradient_seed_into`], M18).
    ///
    /// [`eval`]: Tape::eval
    /// [`gradient_seed_into`]: Tape::gradient_seed_into
    pub fn eval_into(&self, x: &[f64], vals: &mut [f64]) -> f64 {
        let n = self.ops.len();
        if n == 0 {
            return 0.0;
        }
        self.forward_into(x, vals);
        vals[n - 1]
    }

    /// Directional Hessian-vector product: emits
    /// `weight * (∇²f · seed)[k]` into `out[k]` for every problem
    /// variable `k` the tape references. Caller supplies the
    /// forward-pass result `vals` (use [`forward_into`]) plus three
    /// scratch buffers (`dot`, `adj`, `adj_dot`), each at least
    /// `self.ops.len()` long. `out` must be at least one past the
    /// largest variable index in the tape; the routine reads
    /// `seed[k]` for each `Var(k)` and writes `out[k] += weight *
    /// (Hess · seed)[k]`.
    ///
    /// This is one forward-over-reverse AD pass — O(n_ops) work —
    /// regardless of how many variables the tape depends on, which
    /// is what makes Hessian coloring efficient: a single
    /// directional pass recovers a whole color group of columns.
    ///
    /// [`forward_into`]: Tape::forward_into
    pub fn hessian_directional(
        &self,
        vals: &[f64],
        seed: &[f64],
        weight: f64,
        out: &mut [f64],
        dot: &mut [f64],
        adj: &mut [f64],
        adj_dot: &mut [f64],
    ) {
        let n = self.ops.len();
        if n == 0 || weight == 0.0 {
            return;
        }
        debug_assert!(vals.len() >= n);
        debug_assert!(dot.len() >= n);
        debug_assert!(adj.len() >= n);
        debug_assert!(adj_dot.len() >= n);

        // Forward tangent: dot[i] = (∂vals[i] / ∂x · seed). At
        // Var(k) the seed entry feeds in; the rest of the chain
        // rule matches `forward_tangent` exactly.
        for i in 0..n {
            dot[i] = match &self.ops[i] {
                TapeOp::Const(_) => 0.0,
                TapeOp::Var(k) => seed[*k],
                TapeOp::Add(a, b) => dot[*a] + dot[*b],
                TapeOp::Sub(a, b) => dot[*a] - dot[*b],
                TapeOp::Mul(a, b) => dot[*a] * vals[*b] + vals[*a] * dot[*b],
                TapeOp::Div(a, b) => {
                    // (ȧ - q·ḃ)/b, NOT (ȧ·b - a·ḃ)/b². Squaring the denominator
                    // overflows for |b| > 1.3e154 and underflows for |b| < 1.5e-154,
                    // destroying a tangent that is itself perfectly representable:
                    // at a = b = 1e300 with ḃ = 1 the true -a/b² is -1e-300, but
                    // b*b is inf so the squared form returns -0.0. Dividing by b
                    // twice instead of by b² once is the standard Kahan form, and
                    // costs nothing -- q is this node's own value, already computed.
                    (dot[*a] - vals[i] * dot[*b]) / vals[*b]
                }
                TapeOp::Pow(a, b) => {
                    let u = vals[*a];
                    let r = vals[*b];
                    let du = dot[*a];
                    let dr = dot[*b];
                    let mut result = 0.0;
                    // Match the reverse-mode gradient's guard (`rv != 0.0` only): at base
                    // u == 0 the slope is still well defined for r >= 1 (and a
                    // genuine ±inf for r < 1), so it must not be silently dropped,
                    // or the forward tangent disagrees with the reverse gradient.
                    if r != 0.0 {
                        result += r * u.powf(r - 1.0) * du;
                    }
                    if u > 0.0 {
                        result += vals[i] * u.ln() * dr;
                    }
                    result
                }
                TapeOp::Neg(a) => -dot[*a],
                TapeOp::Abs(a) => {
                    if vals[*a] >= 0.0 {
                        dot[*a]
                    } else {
                        -dot[*a]
                    }
                }
                TapeOp::Sqrt(a) => {
                    let sv = vals[i];
                    if sv > 0.0 { dot[*a] * 0.5 / sv } else { 0.0 }
                }
                TapeOp::Exp(a) => vals[i] * dot[*a],
                TapeOp::Log(a) => dot[*a] / vals[*a],
                TapeOp::Log10(a) => dot[*a] / (vals[*a] * std::f64::consts::LN_10),
                TapeOp::Sin(a) => vals[*a].cos() * dot[*a],
                TapeOp::Cos(a) => -vals[*a].sin() * dot[*a],
                TapeOp::Tan(a) => {
                    let t = vals[i];
                    (1.0 + t * t) * dot[*a]
                }
                TapeOp::Atan(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 + u * u)
                }
                TapeOp::Acos(a) => {
                    let u = vals[*a];
                    -dot[*a] / (1.0 - u * u).sqrt()
                }
                TapeOp::Sinh(a) => dot[*a] * vals[*a].cosh(),
                TapeOp::Cosh(a) => dot[*a] * vals[*a].sinh(),
                TapeOp::Tanh(a) => {
                    let t = vals[i];
                    (1.0 - t * t) * dot[*a]
                }
                TapeOp::Asin(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 - u * u).sqrt()
                }
                TapeOp::Acosh(a) => {
                    let u = vals[*a];
                    dot[*a] / (u * u - 1.0).sqrt()
                }
                TapeOp::Asinh(a) => {
                    let u = vals[*a];
                    dot[*a] / (u * u + 1.0).sqrt()
                }
                TapeOp::Atanh(a) => {
                    let u = vals[*a];
                    dot[*a] / (1.0 - u * u)
                }
                TapeOp::Erf(a) => erf_d1(vals[*a]) * dot[*a],
                TapeOp::XLogX(a) => xlogx_d1(vals[*a]) * dot[*a],
                TapeOp::CEntropy(a, b) => {
                    centropy_da(vals[*a], vals[*b]) * dot[*a]
                        + centropy_db(vals[*a], vals[*b]) * dot[*b]
                }
                TapeOp::Atan2(a, b) => {
                    let y = vals[*a];
                    let x = vals[*b];
                    let d = y * y + x * x;
                    (x * dot[*a] - y * dot[*b]) / d
                }
                // min/max: the tangent follows the selected operand.
                TapeOp::Min(a, b) => {
                    if vals[*a] <= vals[*b] {
                        dot[*a]
                    } else {
                        dot[*b]
                    }
                }
                TapeOp::Max(a, b) => {
                    if vals[*a] >= vals[*b] {
                        dot[*a]
                    } else {
                        dot[*b]
                    }
                }
                TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => 0.0,
                TapeOp::Select(c, t, e) => {
                    if vals[*c] != 0.0 {
                        dot[*t]
                    } else {
                        dot[*e]
                    }
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), true, false);
                    let derivs = res.derivs.expect("want_derivs=true returns derivs");
                    let mut acc = 0.0;
                    let mut k = 0usize;
                    for arg in args {
                        if let TapeFuncallArg::Tape(idx) = arg {
                            acc += derivs[k] * dot[*idx];
                            k += 1;
                        }
                    }
                    acc
                }
            };
        }

        // Reverse over tangent. adj[i] = ∂f/∂vals[i],
        // adj_dot[i] = derivative of adj[i] along `seed`
        // direction = (Hess · seed) projected onto slot i.
        for slot in adj.iter_mut().take(n) {
            *slot = 0.0;
        }
        for slot in adj_dot.iter_mut().take(n) {
            *slot = 0.0;
        }
        adj[n - 1] = 1.0;

        for i in (0..n).rev() {
            let w = adj[i];
            let wd = adj_dot[i];
            if w == 0.0 && wd == 0.0 {
                continue;
            }
            match &self.ops[i] {
                TapeOp::Const(_) => {}
                TapeOp::Var(k) => {
                    if wd != 0.0 {
                        out[*k] += weight * wd;
                    }
                }
                TapeOp::Add(a, b) => {
                    adj[*a] += w;
                    adj[*b] += w;
                    adj_dot[*a] += wd;
                    adj_dot[*b] += wd;
                }
                TapeOp::Sub(a, b) => {
                    adj[*a] += w;
                    adj[*b] -= w;
                    adj_dot[*a] += wd;
                    adj_dot[*b] -= wd;
                }
                TapeOp::Mul(a, b) => {
                    adj[*a] += w * vals[*b];
                    adj[*b] += w * vals[*a];
                    adj_dot[*a] += wd * vals[*b] + w * dot[*b];
                    adj_dot[*b] += wd * vals[*a] + w * dot[*a];
                }
                TapeOp::Div(a, b) => {
                    // Kahan form, second order. `vb2`/`vb3` overflow for
                    // |b| > 1.3e154 (and vb2 underflows below 1.5e-154), which
                    // silently zeroed second-order terms that are representable.
                    // Every 1/b² here is regrouped as (·/b)/b, and the
                    // 2a·ḃ/b³ term folds into q̇ = dot[i], already computed:
                    //   -ȧ/b² + 2q·ḃ/b² == (-q̇ + q·(ḃ/b)) / b.
                    let vb = vals[*b];
                    let q = vals[i];
                    let qd = dot[i];
                    adj[*a] += w / vb;
                    adj_dot[*a] += wd / vb - w * (dot[*b] / vb) / vb;
                    adj[*b] -= w * q / vb;
                    adj_dot[*b] += -(wd * q) / vb + (w / vb) * (-qd + q * (dot[*b] / vb));
                }
                TapeOp::Pow(a, b) => {
                    let u = vals[*a];
                    let r = vals[*b];
                    let du = dot[*a];
                    let dr = dot[*b];
                    if r != 0.0 {
                        if u != 0.0 {
                            let p_a = r * u.powf(r - 1.0);
                            adj[*a] += w * p_a;
                            let mut dp_a = dr * u.powf(r - 1.0);
                            if u > 0.0 {
                                dp_a += r * u.powf(r - 1.0) * ((r - 1.0) * du / u + dr * u.ln());
                            } else {
                                dp_a += r * (r - 1.0) * u.powf(r - 2.0) * du;
                            }
                            adj_dot[*a] += wd * p_a + w * dp_a;
                        } else if r >= 2.0 {
                            let p_a = 0.0;
                            adj[*a] += w * p_a;
                            let dp_a = if r == 2.0 {
                                2.0 * du
                            } else {
                                r * (r - 1.0) * (0.0_f64).powf(r - 2.0) * du
                            };
                            adj_dot[*a] += wd * p_a + w * dp_a;
                        }
                    }
                    if u > 0.0 {
                        let ln_u = u.ln();
                        let p_b = vals[i] * ln_u;
                        adj[*b] += w * p_b;
                        let dur = vals[i] * (r * du / u + dr * ln_u);
                        let dp_b = dur * ln_u + vals[i] * du / u;
                        adj_dot[*b] += wd * p_b + w * dp_b;
                    }
                }
                TapeOp::Neg(a) => {
                    adj[*a] -= w;
                    adj_dot[*a] -= wd;
                }
                TapeOp::Abs(a) => {
                    let s = if vals[*a] >= 0.0 { 1.0 } else { -1.0 };
                    adj[*a] += w * s;
                    adj_dot[*a] += wd * s;
                }
                TapeOp::Sqrt(a) => {
                    let sv = vals[i];
                    if sv > 0.0 {
                        let fp = 0.5 / sv;
                        let fpp = -0.25 / (vals[*a] * sv);
                        adj[*a] += w * fp;
                        adj_dot[*a] += wd * fp + w * fpp * dot[*a];
                    }
                }
                TapeOp::Exp(a) => {
                    let ev = vals[i];
                    adj[*a] += w * ev;
                    adj_dot[*a] += wd * ev + w * ev * dot[*a];
                }
                TapeOp::Log(a) => {
                    let u = vals[*a];
                    adj[*a] += w / u;
                    adj_dot[*a] += wd / u + w * (-1.0 / (u * u)) * dot[*a];
                }
                TapeOp::Log10(a) => {
                    let u = vals[*a];
                    let c = std::f64::consts::LN_10;
                    adj[*a] += w / (u * c);
                    adj_dot[*a] += wd / (u * c) + w * (-1.0 / (u * u * c)) * dot[*a];
                }
                TapeOp::Sin(a) => {
                    let u = vals[*a];
                    let cu = u.cos();
                    adj[*a] += w * cu;
                    adj_dot[*a] += wd * cu + w * (-u.sin()) * dot[*a];
                }
                TapeOp::Cos(a) => {
                    let u = vals[*a];
                    let su = u.sin();
                    adj[*a] -= w * su;
                    adj_dot[*a] += wd * (-su) + w * (-u.cos()) * dot[*a];
                }
                TapeOp::Tan(a) => {
                    let t = vals[i];
                    let gp = 1.0 + t * t;
                    let gpp = 2.0 * t * gp;
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Atan(a) => {
                    let u = vals[*a];
                    let d = 1.0 + u * u;
                    let gp = 1.0 / d;
                    let gpp = -2.0 * u / (d * d);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Acos(a) => {
                    let u = vals[*a];
                    let s = 1.0 - u * u;
                    let r = s.sqrt();
                    let gp = -1.0 / r;
                    let gpp = -u / (s * r);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Sinh(a) => {
                    let u = vals[*a];
                    let gp = u.cosh();
                    let gpp = u.sinh();
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Cosh(a) => {
                    let u = vals[*a];
                    let gp = u.sinh();
                    let gpp = u.cosh();
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Tanh(a) => {
                    let t = vals[i];
                    let gp = 1.0 - t * t;
                    let gpp = -2.0 * t * gp;
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Asin(a) => {
                    let u = vals[*a];
                    let s = 1.0 - u * u;
                    let r = s.sqrt();
                    let gp = 1.0 / r;
                    let gpp = u / (s * r);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Acosh(a) => {
                    let u = vals[*a];
                    let s = u * u - 1.0;
                    let r = s.sqrt();
                    let gp = 1.0 / r;
                    let gpp = -u / (s * r);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Asinh(a) => {
                    let u = vals[*a];
                    let s = u * u + 1.0;
                    let r = s.sqrt();
                    let gp = 1.0 / r;
                    let gpp = -u / (s * r);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Atanh(a) => {
                    let u = vals[*a];
                    let d = 1.0 - u * u;
                    let gp = 1.0 / d;
                    let gpp = 2.0 * u / (d * d);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::Erf(a) => {
                    let u = vals[*a];
                    let gp = erf_d1(u);
                    let gpp = erf_d2(u);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::XLogX(a) => {
                    // gpp is 1/u, NOT the -1/u² any decomposition through
                    // `ln''` would produce. That is the whole point of the op.
                    let u = vals[*a];
                    let gp = xlogx_d1(u);
                    let gpp = xlogx_d2(u);
                    adj[*a] += w * gp;
                    adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                }
                TapeOp::CEntropy(a, b) => {
                    let ua = vals[*a];
                    let ub = vals[*b];
                    let fa = centropy_da(ua, ub);
                    let fb = centropy_db(ua, ub);
                    let faa = centropy_daa(ua);
                    let fab = centropy_dab(ub);
                    let fbb = centropy_dbb(ua, ub);
                    adj[*a] += w * fa;
                    adj[*b] += w * fb;
                    adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
                    adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
                }
                TapeOp::Atan2(a, b) => {
                    let y = vals[*a];
                    let x = vals[*b];
                    let d = y * y + x * x;
                    let d2 = d * d;
                    let fa = x / d;
                    let fb = -y / d;
                    let faa = -2.0 * y * x / d2;
                    let fab = (y * y - x * x) / d2;
                    let fbb = 2.0 * y * x / d2;
                    adj[*a] += w * fa;
                    adj[*b] += w * fb;
                    adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
                    adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
                }
                // min/max are piecewise linear (zero second derivative):
                // route the adjoint and its tangent into the selected
                // operand, exactly like the active branch of a Select.
                TapeOp::Min(a, b) => {
                    let br = if vals[*a] <= vals[*b] { *a } else { *b };
                    adj[br] += w;
                    adj_dot[br] += wd;
                }
                TapeOp::Max(a, b) => {
                    let br = if vals[*a] >= vals[*b] { *a } else { *b };
                    adj[br] += w;
                    adj_dot[br] += wd;
                }
                // Zero derivative: no first- or second-order adjoint.
                TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => {}
                // Route both the adjoint and its tangent into the
                // active branch; the condition contributes nothing.
                TapeOp::Select(c, t, e) => {
                    let br = if vals[*c] != 0.0 { *t } else { *e };
                    adj[br] += w;
                    adj_dot[br] += wd;
                }
                TapeOp::Funcall(fc) => {
                    let FuncallData { lib, name, args } = fc.as_ref();
                    let call_args = funcall_to_ext_args(args, vals);
                    let res = ext_eval_or_nan(lib, name, &call_args, args.len(), true, true);
                    let derivs = res.derivs.expect("want_derivs=true returns derivs");
                    let hes = res.hessian.expect("want_hes=true returns hessian");
                    let real_tape: Vec<usize> = args
                        .iter()
                        .filter_map(|a| match a {
                            TapeFuncallArg::Tape(t) => Some(*t),
                            TapeFuncallArg::Str(_) => None,
                        })
                        .collect();
                    for (k, &tk) in real_tape.iter().enumerate() {
                        adj[tk] += w * derivs[k];
                        let mut second_term = 0.0;
                        for (l, &tl) in real_tape.iter().enumerate() {
                            let (lo, hi) = if k <= l { (k, l) } else { (l, k) };
                            let h_kl = hes[lo + hi * (hi + 1) / 2];
                            second_term += h_kl * dot[tl];
                        }
                        adj_dot[tk] += wd * derivs[k] + w * second_term;
                    }
                }
            }
        }
    }

    /// Forward-over-reverse Hessian: for each variable `j` the tape
    /// depends on, accumulate `weight * (d²f / dx_i dx_j)` into
    /// `values[hess_map[(i, j)]]` for every `(i, j)` lower-triangle
    /// pair in the map. The same routine is used for the objective
    /// (with `weight = obj_factor`) and each active constraint (with
    /// `weight = lambda[k]`); contributions sum into the shared map.
    pub fn hessian_accumulate(
        &self,
        x: &[f64],
        weight: f64,
        hess_map: &HashMap<(usize, usize), usize>,
        values: &mut [f64],
    ) {
        let n = self.ops.len();
        if n == 0 || weight == 0.0 {
            return;
        }
        let v = self.forward(x);
        let var_indices = self.variables();

        // Hoist scratch allocations out of the per-variable loop —
        // each was costing O(n) per j on every hessian_accumulate
        // call, which dominated runtime on large tapes (the dense-
        // Hessian Mittelmann problems). `forward_tangent` fully
        // overwrites `dot`, so no reset is needed there. `adj` and
        // `adj_dot` are mutated additively, so we zero them per j.
        let mut dot = vec![0.0f64; n];
        let mut adj = vec![0.0f64; n];
        let mut adj_dot = vec![0.0f64; n];
        for &j in &var_indices {
            self.forward_tangent(&v, j, &mut dot);

            // adj[i] = standard adjoint (∂f/∂slot_i)
            // adj_dot[i] = derivative of adj[i] w.r.t. x_j = ∂²f/(∂slot_i ∂x_j)
            adj.fill(0.0);
            adj_dot.fill(0.0);
            adj[n - 1] = 1.0;

            for i in (0..n).rev() {
                let w = adj[i];
                let wd = adj_dot[i];
                if w == 0.0 && wd == 0.0 {
                    continue;
                }
                match &self.ops[i] {
                    TapeOp::Const(_) => {}
                    TapeOp::Var(k) => {
                        // Lower-triangle: only emit when row k >= col j
                        // so an off-diagonal pair appears once.
                        if wd != 0.0 && *k >= j {
                            if let Some(&pos) = hess_map.get(&(*k, j)) {
                                values[pos] += weight * wd;
                            }
                        }
                    }
                    TapeOp::Add(a, b) => {
                        adj[*a] += w;
                        adj[*b] += w;
                        adj_dot[*a] += wd;
                        adj_dot[*b] += wd;
                    }
                    TapeOp::Sub(a, b) => {
                        adj[*a] += w;
                        adj[*b] -= w;
                        adj_dot[*a] += wd;
                        adj_dot[*b] -= wd;
                    }
                    TapeOp::Mul(a, b) => {
                        adj[*a] += w * v[*b];
                        adj[*b] += w * v[*a];
                        adj_dot[*a] += wd * v[*b] + w * dot[*b];
                        adj_dot[*b] += wd * v[*a] + w * dot[*a];
                    }
                    TapeOp::Div(a, b) => {
                        // Kahan form -- see the identical arm in the dense
                        // reverse-over-forward sweep for the derivation.
                        let vb = v[*b];
                        let q = v[i];
                        let qd = dot[i];
                        adj[*a] += w / vb;
                        adj_dot[*a] += wd / vb - w * (dot[*b] / vb) / vb;
                        adj[*b] -= w * q / vb;
                        adj_dot[*b] += -(wd * q) / vb + (w / vb) * (-qd + q * (dot[*b] / vb));
                    }
                    TapeOp::Pow(a, b) => {
                        let u = v[*a];
                        let r = v[*b];
                        let du = dot[*a];
                        let dr = dot[*b];
                        if r != 0.0 {
                            if u != 0.0 {
                                let p_a = r * u.powf(r - 1.0);
                                adj[*a] += w * p_a;
                                let mut dp_a = dr * u.powf(r - 1.0);
                                if u > 0.0 {
                                    dp_a +=
                                        r * u.powf(r - 1.0) * ((r - 1.0) * du / u + dr * u.ln());
                                } else {
                                    dp_a += r * (r - 1.0) * u.powf(r - 2.0) * du;
                                }
                                adj_dot[*a] += wd * p_a + w * dp_a;
                            } else if r >= 2.0 {
                                let p_a = 0.0;
                                adj[*a] += w * p_a;
                                let dp_a = if r == 2.0 {
                                    2.0 * du
                                } else {
                                    r * (r - 1.0) * (0.0_f64).powf(r - 2.0) * du
                                };
                                adj_dot[*a] += wd * p_a + w * dp_a;
                            }
                        }
                        if u > 0.0 {
                            let ln_u = u.ln();
                            let p_b = v[i] * ln_u;
                            adj[*b] += w * p_b;
                            let dur = v[i] * (r * du / u + dr * ln_u);
                            let dp_b = dur * ln_u + v[i] * du / u;
                            adj_dot[*b] += wd * p_b + w * dp_b;
                        }
                    }
                    TapeOp::Neg(a) => {
                        adj[*a] -= w;
                        adj_dot[*a] -= wd;
                    }
                    TapeOp::Abs(a) => {
                        let s = if v[*a] >= 0.0 { 1.0 } else { -1.0 };
                        adj[*a] += w * s;
                        adj_dot[*a] += wd * s;
                    }
                    TapeOp::Sqrt(a) => {
                        let sv = v[i];
                        if sv > 0.0 {
                            let fp = 0.5 / sv;
                            let fpp = -0.25 / (v[*a] * sv);
                            adj[*a] += w * fp;
                            adj_dot[*a] += wd * fp + w * fpp * dot[*a];
                        }
                    }
                    TapeOp::Exp(a) => {
                        let ev = v[i];
                        adj[*a] += w * ev;
                        adj_dot[*a] += wd * ev + w * ev * dot[*a];
                    }
                    TapeOp::Log(a) => {
                        let u = v[*a];
                        adj[*a] += w / u;
                        adj_dot[*a] += wd / u + w * (-1.0 / (u * u)) * dot[*a];
                    }
                    TapeOp::Log10(a) => {
                        let u = v[*a];
                        let c = std::f64::consts::LN_10;
                        adj[*a] += w / (u * c);
                        adj_dot[*a] += wd / (u * c) + w * (-1.0 / (u * u * c)) * dot[*a];
                    }
                    TapeOp::Sin(a) => {
                        let u = v[*a];
                        let cu = u.cos();
                        adj[*a] += w * cu;
                        adj_dot[*a] += wd * cu + w * (-u.sin()) * dot[*a];
                    }
                    TapeOp::Cos(a) => {
                        let u = v[*a];
                        let su = u.sin();
                        adj[*a] -= w * su;
                        adj_dot[*a] += wd * (-su) + w * (-u.cos()) * dot[*a];
                    }
                    TapeOp::Tan(a) => {
                        let t = v[i];
                        let gp = 1.0 + t * t;
                        let gpp = 2.0 * t * gp;
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Atan(a) => {
                        let u = v[*a];
                        let d = 1.0 + u * u;
                        let gp = 1.0 / d;
                        let gpp = -2.0 * u / (d * d);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Acos(a) => {
                        let u = v[*a];
                        let s = 1.0 - u * u;
                        let r = s.sqrt();
                        let gp = -1.0 / r;
                        let gpp = -u / (s * r);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Sinh(a) => {
                        let u = v[*a];
                        let gp = u.cosh();
                        let gpp = u.sinh();
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Cosh(a) => {
                        let u = v[*a];
                        let gp = u.sinh();
                        let gpp = u.cosh();
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Tanh(a) => {
                        let t = v[i];
                        let gp = 1.0 - t * t;
                        let gpp = -2.0 * t * gp;
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Asin(a) => {
                        let u = v[*a];
                        let s = 1.0 - u * u;
                        let r = s.sqrt();
                        let gp = 1.0 / r;
                        let gpp = u / (s * r);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Acosh(a) => {
                        let u = v[*a];
                        let s = u * u - 1.0;
                        let r = s.sqrt();
                        let gp = 1.0 / r;
                        let gpp = -u / (s * r);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Asinh(a) => {
                        let u = v[*a];
                        let s = u * u + 1.0;
                        let r = s.sqrt();
                        let gp = 1.0 / r;
                        let gpp = -u / (s * r);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Atanh(a) => {
                        let u = v[*a];
                        let d = 1.0 - u * u;
                        let gp = 1.0 / d;
                        let gpp = 2.0 * u / (d * d);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::Erf(a) => {
                        let u = v[*a];
                        let gp = erf_d1(u);
                        let gpp = erf_d2(u);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::XLogX(a) => {
                        // See the identical arm in `hessian_accumulate`: gpp is
                        // 1/u, never the unrepresentable -1/u² of `ln''`.
                        let u = v[*a];
                        let gp = xlogx_d1(u);
                        let gpp = xlogx_d2(u);
                        adj[*a] += w * gp;
                        adj_dot[*a] += wd * gp + w * gpp * dot[*a];
                    }
                    TapeOp::CEntropy(a, b) => {
                        let ua = v[*a];
                        let ub = v[*b];
                        let fa = centropy_da(ua, ub);
                        let fb = centropy_db(ua, ub);
                        let faa = centropy_daa(ua);
                        let fab = centropy_dab(ub);
                        let fbb = centropy_dbb(ua, ub);
                        adj[*a] += w * fa;
                        adj[*b] += w * fb;
                        adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
                        adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
                    }
                    TapeOp::Atan2(a, b) => {
                        let y = v[*a];
                        let x = v[*b];
                        let d = y * y + x * x;
                        let d2 = d * d;
                        let fa = x / d;
                        let fb = -y / d;
                        let faa = -2.0 * y * x / d2;
                        let fab = (y * y - x * x) / d2;
                        let fbb = 2.0 * y * x / d2;
                        adj[*a] += w * fa;
                        adj[*b] += w * fb;
                        adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
                        adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
                    }
                    // min/max are piecewise linear (zero second
                    // derivative): route adjoint and its tangent into
                    // the selected operand, like an active Select branch.
                    TapeOp::Min(a, b) => {
                        let br = if v[*a] <= v[*b] { *a } else { *b };
                        adj[br] += w;
                        adj_dot[br] += wd;
                    }
                    TapeOp::Max(a, b) => {
                        let br = if v[*a] >= v[*b] { *a } else { *b };
                        adj[br] += w;
                        adj_dot[br] += wd;
                    }
                    // Zero derivative: no first- or second-order adjoint.
                    TapeOp::Cmp(_, _, _)
                    | TapeOp::And(_, _)
                    | TapeOp::Or(_, _)
                    | TapeOp::Not(_) => {}
                    // Route adjoint and its tangent into the active
                    // branch only; the condition contributes nothing.
                    TapeOp::Select(c, t, e) => {
                        let br = if v[*c] != 0.0 { *t } else { *e };
                        adj[br] += w;
                        adj_dot[br] += wd;
                    }
                    TapeOp::Funcall(fc) => {
                        let FuncallData { lib, name, args } = fc.as_ref();
                        let call_args = funcall_to_ext_args(args, &v);
                        let res = ext_eval_or_nan(lib, name, &call_args, args.len(), true, true);
                        let derivs = res.derivs.expect("want_derivs=true returns derivs");
                        let hes = res.hessian.expect("want_hes=true returns hessian");
                        let real_tape: Vec<usize> = args
                            .iter()
                            .filter_map(|a| match a {
                                TapeFuncallArg::Tape(t) => Some(*t),
                                TapeFuncallArg::Str(_) => None,
                            })
                            .collect();
                        for (k, &tk) in real_tape.iter().enumerate() {
                            adj[tk] += w * derivs[k];
                            let mut second_term = 0.0;
                            for (l, &tl) in real_tape.iter().enumerate() {
                                let (lo, hi) = if k <= l { (k, l) } else { (l, k) };
                                let h_kl = hes[lo + hi * (hi + 1) / 2];
                                second_term += h_kl * dot[tl];
                            }
                            adj_dot[tk] += wd * derivs[k] + w * second_term;
                        }
                    }
                }
            }
        }
    }

    /// Structural Hessian sparsity (lower triangle, row >= col).
    /// Propagates per-slot variable-dependence sets forward; each
    /// nonlinear op emits the cross/self products of its operand sets.
    /// Linear ops contribute no second-derivative pairs.
    pub fn hessian_sparsity(&self) -> BTreeSet<(usize, usize)> {
        let n = self.ops.len();
        let mut var_sets: Vec<BTreeSet<usize>> = Vec::with_capacity(n);
        let mut pairs: BTreeSet<(usize, usize)> = BTreeSet::new();

        // Slot lifetimes: `last_use[i]` is the highest-numbered op that
        // reads slot `i` (or `i` itself when nothing does). A slot's
        // variable set is dead the moment its last reader has been
        // processed, so it can be *moved into* that reader's result
        // instead of copied, and dropped otherwise.
        //
        // Without this the pass is O(n²) in both time and memory on any
        // tape carrying a long additive chain — `v0 * (v0 + v1 + … + vk)`,
        // the shape a dense Hessian row comes from — because every
        // partial sum leaves behind its own full-size `BTreeSet` and each
        // union copies the whole running set. `split_top_sums` keeps
        // *top-level* sums out of a single tape, so this only bites when
        // the sum feeds an enclosing operator, which is exactly the dense
        // case. With reuse, a left-leaning chain extends one set in place.
        let mut last_use: Vec<usize> = (0..n).collect();
        for (i, op) in self.ops.iter().enumerate() {
            for_each_input(op, |a| last_use[a] = i);
        }
        // Union of two operand sets, taking ownership of whichever dies
        // at this op rather than copying. `a == b` is handled first:
        // taking one would empty the other.
        macro_rules! merge {
            ($a:expr, $b:expr, $i:expr) => {{
                let (a, b, i) = ($a, $b, $i);
                if a == b {
                    if last_use[a] == i {
                        std::mem::take(&mut var_sets[a])
                    } else {
                        var_sets[a].clone()
                    }
                } else {
                    // Prefer moving the larger of the two dead sets.
                    let take_a = last_use[a] == i
                        && (last_use[b] != i || var_sets[a].len() >= var_sets[b].len());
                    if take_a {
                        let mut s = std::mem::take(&mut var_sets[a]);
                        s.extend(var_sets[b].iter().copied());
                        s
                    } else if last_use[b] == i {
                        let mut s = std::mem::take(&mut var_sets[b]);
                        s.extend(var_sets[a].iter().copied());
                        s
                    } else {
                        var_sets[a].union(&var_sets[b]).copied().collect()
                    }
                }
            }};
        }
        // Same, for the one-operand ops.
        macro_rules! carry {
            ($a:expr, $i:expr) => {{
                let (a, i) = ($a, $i);
                if last_use[a] == i {
                    std::mem::take(&mut var_sets[a])
                } else {
                    var_sets[a].clone()
                }
            }};
        }

        let emit_cross =
            |s1: &BTreeSet<usize>, s2: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
                for &v1 in s1 {
                    for &v2 in s2 {
                        let (r, c) = if v1 >= v2 { (v1, v2) } else { (v2, v1) };
                        pairs.insert((r, c));
                    }
                }
            };
        let emit_self = |s: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
            let vars: Vec<usize> = s.iter().copied().collect();
            for (ai, &vi) in vars.iter().enumerate() {
                for &vj in &vars[..=ai] {
                    let (r, c) = if vi >= vj { (vi, vj) } else { (vj, vi) };
                    pairs.insert((r, c));
                }
            }
        };

        for (i, op) in self.ops.iter().enumerate() {
            let vset = match op {
                TapeOp::Const(_) => BTreeSet::new(),
                TapeOp::Var(j) => {
                    let mut s = BTreeSet::new();
                    s.insert(*j);
                    s
                }
                TapeOp::Add(a, b) | TapeOp::Sub(a, b) => merge!(*a, *b, i),
                TapeOp::Neg(a) | TapeOp::Abs(a) => carry!(*a, i),
                TapeOp::Mul(a, b) => {
                    emit_cross(&var_sets[*a], &var_sets[*b], &mut pairs);
                    merge!(*a, *b, i)
                }
                TapeOp::Div(a, b) => {
                    emit_cross(&var_sets[*a], &var_sets[*b], &mut pairs);
                    emit_self(&var_sets[*b], &mut pairs);
                    merge!(*a, *b, i)
                }
                TapeOp::Pow(a, b) => {
                    let combined = merge!(*a, *b, i);
                    emit_self(&combined, &mut pairs);
                    combined
                }
                TapeOp::Sqrt(a)
                | TapeOp::Exp(a)
                | TapeOp::Log(a)
                | TapeOp::Log10(a)
                | TapeOp::Sin(a)
                | TapeOp::Cos(a)
                | TapeOp::Tan(a)
                | TapeOp::Atan(a)
                | TapeOp::Acos(a)
                | TapeOp::Sinh(a)
                | TapeOp::Cosh(a)
                | TapeOp::Tanh(a)
                | TapeOp::Asin(a)
                | TapeOp::Acosh(a)
                | TapeOp::Asinh(a)
                | TapeOp::Erf(a)
                | TapeOp::XLogX(a)
                | TapeOp::Atanh(a) => {
                    emit_self(&var_sets[*a], &mut pairs);
                    carry!(*a, i)
                }
                // atan2(y, x) and centropy(a, b) are nonlinear in both
                // operands with a full 2×2 second-derivative block; the
                // structural superset is every self/cross pair within the
                // combined operand set.
                TapeOp::Atan2(a, b) | TapeOp::CEntropy(a, b) => {
                    let combined = merge!(*a, *b, i);
                    emit_self(&combined, &mut pairs);
                    combined
                }
                // Comparisons / logical connectives are piecewise
                // constant: identically-zero derivative, so they
                // introduce no second-derivative pairs and carry no
                // variable dependence downstream (their result is a
                // constant as far as AD is concerned).
                TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => {
                    BTreeSet::new()
                }
                // Select passes through the active branch's value with
                // unit derivative, so it emits no pairs of its own; its
                // dependence set is the union of *both* branches
                // (either may become active as x varies — conservative
                // and correct for a structural superset). The condition
                // contributes no derivative and is excluded.
                TapeOp::Select(_c, t, e) => merge!(*t, *e, i),
                // min/max are piecewise linear: the active operand passes
                // through with unit derivative, so the second derivative is
                // identically zero (no pairs). Their dependence set is the
                // union of both operands (either may become active as x
                // varies — conservative and correct for a structural
                // superset), mirroring Select.
                TapeOp::Min(a, b) | TapeOp::Max(a, b) => merge!(*a, *b, i),
                TapeOp::Funcall(fc) => {
                    let args = &fc.args;
                    let mut combined: BTreeSet<usize> = BTreeSet::new();
                    for arg in args {
                        if let TapeFuncallArg::Tape(t) = arg {
                            for &vv in &var_sets[*t] {
                                combined.insert(vv);
                            }
                        }
                    }
                    emit_self(&combined, &mut pairs);
                    combined
                }
            };
            // Release any operand whose last reader was this op and which
            // `merge!` / `carry!` did not already move out (already-moved
            // slots are empty, so this is a no-op for them). Without it a
            // tape's peak memory is the sum of every intermediate set
            // rather than the live ones.
            for_each_input(op, |a| {
                if last_use[a] == i {
                    var_sets[a].clear();
                }
            });
            var_sets.push(vset);
        }
        pairs
    }
}

/// Call `f` on every tape slot `op` reads.
///
/// Deliberately written as an exhaustive match with **no catch-all arm**:
/// a new `TapeOp` variant must be added here or this stops compiling.
/// A silent "reads nothing" default would let `hessian_sparsity`'s
/// liveness analysis drop an operand's variable set while it is still
/// needed, which would quietly under-report Hessian sparsity — the kind
/// of wrong answer that surfaces as a bad search direction, not a panic.
/// Over-reporting an input is merely a missed optimization, so when in
/// doubt list the slot.
fn for_each_input(op: &TapeOp, mut f: impl FnMut(usize)) {
    match op {
        TapeOp::Const(_) | TapeOp::Var(_) => {}
        TapeOp::Neg(a)
        | TapeOp::Abs(a)
        | TapeOp::Sqrt(a)
        | TapeOp::Exp(a)
        | TapeOp::Log(a)
        | TapeOp::Log10(a)
        | TapeOp::Sin(a)
        | TapeOp::Cos(a)
        | TapeOp::Tan(a)
        | TapeOp::Atan(a)
        | TapeOp::Acos(a)
        | TapeOp::Sinh(a)
        | TapeOp::Cosh(a)
        | TapeOp::Tanh(a)
        | TapeOp::Asin(a)
        | TapeOp::Acosh(a)
        | TapeOp::Asinh(a)
        | TapeOp::Atanh(a)
        | TapeOp::Erf(a)
        | TapeOp::XLogX(a)
        | TapeOp::Not(a) => f(*a),
        TapeOp::Add(a, b)
        | TapeOp::Sub(a, b)
        | TapeOp::Mul(a, b)
        | TapeOp::Div(a, b)
        | TapeOp::Pow(a, b)
        | TapeOp::CEntropy(a, b)
        | TapeOp::Atan2(a, b)
        | TapeOp::Min(a, b)
        | TapeOp::Max(a, b)
        | TapeOp::And(a, b)
        | TapeOp::Or(a, b)
        | TapeOp::Cmp(_, a, b) => {
            f(*a);
            f(*b);
        }
        TapeOp::Select(c, t, e) => {
            f(*c);
            f(*t);
            f(*e);
        }
        TapeOp::Funcall(fc) => {
            for arg in &fc.args {
                if let TapeFuncallArg::Tape(t) = arg {
                    f(*t);
                }
            }
        }
    }
}

fn build_recursive(
    expr: &Expr,
    ops: &mut Vec<TapeOp>,
    cache: &mut HashMap<*const Expr, usize>,
    resolver: &ExternalResolver,
) -> usize {
    match expr {
        Expr::Const(c) => {
            let idx = ops.len();
            ops.push(TapeOp::Const(*c));
            idx
        }
        Expr::Var(i) => {
            let idx = ops.len();
            ops.push(TapeOp::Var(*i));
            idx
        }
        Expr::Binary(op, a, b) => {
            // Pow(x, const) is the dominant libm/dispatch cost in
            // transcendental-heavy AMPL tapes (henon, lane_emden, …):
            // `powf` itself is ~30–50 cycles AND the reverse-mode arm
            // for `Pow` carries an extra `ln(x)` branch. Rewriting
            // small integer / half-integer exponents into mul/sqrt
            // chains drops these calls entirely and reroutes the AD
            // through the much cheaper `Mul`/`Sqrt` arms.
            if let BinOp::Pow = op {
                if let Some(c) = peek_const(b) {
                    if let Some(idx) = try_emit_const_pow(a, c, ops, cache, resolver) {
                        return idx;
                    }
                }
            }
            let l = build_recursive(a, ops, cache, resolver);
            let r = build_recursive(b, ops, cache, resolver);
            let idx = ops.len();
            ops.push(match op {
                BinOp::Add => TapeOp::Add(l, r),
                BinOp::Sub => TapeOp::Sub(l, r),
                BinOp::Mul => TapeOp::Mul(l, r),
                BinOp::Div => TapeOp::Div(l, r),
                BinOp::Pow => TapeOp::Pow(l, r),
                BinOp::Atan2 => TapeOp::Atan2(l, r),
                BinOp::CEntropy => TapeOp::CEntropy(l, r),
            });
            idx
        }
        Expr::Unary(op, a) => {
            let v = build_recursive(a, ops, cache, resolver);
            let idx = ops.len();
            ops.push(match op {
                UnaryOp::Neg => TapeOp::Neg(v),
                UnaryOp::Sqrt => TapeOp::Sqrt(v),
                UnaryOp::Log => TapeOp::Log(v),
                UnaryOp::Log10 => TapeOp::Log10(v),
                UnaryOp::Exp => TapeOp::Exp(v),
                UnaryOp::Abs => TapeOp::Abs(v),
                UnaryOp::Sin => TapeOp::Sin(v),
                UnaryOp::Cos => TapeOp::Cos(v),
                UnaryOp::Tan => TapeOp::Tan(v),
                UnaryOp::Atan => TapeOp::Atan(v),
                UnaryOp::Acos => TapeOp::Acos(v),
                UnaryOp::Sinh => TapeOp::Sinh(v),
                UnaryOp::Cosh => TapeOp::Cosh(v),
                UnaryOp::Tanh => TapeOp::Tanh(v),
                UnaryOp::Asin => TapeOp::Asin(v),
                UnaryOp::Acosh => TapeOp::Acosh(v),
                UnaryOp::Asinh => TapeOp::Asinh(v),
                UnaryOp::Atanh => TapeOp::Atanh(v),
                UnaryOp::Erf => TapeOp::Erf(v),
                UnaryOp::XLogX => TapeOp::XLogX(v),
            });
            idx
        }
        Expr::Sum(args) => {
            if args.is_empty() {
                let idx = ops.len();
                ops.push(TapeOp::Const(0.0));
                return idx;
            }
            let mut acc = build_recursive(&args[0], ops, cache, resolver);
            for a in &args[1..] {
                let next = build_recursive(a, ops, cache, resolver);
                let idx = ops.len();
                ops.push(TapeOp::Add(acc, next));
                acc = idx;
            }
            acc
        }
        // n-ary min/max fold to a left-associative chain of binary
        // Min/Max TapeOps. The chain reproduces the list extremum, and
        // the binary Min/Max AD arms route the (sub)gradient to the
        // active operand at each step — equivalent to selecting the one
        // active operand of the whole list. An empty list cannot arise
        // from a well-formed `.nl` MINLIST/MAXLIST (count >= 1); guard
        // with a 0 constant for safety rather than panicking.
        Expr::MinList(args) | Expr::MaxList(args) => {
            let is_min = matches!(expr, Expr::MinList(_));
            if args.is_empty() {
                let idx = ops.len();
                ops.push(TapeOp::Const(0.0));
                return idx;
            }
            let mut acc = build_recursive(&args[0], ops, cache, resolver);
            for a in &args[1..] {
                let next = build_recursive(a, ops, cache, resolver);
                let idx = ops.len();
                ops.push(if is_min {
                    TapeOp::Min(acc, next)
                } else {
                    TapeOp::Max(acc, next)
                });
                acc = idx;
            }
            acc
        }
        Expr::Cse(body) => {
            // Cache by Arc identity so each shared body is emitted into
            // the tape exactly once and every reference resolves to the
            // same result-slot index. Forward computes the body once;
            // reverse-mode adjoint sums contributions from every ref
            // into that shared slot — exact chain rule for shared
            // sub-expressions.
            let key = Arc::as_ptr(body) as *const Expr;
            if let Some(&idx) = cache.get(&key) {
                idx
            } else {
                let idx = build_recursive(body, ops, cache, resolver);
                cache.insert(key, idx);
                idx
            }
        }
        Expr::Compare(op, a, b) => {
            let l = build_recursive(a, ops, cache, resolver);
            let r = build_recursive(b, ops, cache, resolver);
            let idx = ops.len();
            ops.push(TapeOp::Cmp(*op, l, r));
            idx
        }
        Expr::And(a, b) => {
            let l = build_recursive(a, ops, cache, resolver);
            let r = build_recursive(b, ops, cache, resolver);
            let idx = ops.len();
            ops.push(TapeOp::And(l, r));
            idx
        }
        Expr::Or(a, b) => {
            let l = build_recursive(a, ops, cache, resolver);
            let r = build_recursive(b, ops, cache, resolver);
            let idx = ops.len();
            ops.push(TapeOp::Or(l, r));
            idx
        }
        Expr::Not(a) => {
            let v = build_recursive(a, ops, cache, resolver);
            let idx = ops.len();
            ops.push(TapeOp::Not(v));
            idx
        }
        Expr::Cond { cond, then_, else_ } => {
            let c = build_recursive(cond, ops, cache, resolver);
            let t = build_recursive(then_, ops, cache, resolver);
            let e = build_recursive(else_, ops, cache, resolver);
            let idx = ops.len();
            ops.push(TapeOp::Select(c, t, e));
            idx
        }
        Expr::Funcall { id, args } => {
            let (lib, name) = resolver
                .funcs_by_id
                .get(id)
                .unwrap_or_else(|| panic!("unresolved AMPL funcall id {id}"));
            let tape_args: Vec<TapeFuncallArg> = args
                .iter()
                .map(|a| match a {
                    FuncallArg::Real(e) => {
                        TapeFuncallArg::Tape(build_recursive(e, ops, cache, resolver))
                    }
                    FuncallArg::Str(s) => TapeFuncallArg::Str(s.clone()),
                })
                .collect();
            let idx = ops.len();
            ops.push(TapeOp::Funcall(Box::new(FuncallData {
                lib: Arc::clone(lib),
                name: name.clone(),
                args: tape_args,
            })));
            idx
        }
    }
}

/// Resolve `e` to a literal constant if it is one (transparently
/// peering through `Cse` wrappers, which AMPL emits around shared
/// constants in CSE-heavy problems).
fn peek_const(e: &Expr) -> Option<f64> {
    match e {
        Expr::Const(c) => Some(*c),
        Expr::Cse(body) => peek_const(body),
        _ => None,
    }
}

/// Try to rewrite `base ^ exponent_const` into cheaper ops. Returns
/// the result tape-slot on success; `None` means "fall through to
/// generic Pow." Handles the cases that account for the bulk of
/// AMPL-emitted Pow nodes: integer exponents up to ±8 and the
/// `Sqrt`/passthrough/one specials. Half-integer exponents (e.g.
/// `^1.5`) and larger integers are left to generic `Pow` since the
/// resulting mul chain grows the tape faster than it saves work.
fn try_emit_const_pow(
    base_expr: &Expr,
    c: f64,
    ops: &mut Vec<TapeOp>,
    cache: &mut HashMap<*const Expr, usize>,
    resolver: &ExternalResolver,
) -> Option<usize> {
    if c == 0.0 {
        let idx = ops.len();
        ops.push(TapeOp::Const(1.0));
        return Some(idx);
    }
    if c == 1.0 {
        return Some(build_recursive(base_expr, ops, cache, resolver));
    }
    if c == 0.5 {
        let b = build_recursive(base_expr, ops, cache, resolver);
        let idx = ops.len();
        ops.push(TapeOp::Sqrt(b));
        return Some(idx);
    }
    // Integer exponents: bounded so a bad tape can't blow up the
    // op count. 8 covers everything AMPL typically emits for
    // polynomial models; beyond that the binary-expansion mul
    // chain (≥4 ops) starts to lose to a single `powf`.
    if c.is_finite() && c.fract() == 0.0 && c.abs() <= 8.0 {
        let n = c.abs() as u32;
        if n == 0 {
            // Already handled above, but guard.
            let idx = ops.len();
            ops.push(TapeOp::Const(1.0));
            return Some(idx);
        }
        let b = build_recursive(base_expr, ops, cache, resolver);
        let pos = emit_int_pow(b, n, ops);
        if c < 0.0 {
            // x^-n = 1 / x^n. Saves the powf and its ln branch in
            // reverse mode; cost is one Div in their place.
            let one_idx = ops.len();
            ops.push(TapeOp::Const(1.0));
            let idx = ops.len();
            ops.push(TapeOp::Div(one_idx, pos));
            return Some(idx);
        }
        return Some(pos);
    }
    None
}

/// Emit `base^n` for `n >= 1` as a binary-expansion mul chain.
/// Worst-case op count is `2·floor(log2(n))` — i.e. 1 op for n=2, 2
/// for n=3/4, 3 for n=5..8.
fn emit_int_pow(base: usize, n: u32, ops: &mut Vec<TapeOp>) -> usize {
    debug_assert!(n >= 1);
    if n == 1 {
        return base;
    }
    let half = emit_int_pow(base, n / 2, ops);
    let squared = ops.len();
    ops.push(TapeOp::Mul(half, half));
    if n % 2 == 1 {
        let idx = ops.len();
        ops.push(TapeOp::Mul(squared, base));
        idx
    } else {
        squared
    }
}

// ============================================================
// HybridTape: per-summand local tapes + shared CSE prelude.
//
// Partial separability — the .nl Sum/Add structure — gets each
// summand its own local Vec<SummandOp>. CSE bodies (V-segments
// in .nl) that appear in two or more summands are promoted into
// a single shared `prelude: Vec<TapeOp>`; per-summand references
// to a promoted CSE are SummandOp::Shared(prelude_slot).
//
// This is strictly better than either extreme:
//   - per-summand Tape (no cross-summand sharing): re-inlines
//     every shared CSE, blows up tape size when many constraints
//     share a stencil derivative (Mittelmann *120 problems).
//   - GlobalTape (single shared Vec<TapeOp> for everything):
//     per-root reverse sweeps scatter across a many-MB buffer,
//     thrashing cache when no CSE is actually shared (lane_emden
//     120: each constraint owns its own ops → 50% regression
//     vs per-summand tapes).
//
// Forward: prelude once, then each summand's local pass.
// Reverse / forward-over-reverse: per-summand sweep over local
// reach (which propagates adjoints into prelude_adj at Shared
// boundaries), then a small reverse pass over the summand's
// prelude_reach to fold those into grad / Hessian.
// ============================================================

/// One slot in a per-summand local tape.
#[derive(Debug, Clone)]
pub enum SummandOp {
    /// Local op — operand indices reference other slots in the
    /// same per-summand vector.
    Local(TapeOp),
    /// Pull a value from the shared prelude at slot `usize`. No
    /// downstream cost beyond the lookup; adjoints flowing into
    /// this slot accumulate into the prelude adjoint buffer.
    Shared(usize),
}

#[derive(Debug, Clone)]
pub struct Summand {
    pub ops: Vec<SummandOp>,
    /// Local slot holding the summand's final value.
    pub root_slot: usize,
    /// Local slots reachable from `root_slot`, ascending (topo).
    pub local_reach: Vec<usize>,
    /// Prelude slots reachable from the summand's Shared refs,
    /// ascending (topo in prelude's operand DAG).
    pub prelude_reach: Vec<usize>,
    /// Variables touched by Var ops inside `local_reach`.
    pub local_vars: Vec<usize>,
    /// Variables touched by Var ops inside `prelude_reach`.
    pub prelude_vars: Vec<usize>,
    /// `local_vars ∪ prelude_vars`, sorted — every problem variable this
    /// summand can contribute a derivative for. `eval_h` maps it through
    /// the Hessian coloring to decide which colors the summand
    /// participates in (issue #557); it must include `prelude_vars` or a
    /// summand would be skipped for colors it reaches only through a
    /// shared CSE body.
    pub all_vars: Vec<usize>,
}

#[derive(Debug, Clone)]
pub struct HybridTape {
    /// Shared CSE bodies. Slot indices in `SummandOp::Shared`
    /// point here; this Vec is built bottom-up by `build_recursive`,
    /// so operand indices are always less than the consumer's
    /// index (topo in ascending order).
    pub prelude: Vec<TapeOp>,
    pub summands: Vec<Summand>,
}

impl HybridTape {
    /// Build hybrid tape from a list of root expressions. CSE
    /// bodies referenced from ≥ 2 roots are promoted into the
    /// shared prelude; CSEs touched by only one root are inlined
    /// into that summand's local ops.
    pub fn build_multi(exprs: &[Expr]) -> Self {
        // Pass 1: per-Cse-pointer count of how many roots reference
        // it (each root contributes at most 1 to the count). The
        // ≥2 threshold means a CSE is shared across summands.
        let mut cse_count: HashMap<*const Expr, usize> = HashMap::new();
        for e in exprs {
            let mut seen_in_root: HashSet<*const Expr> = HashSet::new();
            count_cse_appearances(e, &mut seen_in_root, &mut cse_count);
        }

        // Pass 2: build prelude + each summand. The summand builder
        // hits the prelude path lazily — only when it encounters a
        // promoted Cse — so the prelude grows only with bodies that
        // are actually referenced from multiple summands.
        let mut prelude: Vec<TapeOp> = Vec::new();
        let mut prelude_map: HashMap<*const Expr, usize> = HashMap::new();
        let mut summands: Vec<Summand> = Vec::with_capacity(exprs.len());
        for e in exprs {
            let mut local: Vec<SummandOp> = Vec::new();
            let mut local_cache: HashMap<*const Expr, usize> = HashMap::new();
            let root_slot = build_into_summand(
                e,
                &mut local,
                &mut local_cache,
                &mut prelude,
                &mut prelude_map,
                &cse_count,
            );
            summands.push(Summand {
                ops: local,
                root_slot,
                local_reach: Vec::new(),
                prelude_reach: Vec::new(),
                local_vars: Vec::new(),
                prelude_vars: Vec::new(),
                all_vars: Vec::new(),
            });
        }

        // Pass 3: per-summand reach / vars. Prelude reach uses an
        // epoch-tagged shared visited buffer so total cost stays
        // O(Σ |prelude_reach_i|) rather than O(n_summands × |prelude|).
        let mut p_visited: Vec<u32> = vec![0; prelude.len()];
        let mut p_epoch: u32 = 0;
        let mut p_stack: Vec<usize> = Vec::new();
        for s in &mut summands {
            let (local_reach, shared_refs) = compute_local_reach(&s.ops, s.root_slot);
            s.local_reach = local_reach;

            let mut lv: BTreeSet<usize> = BTreeSet::new();
            for &i in &s.local_reach {
                if let SummandOp::Local(TapeOp::Var(j)) = &s.ops[i] {
                    lv.insert(*j);
                }
            }
            s.local_vars = lv.iter().copied().collect();

            if !shared_refs.is_empty() {
                p_epoch += 1;
                let mut preach: Vec<usize> = Vec::new();
                for &start in &shared_refs {
                    bfs_prelude(
                        &prelude,
                        start,
                        &mut p_visited,
                        p_epoch,
                        &mut p_stack,
                        &mut preach,
                    );
                }
                preach.sort_unstable();
                s.prelude_vars = vars_in(&prelude, &preach);
                s.prelude_reach = preach;
            }

            let mut av: BTreeSet<usize> = lv;
            for &v in &s.prelude_vars {
                av.insert(v);
            }
            s.all_vars = av.into_iter().collect();
        }

        HybridTape { prelude, summands }
    }

    pub fn n_prelude_ops(&self) -> usize {
        self.prelude.len()
    }
    pub fn n_summands(&self) -> usize {
        self.summands.len()
    }
    pub fn max_summand_ops(&self) -> usize {
        self.summands.iter().map(|s| s.ops.len()).max().unwrap_or(0)
    }
    pub fn total_local_ops(&self) -> usize {
        self.summands.iter().map(|s| s.ops.len()).sum()
    }

    /// Forward sweep over the shared prelude. `prelude_vals` must
    /// have length `n_prelude_ops`.
    pub fn forward_prelude(&self, x: &[f64], prelude_vals: &mut [f64]) {
        debug_assert_eq!(prelude_vals.len(), self.prelude.len());
        for i in 0..self.prelude.len() {
            prelude_vals[i] = fwd_step(&self.prelude[i], x, prelude_vals);
        }
    }

    /// Forward sweep over one summand. `local_vals` must hold at
    /// least `s.ops.len()` entries.
    pub fn forward_summand(
        &self,
        s: &Summand,
        x: &[f64],
        prelude_vals: &[f64],
        local_vals: &mut [f64],
    ) {
        debug_assert!(local_vals.len() >= s.ops.len());
        for i in 0..s.ops.len() {
            local_vals[i] = match &s.ops[i] {
                SummandOp::Local(op) => fwd_step(op, x, local_vals),
                SummandOp::Shared(k) => prelude_vals[*k],
            };
        }
    }

    /// Value at the summand root after `forward_summand`.
    #[inline]
    pub fn root_value(&self, s: &Summand, local_vals: &[f64]) -> f64 {
        local_vals[s.root_slot]
    }

    /// Reverse-mode gradient for one summand. Walks `local_reach`
    /// in reverse — propagating adjoints into `prelude_adj` at
    /// Shared boundaries — and then walks `prelude_reach` in
    /// reverse to land contributions in `grad`. Scratch arrays
    /// `local_adj` and `prelude_adj` are zeroed only at the slots
    /// actually touched.
    #[allow(clippy::too_many_arguments)]
    pub fn gradient_summand(
        &self,
        s: &Summand,
        prelude_vals: &[f64],
        local_vals: &[f64],
        seed: f64,
        grad: &mut [f64],
        local_adj: &mut [f64],
        prelude_adj: &mut [f64],
    ) {
        if seed == 0.0 || s.local_reach.is_empty() {
            return;
        }
        for &i in &s.local_reach {
            local_adj[i] = 0.0;
        }
        for &i in &s.prelude_reach {
            prelude_adj[i] = 0.0;
        }
        local_adj[s.root_slot] = seed;
        for &i in s.local_reach.iter().rev() {
            let a = local_adj[i];
            if a == 0.0 {
                continue;
            }
            match &s.ops[i] {
                SummandOp::Local(op) => rev_step(op, i, local_vals, local_adj, a, grad),
                SummandOp::Shared(k) => {
                    prelude_adj[*k] += a;
                }
            }
        }
        for &i in s.prelude_reach.iter().rev() {
            let a = prelude_adj[i];
            if a == 0.0 {
                continue;
            }
            rev_step(&self.prelude[i], i, prelude_vals, prelude_adj, a, grad);
        }
    }

    /// Forward tangent over the whole prelude for one dense seed
    /// vector (a Hessian color): `prelude_dot[i] = (∂prelude[i]/∂x) ·
    /// seed`. Runs **once per color for the entire constraint block**
    /// — this is the sweep the coloring lets every summand of that
    /// color share, and the reason the directional Hessian path
    /// exists at all (issue #557): a per-summand-per-variable seeding
    /// strategy would repeat it for every referencing summand.
    /// `reach` is the set of prelude slots this color actually uses —
    /// the union of `prelude_reach` over the color's summands. It must
    /// be **ascending** (the prelude is emitted bottom-up, so operand
    /// indices are always below their consumer's) and **closed under
    /// operands**, which a union of `prelude_reach` sets is by
    /// construction. Slots outside it are neither written nor read:
    /// a summand only pulls `prelude_dot[k]` for `k` in its own
    /// `prelude_reach`, so a stale tangent left in an untouched slot by
    /// a previous color can never be observed. Passing every slot is
    /// therefore always correct, just not always cheap — walking the
    /// whole prelude once per color is `n_colors × |prelude|` work
    /// against the `|prelude|` the op-ratio gate assumes.
    pub fn prelude_tangent(
        &self,
        prelude_vals: &[f64],
        seed: &[f64],
        reach: &[u32],
        prelude_dot: &mut [f64],
    ) {
        debug_assert!(prelude_dot.len() >= self.prelude.len());
        for &i in reach {
            let i = i as usize;
            prelude_dot[i] = fwd_dir_step(&self.prelude[i], seed, prelude_vals, prelude_dot, i);
        }
    }

    /// Directional forward-over-reverse Hessian pass for one summand
    /// with multiplier `weight`, sharing the per-color prelude tangent
    /// computed by [`prelude_tangent`]: forward-tangent the local ops
    /// (pulling `prelude_dot` at `Shared` boundaries), then
    /// reverse-over-tangent them, writing `weight * (H · seed)[k]`
    /// into the dense `out[k]` at each `Var(k)` — the same contract as
    /// [`Tape::hessian_directional`], so contributions land straight
    /// in the coloring's `compressed[c]` buffer.
    ///
    /// Adjoints crossing a `Shared` boundary are folded into
    /// `prelude_adj` / `prelude_adj_dot` **scaled by `weight`**, and
    /// left there: reverse-over-tangent is linear in its adjoint
    /// seeds, so accumulating `λ_k`-weighted adjoints across every
    /// summand of a color and running [`prelude_reverse_directional`]
    /// once with unit weight computes the same second-order prelude
    /// contribution as a per-summand sweep — while paying for the
    /// prelude walk once per color instead of once per (summand,
    /// color) pair.
    ///
    /// `local_vals` must hold this summand's forward values (see
    /// [`forward_summand`]); `local_dot` / `local_adj` /
    /// `local_adj_dot` are scratch arenas (≥ `s.ops.len()`), zeroed
    /// here only at the slots the summand reaches.
    ///
    /// [`prelude_tangent`]: HybridTape::prelude_tangent
    /// [`prelude_reverse_directional`]: HybridTape::prelude_reverse_directional
    /// [`forward_summand`]: HybridTape::forward_summand
    #[allow(clippy::too_many_arguments)]
    pub fn hessian_summand_directional(
        &self,
        s: &Summand,
        local_vals: &[f64],
        prelude_dot: &[f64],
        seed: &[f64],
        weight: f64,
        out: &mut [f64],
        local_dot: &mut [f64],
        local_adj: &mut [f64],
        local_adj_dot: &mut [f64],
        prelude_adj: &mut [f64],
        prelude_adj_dot: &mut [f64],
    ) {
        if weight == 0.0 || s.local_reach.is_empty() {
            return;
        }
        for &i in &s.local_reach {
            local_adj[i] = 0.0;
            local_adj_dot[i] = 0.0;
        }
        // `local_dot` needs no pre-zeroing: `local_reach` is ascending
        // and every operand of a reached op is itself reached, so each
        // slot is written before any read.
        for &i in &s.local_reach {
            local_dot[i] = match &s.ops[i] {
                SummandOp::Local(op) => fwd_dir_step(op, seed, local_vals, local_dot, i),
                SummandOp::Shared(k) => prelude_dot[*k],
            };
        }
        local_adj[s.root_slot] = 1.0;
        for &i in s.local_reach.iter().rev() {
            let w = local_adj[i];
            let wd = local_adj_dot[i];
            if w == 0.0 && wd == 0.0 {
                continue;
            }
            match &s.ops[i] {
                SummandOp::Local(op) => {
                    ror_dir_step(
                        op,
                        i,
                        local_vals,
                        local_dot,
                        local_adj,
                        local_adj_dot,
                        w,
                        wd,
                        weight,
                        out,
                    );
                }
                SummandOp::Shared(k) => {
                    prelude_adj[*k] += weight * w;
                    prelude_adj_dot[*k] += weight * wd;
                }
            }
        }
    }

    /// Reverse-over-tangent over the prelude with unit weight,
    /// consuming the adjoint accumulators built by
    /// [`hessian_summand_directional`] across all summands of one
    /// color and writing variable contributions into the dense `out`.
    ///
    /// `prelude_adj` / `prelude_adj_dot` must be all-zero except for
    /// the accumulated seeds on entry (allocate them zeroed and this
    /// invariant maintains itself); each slot is re-zeroed as it is
    /// consumed — adjoints only ever propagate to lower slots, which
    /// the reverse walk has not visited yet — so the buffers come back
    /// all-zero, ready for the next color, without an O(prelude) fill.
    ///
    /// [`hessian_summand_directional`]: HybridTape::hessian_summand_directional
    pub fn prelude_reverse_directional(
        &self,
        prelude_vals: &[f64],
        prelude_dot: &[f64],
        reach: &[u32],
        out: &mut [f64],
        prelude_adj: &mut [f64],
        prelude_adj_dot: &mut [f64],
    ) {
        for &i in reach.iter().rev() {
            let i = i as usize;
            let w = prelude_adj[i];
            let wd = prelude_adj_dot[i];
            if w == 0.0 && wd == 0.0 {
                continue;
            }
            prelude_adj[i] = 0.0;
            prelude_adj_dot[i] = 0.0;
            ror_dir_step(
                &self.prelude[i],
                i,
                prelude_vals,
                prelude_dot,
                prelude_adj,
                prelude_adj_dot,
                w,
                wd,
                1.0,
                out,
            );
        }
    }

    /// Structural Hessian sparsity over the whole hybrid tape:
    /// every pair the prelude or any summand can produce.
    pub fn hessian_sparsity_all(&self) -> BTreeSet<(usize, usize)> {
        let mut pairs = hessian_sparsity_impl(&self.prelude);

        // Per-prelude-slot var-set, reused across summands as the
        // var-set carrier for Shared refs.
        let prelude_var_sets = compute_var_sets(&self.prelude);

        for s in &self.summands {
            summand_sparsity(&s.ops, &prelude_var_sets, &mut pairs);
        }
        pairs
    }
}

/// Pass-1 helper: per-root walk that increments `counts[ptr]` the
/// first time a Cse pointer is encountered in this root. Recursing
/// into the body is gated on the first visit to avoid quadratic
/// blowup on heavily shared CSE DAGs.
/// True when `expr` (or any subexpression) is an AMPL external function
/// call. The hybrid summand path rejects funcalls outright, but the
/// *promoted*-CSE branch emits a shared CSE body via `build_recursive`
/// with an **empty** `ExternalResolver::default()` — it has no resolver
/// of its own. Without this pre-scan a funcall buried in a promoted CSE
/// would reach `build_recursive`'s `Expr::Funcall` arm and panic with the
/// misleading `unresolved AMPL funcall id <n>` message, instead of the
/// clear "not supported on the hybrid path" message the non-promoted
/// summand path raises. Pre-scanning makes both paths report the same
/// reason. (Funcalls are unsupported on the hybrid path regardless of
/// whether the id would resolve, so this never rejects a buildable tape.)
/// Whether [`HybridTape::build_multi`] can build `exprs`, i.e. none of
/// them uses an opcode the hybrid (partial-separability) path rejects:
/// comparisons, AND/OR/NOT, if-then-else, min/max lists, or AMPL external
/// function calls. `build_into_summand` *panics* on those, so a caller
/// choosing between the hybrid path and the flat [`Tape`] path has to ask
/// first — there is nothing to catch.
///
/// Walks with an explicit stack (expression DAGs from `.nl` files can be
/// deeper than the default thread stack) and visits each shared CSE body
/// once, so the cost is linear in the DAG rather than in its unfolding.
pub fn hybrid_supported(exprs: &[Expr]) -> bool {
    let mut stack: Vec<&Expr> = exprs.iter().collect();
    let mut seen_cse: HashSet<*const Expr> = HashSet::new();
    while let Some(e) = stack.pop() {
        match e {
            Expr::Const(_) | Expr::Var(_) => {}
            Expr::Binary(_, a, b) => {
                stack.push(a);
                stack.push(b);
            }
            Expr::Unary(_, a) => stack.push(a),
            Expr::Sum(args) => stack.extend(args.iter()),
            Expr::Cse(body) => {
                if seen_cse.insert(Arc::as_ptr(body)) {
                    stack.push(body);
                }
            }
            Expr::Compare(..)
            | Expr::And(..)
            | Expr::Or(..)
            | Expr::Not(_)
            | Expr::Cond { .. }
            | Expr::MinList(_)
            | Expr::MaxList(_)
            | Expr::Funcall { .. } => return false,
        }
    }
    true
}

fn cse_contains_funcall(expr: &Expr) -> bool {
    match expr {
        Expr::Funcall { .. } => true,
        Expr::Const(_) | Expr::Var(_) => false,
        Expr::Binary(_, a, b) => cse_contains_funcall(a) || cse_contains_funcall(b),
        Expr::Unary(_, a) => cse_contains_funcall(a),
        Expr::Sum(args) | Expr::MinList(args) | Expr::MaxList(args) => {
            args.iter().any(cse_contains_funcall)
        }
        Expr::Compare(_, a, b) | Expr::And(a, b) | Expr::Or(a, b) => {
            cse_contains_funcall(a) || cse_contains_funcall(b)
        }
        Expr::Not(a) => cse_contains_funcall(a),
        Expr::Cond { cond, then_, else_ } => {
            cse_contains_funcall(cond) || cse_contains_funcall(then_) || cse_contains_funcall(else_)
        }
        Expr::Cse(body) => cse_contains_funcall(body),
    }
}

fn count_cse_appearances(
    e: &Expr,
    seen_in_root: &mut HashSet<*const Expr>,
    counts: &mut HashMap<*const Expr, usize>,
) {
    match e {
        Expr::Const(_) | Expr::Var(_) => {}
        Expr::Binary(_, a, b) => {
            count_cse_appearances(a, seen_in_root, counts);
            count_cse_appearances(b, seen_in_root, counts);
        }
        Expr::Unary(_, a) => count_cse_appearances(a, seen_in_root, counts),
        Expr::Sum(args) | Expr::MinList(args) | Expr::MaxList(args) => {
            for a in args {
                count_cse_appearances(a, seen_in_root, counts);
            }
        }
        Expr::Compare(_, a, b) | Expr::And(a, b) | Expr::Or(a, b) => {
            count_cse_appearances(a, seen_in_root, counts);
            count_cse_appearances(b, seen_in_root, counts);
        }
        Expr::Not(a) => count_cse_appearances(a, seen_in_root, counts),
        Expr::Cond { cond, then_, else_ } => {
            count_cse_appearances(cond, seen_in_root, counts);
            count_cse_appearances(then_, seen_in_root, counts);
            count_cse_appearances(else_, seen_in_root, counts);
        }
        Expr::Cse(body) => {
            let key = Arc::as_ptr(body) as *const Expr;
            if seen_in_root.insert(key) {
                *counts.entry(key).or_insert(0) += 1;
                count_cse_appearances(body, seen_in_root, counts);
            }
        }
        Expr::Funcall { args, .. } => {
            for arg in args {
                if let FuncallArg::Real(e) = arg {
                    count_cse_appearances(e, seen_in_root, counts);
                }
            }
        }
    }
}

/// Recursive summand builder. CSEs that meet the promotion bar
/// (≥ 2 roots reference them per `cse_count`) get a single prelude
/// emission via `build_recursive`; the summand records a Shared op
/// pointing at the prelude slot. Non-promoted CSEs are inlined
/// into the summand with intra-summand Arc-pointer dedup.
fn build_into_summand(
    expr: &Expr,
    local: &mut Vec<SummandOp>,
    local_cache: &mut HashMap<*const Expr, usize>,
    prelude: &mut Vec<TapeOp>,
    prelude_map: &mut HashMap<*const Expr, usize>,
    cse_count: &HashMap<*const Expr, usize>,
) -> usize {
    match expr {
        Expr::Const(c) => {
            let i = local.len();
            local.push(SummandOp::Local(TapeOp::Const(*c)));
            i
        }
        Expr::Var(j) => {
            let i = local.len();
            local.push(SummandOp::Local(TapeOp::Var(*j)));
            i
        }
        Expr::Binary(op, a, b) => {
            if let BinOp::Pow = op {
                if let Some(c) = peek_const(b) {
                    if let Some(i) = try_emit_const_pow_summand(
                        a,
                        c,
                        local,
                        local_cache,
                        prelude,
                        prelude_map,
                        cse_count,
                    ) {
                        return i;
                    }
                }
            }
            let l = build_into_summand(a, local, local_cache, prelude, prelude_map, cse_count);
            let r = build_into_summand(b, local, local_cache, prelude, prelude_map, cse_count);
            let i = local.len();
            local.push(SummandOp::Local(match op {
                BinOp::Add => TapeOp::Add(l, r),
                BinOp::Sub => TapeOp::Sub(l, r),
                BinOp::Mul => TapeOp::Mul(l, r),
                BinOp::Div => TapeOp::Div(l, r),
                BinOp::Pow => TapeOp::Pow(l, r),
                BinOp::Atan2 => TapeOp::Atan2(l, r),
                BinOp::CEntropy => TapeOp::CEntropy(l, r),
            }));
            i
        }
        Expr::Unary(op, a) => {
            let v = build_into_summand(a, local, local_cache, prelude, prelude_map, cse_count);
            let i = local.len();
            local.push(SummandOp::Local(match op {
                UnaryOp::Neg => TapeOp::Neg(v),
                UnaryOp::Sqrt => TapeOp::Sqrt(v),
                UnaryOp::Log => TapeOp::Log(v),
                UnaryOp::Log10 => TapeOp::Log10(v),
                UnaryOp::Exp => TapeOp::Exp(v),
                UnaryOp::Abs => TapeOp::Abs(v),
                UnaryOp::Sin => TapeOp::Sin(v),
                UnaryOp::Cos => TapeOp::Cos(v),
                UnaryOp::Tan => TapeOp::Tan(v),
                UnaryOp::Atan => TapeOp::Atan(v),
                UnaryOp::Acos => TapeOp::Acos(v),
                UnaryOp::Sinh => TapeOp::Sinh(v),
                UnaryOp::Cosh => TapeOp::Cosh(v),
                UnaryOp::Tanh => TapeOp::Tanh(v),
                UnaryOp::Asin => TapeOp::Asin(v),
                UnaryOp::Acosh => TapeOp::Acosh(v),
                UnaryOp::Asinh => TapeOp::Asinh(v),
                UnaryOp::Atanh => TapeOp::Atanh(v),
                UnaryOp::Erf => TapeOp::Erf(v),
                UnaryOp::XLogX => TapeOp::XLogX(v),
            }));
            i
        }
        Expr::Sum(args) => {
            if args.is_empty() {
                let i = local.len();
                local.push(SummandOp::Local(TapeOp::Const(0.0)));
                return i;
            }
            let mut acc = build_into_summand(
                &args[0],
                local,
                local_cache,
                prelude,
                prelude_map,
                cse_count,
            );
            for a in &args[1..] {
                let nxt =
                    build_into_summand(a, local, local_cache, prelude, prelude_map, cse_count);
                let i = local.len();
                local.push(SummandOp::Local(TapeOp::Add(acc, nxt)));
                acc = i;
            }
            acc
        }
        Expr::Cse(body) => {
            let key = Arc::as_ptr(body) as *const Expr;
            if let Some(&li) = local_cache.get(&key) {
                return li;
            }
            let promoted = cse_count.get(&key).copied().unwrap_or(0) >= 2;
            if promoted {
                // `build_recursive` below runs with an empty resolver, so a
                // funcall hidden inside the promoted body would panic with the
                // misleading "unresolved AMPL funcall id" message rather than
                // the clear hybrid-unsupported message the non-promoted summand
                // path (and the `Expr::Funcall` arm at the bottom) raises.
                // Reject it up front so both CSE paths report the same reason.
                if cse_contains_funcall(body) {
                    panic!(
                        "HybridTape: AMPL external function calls are not supported on the \
                         hybrid (partial-separability) tape path. Build with \
                         Tape::build_with_externals instead."
                    );
                }
                // Build (or reuse) the prelude slot for this CSE.
                // `build_recursive(expr, ...)` hits the Cse arm,
                // emits the body once into prelude, and caches it
                // in `prelude_map` keyed by this Arc pointer.
                let pslot =
                    build_recursive(expr, prelude, prelude_map, &ExternalResolver::default());
                let li = local.len();
                local.push(SummandOp::Shared(pslot));
                local_cache.insert(key, li);
                li
            } else {
                let li =
                    build_into_summand(body, local, local_cache, prelude, prelude_map, cse_count);
                local_cache.insert(key, li);
                li
            }
        }
        Expr::Compare(_, _, _)
        | Expr::And(_, _)
        | Expr::Or(_, _)
        | Expr::Not(_)
        | Expr::Cond { .. }
        | Expr::MinList(_)
        | Expr::MaxList(_) => {
            panic!(
                "HybridTape: conditional / logical / min-max opcodes (comparisons, \
                 AND/OR/NOT, if-then-else, min/max lists) are not supported on the \
                 hybrid (partial-separability) tape path. Build with \
                 Tape::build_with_externals instead."
            );
        }
        Expr::Funcall { .. } => {
            panic!(
                "HybridTape: AMPL external function calls are not supported on the \
                 hybrid (partial-separability) tape path. Build with Tape::build_with_externals \
                 instead."
            );
        }
    }
}

/// Pow-lowering specialised for summand builds. Mirrors
/// `try_emit_const_pow` but with summand-flavoured emission.
fn try_emit_const_pow_summand(
    base_expr: &Expr,
    c: f64,
    local: &mut Vec<SummandOp>,
    local_cache: &mut HashMap<*const Expr, usize>,
    prelude: &mut Vec<TapeOp>,
    prelude_map: &mut HashMap<*const Expr, usize>,
    cse_count: &HashMap<*const Expr, usize>,
) -> Option<usize> {
    if c == 0.0 {
        let i = local.len();
        local.push(SummandOp::Local(TapeOp::Const(1.0)));
        return Some(i);
    }
    if c == 1.0 {
        return Some(build_into_summand(
            base_expr,
            local,
            local_cache,
            prelude,
            prelude_map,
            cse_count,
        ));
    }
    if c == 0.5 {
        let b = build_into_summand(
            base_expr,
            local,
            local_cache,
            prelude,
            prelude_map,
            cse_count,
        );
        let i = local.len();
        local.push(SummandOp::Local(TapeOp::Sqrt(b)));
        return Some(i);
    }
    if c.is_finite() && c.fract() == 0.0 && c.abs() <= 8.0 {
        let n = c.abs() as u32;
        if n == 0 {
            let i = local.len();
            local.push(SummandOp::Local(TapeOp::Const(1.0)));
            return Some(i);
        }
        let b = build_into_summand(
            base_expr,
            local,
            local_cache,
            prelude,
            prelude_map,
            cse_count,
        );
        let pos = emit_int_pow_summand(b, n, local);
        if c < 0.0 {
            let one_idx = local.len();
            local.push(SummandOp::Local(TapeOp::Const(1.0)));
            let i = local.len();
            local.push(SummandOp::Local(TapeOp::Div(one_idx, pos)));
            return Some(i);
        }
        return Some(pos);
    }
    None
}

fn emit_int_pow_summand(base: usize, n: u32, local: &mut Vec<SummandOp>) -> usize {
    debug_assert!(n >= 1);
    if n == 1 {
        return base;
    }
    let half = emit_int_pow_summand(base, n / 2, local);
    let squared = local.len();
    local.push(SummandOp::Local(TapeOp::Mul(half, half)));
    if n % 2 == 1 {
        let i = local.len();
        local.push(SummandOp::Local(TapeOp::Mul(squared, base)));
        i
    } else {
        squared
    }
}

/// Walk a summand's local op DAG from `root`, returning the
/// reachable local slots (sorted ascending) plus the distinct
/// prelude slots referenced by any Shared op along the way.
fn compute_local_reach(ops: &[SummandOp], root: usize) -> (Vec<usize>, Vec<usize>) {
    let mut visited = vec![false; ops.len()];
    let mut reach: Vec<usize> = Vec::new();
    let mut shared: BTreeSet<usize> = BTreeSet::new();
    let mut stack: Vec<usize> = Vec::with_capacity(16);
    visited[root] = true;
    reach.push(root);
    stack.push(root);
    while let Some(s) = stack.pop() {
        match &ops[s] {
            SummandOp::Local(op) => {
                let (a, b) = op_operands(op);
                if let Some(a) = a {
                    if !visited[a] {
                        visited[a] = true;
                        reach.push(a);
                        stack.push(a);
                    }
                }
                if let Some(b) = b {
                    if !visited[b] {
                        visited[b] = true;
                        reach.push(b);
                        stack.push(b);
                    }
                }
            }
            SummandOp::Shared(k) => {
                shared.insert(*k);
            }
        }
    }
    reach.sort_unstable();
    (reach, shared.into_iter().collect())
}

/// Epoch-tagged BFS over the prelude operand DAG, accumulating
/// reachable slots into `out`. Caller is responsible for sorting
/// `out` after a batch of starts has been processed.
fn bfs_prelude(
    prelude: &[TapeOp],
    start: usize,
    visited: &mut [u32],
    cur: u32,
    stack: &mut Vec<usize>,
    out: &mut Vec<usize>,
) {
    if visited[start] == cur {
        return;
    }
    visited[start] = cur;
    out.push(start);
    stack.push(start);
    while let Some(s) = stack.pop() {
        let (a, b) = op_operands(&prelude[s]);
        if let Some(a) = a {
            if visited[a] != cur {
                visited[a] = cur;
                out.push(a);
                stack.push(a);
            }
        }
        if let Some(b) = b {
            if visited[b] != cur {
                visited[b] = cur;
                out.push(b);
                stack.push(b);
            }
        }
    }
}

/// Per-op var-set for the prelude — every slot's transitive
/// variable footprint. Used by `summand_sparsity` to expand
/// `SummandOp::Shared(k)` into its var-set carrier.
fn compute_var_sets(ops: &[TapeOp]) -> Vec<BTreeSet<usize>> {
    let mut out: Vec<BTreeSet<usize>> = Vec::with_capacity(ops.len());
    for op in ops {
        let vs: BTreeSet<usize> = match op {
            TapeOp::Const(_) => BTreeSet::new(),
            TapeOp::Var(j) => {
                let mut s = BTreeSet::new();
                s.insert(*j);
                s
            }
            TapeOp::Add(a, b)
            | TapeOp::Sub(a, b)
            | TapeOp::Mul(a, b)
            | TapeOp::Div(a, b)
            | TapeOp::Pow(a, b)
            | TapeOp::Atan2(a, b)
            | TapeOp::CEntropy(a, b) => out[*a].union(&out[*b]).copied().collect(),
            TapeOp::Neg(a)
            | TapeOp::Abs(a)
            | TapeOp::Sqrt(a)
            | TapeOp::Exp(a)
            | TapeOp::Log(a)
            | TapeOp::Log10(a)
            | TapeOp::Sin(a)
            | TapeOp::Cos(a)
            | TapeOp::Tan(a)
            | TapeOp::Atan(a)
            | TapeOp::Acos(a)
            | TapeOp::Sinh(a)
            | TapeOp::Cosh(a)
            | TapeOp::Tanh(a)
            | TapeOp::Asin(a)
            | TapeOp::Acosh(a)
            | TapeOp::Asinh(a)
            | TapeOp::Erf(a)
            | TapeOp::XLogX(a)
            | TapeOp::Atanh(a) => out[*a].clone(),
            TapeOp::Cmp(_, _, _)
            | TapeOp::And(_, _)
            | TapeOp::Or(_, _)
            | TapeOp::Not(_)
            | TapeOp::Select(_, _, _)
            | TapeOp::Min(_, _)
            | TapeOp::Max(_, _) => unreachable!(
                "HybridTape prelude cannot contain conditional / logical / min-max \
                 TapeOps; build_into_summand panics on those Expr variants."
            ),
            TapeOp::Funcall(_) => unreachable!(
                "HybridTape prelude cannot contain TapeOp::Funcall; \
                 build_into_summand panics on Expr::Funcall."
            ),
        };
        out.push(vs);
    }
    out
}

/// Per-op Hessian-sparsity propagation over a summand's mixed
/// SummandOp slice. Shared refs contribute their prelude var-set
/// but do not themselves emit pairs (those came from
/// `hessian_sparsity_impl(&prelude)`).
fn summand_sparsity(
    ops: &[SummandOp],
    prelude_var_sets: &[BTreeSet<usize>],
    pairs: &mut BTreeSet<(usize, usize)>,
) {
    let mut var_sets: Vec<BTreeSet<usize>> = Vec::with_capacity(ops.len());
    let emit_cross =
        |s1: &BTreeSet<usize>, s2: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
            for &v1 in s1 {
                for &v2 in s2 {
                    let (r, c) = if v1 >= v2 { (v1, v2) } else { (v2, v1) };
                    pairs.insert((r, c));
                }
            }
        };
    let emit_self = |s: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
        let vars: Vec<usize> = s.iter().copied().collect();
        for (ai, &vi) in vars.iter().enumerate() {
            for &vj in &vars[..=ai] {
                let (r, c) = if vi >= vj { (vi, vj) } else { (vj, vi) };
                pairs.insert((r, c));
            }
        }
    };
    for so in ops {
        let vset: BTreeSet<usize> = match so {
            SummandOp::Shared(k) => prelude_var_sets[*k].clone(),
            SummandOp::Local(op) => match op {
                TapeOp::Const(_) => BTreeSet::new(),
                TapeOp::Var(j) => {
                    let mut s = BTreeSet::new();
                    s.insert(*j);
                    s
                }
                TapeOp::Add(a, b) | TapeOp::Sub(a, b) => {
                    var_sets[*a].union(&var_sets[*b]).copied().collect()
                }
                TapeOp::Neg(a) | TapeOp::Abs(a) => var_sets[*a].clone(),
                TapeOp::Mul(a, b) => {
                    emit_cross(&var_sets[*a], &var_sets[*b], pairs);
                    var_sets[*a].union(&var_sets[*b]).copied().collect()
                }
                TapeOp::Div(a, b) => {
                    emit_cross(&var_sets[*a], &var_sets[*b], pairs);
                    emit_self(&var_sets[*b], pairs);
                    var_sets[*a].union(&var_sets[*b]).copied().collect()
                }
                TapeOp::Pow(a, b) | TapeOp::Atan2(a, b) | TapeOp::CEntropy(a, b) => {
                    let combined: BTreeSet<usize> =
                        var_sets[*a].union(&var_sets[*b]).copied().collect();
                    emit_self(&combined, pairs);
                    combined
                }
                TapeOp::Sqrt(a)
                | TapeOp::Exp(a)
                | TapeOp::Log(a)
                | TapeOp::Log10(a)
                | TapeOp::Sin(a)
                | TapeOp::Cos(a)
                | TapeOp::Tan(a)
                | TapeOp::Atan(a)
                | TapeOp::Acos(a)
                | TapeOp::Sinh(a)
                | TapeOp::Cosh(a)
                | TapeOp::Tanh(a)
                | TapeOp::Asin(a)
                | TapeOp::Acosh(a)
                | TapeOp::Asinh(a)
                | TapeOp::Erf(a)
                | TapeOp::XLogX(a)
                | TapeOp::Atanh(a) => {
                    emit_self(&var_sets[*a], pairs);
                    var_sets[*a].clone()
                }
                TapeOp::Cmp(_, _, _)
                | TapeOp::And(_, _)
                | TapeOp::Or(_, _)
                | TapeOp::Not(_)
                | TapeOp::Select(_, _, _)
                | TapeOp::Min(_, _)
                | TapeOp::Max(_, _) => unreachable!(
                    "HybridTape summand cannot contain conditional / logical / min-max \
                     TapeOps; build_into_summand panics on those Expr variants."
                ),
                TapeOp::Funcall(_) => unreachable!(
                    "HybridTape summand cannot contain TapeOp::Funcall; \
                     build_into_summand panics on Expr::Funcall."
                ),
            },
        };
        var_sets.push(vset);
    }
}

/// Operand indices of a `TapeOp`, normalized into a fixed-length
/// array so callers don't need to re-match every site.
#[inline]
pub(crate) fn op_operands(op: &TapeOp) -> (Option<usize>, Option<usize>) {
    match op {
        TapeOp::Const(_) | TapeOp::Var(_) => (None, None),
        TapeOp::Add(a, b)
        | TapeOp::Sub(a, b)
        | TapeOp::Mul(a, b)
        | TapeOp::Div(a, b)
        | TapeOp::Pow(a, b)
        | TapeOp::Atan2(a, b)
        | TapeOp::CEntropy(a, b) => (Some(*a), Some(*b)),
        TapeOp::Neg(a)
        | TapeOp::Abs(a)
        | TapeOp::Sqrt(a)
        | TapeOp::Exp(a)
        | TapeOp::Log(a)
        | TapeOp::Log10(a)
        | TapeOp::Sin(a)
        | TapeOp::Cos(a)
        | TapeOp::Tan(a)
        | TapeOp::Atan(a)
        | TapeOp::Acos(a)
        | TapeOp::Sinh(a)
        | TapeOp::Cosh(a)
        | TapeOp::Tanh(a)
        | TapeOp::Asin(a)
        | TapeOp::Acosh(a)
        | TapeOp::Asinh(a)
        | TapeOp::Erf(a)
        | TapeOp::XLogX(a)
        | TapeOp::Atanh(a) => (Some(*a), None),
        // Conditional / logical TapeOps never reach the HybridTape
        // operand-walk (build_into_summand rejects them). Cmp/And/Or
        // have two operands; Not has one; Select's three can't be
        // expressed in this two-slot shape, so it would be a bug to
        // see it here.
        TapeOp::Cmp(_, a, b) | TapeOp::And(a, b) | TapeOp::Or(a, b) => (Some(*a), Some(*b)),
        TapeOp::Not(a) => (Some(*a), None),
        TapeOp::Select(_, _, _) => unreachable!(
            "op_operands: TapeOp::Select has three operands and is unsupported on \
             the HybridTape path"
        ),
        TapeOp::Min(_, _) | TapeOp::Max(_, _) => unreachable!(
            "op_operands: TapeOp::Min/Max are unsupported on the HybridTape path \
             (build_into_summand rejects min/max lists)"
        ),
        // Returning `(None, None)` here would be a silent wrong answer, not a
        // conservative one: a Funcall's tape arguments would be missing from
        // `local_reach`, and the directional Hessian deliberately does NOT
        // pre-zero `local_dot`, so those slots would be read as stale scratch
        // from a previous summand. Unreachable today —
        // `build_into_summand` panics on `Expr::Funcall` before any tape is
        // built — and this keeps it loud if that ever changes.
        TapeOp::Funcall(_) => unreachable!(
            "op_operands: TapeOp::Funcall is unsupported on the HybridTape path \
             (build_into_summand rejects external function calls)"
        ),
    }
}

fn vars_in(ops: &[TapeOp], reach: &[usize]) -> Vec<usize> {
    let mut s: BTreeSet<usize> = BTreeSet::new();
    for &i in reach {
        if let TapeOp::Var(j) = &ops[i] {
            s.insert(*j);
        }
    }
    s.into_iter().collect()
}

// ----- Free-function AD step kernels used by GlobalTape -----

#[inline]
fn fwd_step(op: &TapeOp, x: &[f64], vals: &[f64]) -> f64 {
    match op {
        TapeOp::Const(c) => *c,
        TapeOp::Var(i) => x[*i],
        TapeOp::Add(a, b) => vals[*a] + vals[*b],
        TapeOp::Sub(a, b) => vals[*a] - vals[*b],
        TapeOp::Mul(a, b) => vals[*a] * vals[*b],
        TapeOp::Div(a, b) => vals[*a] / vals[*b],
        TapeOp::Pow(a, b) => vals[*a].powf(vals[*b]),
        TapeOp::Neg(a) => -vals[*a],
        TapeOp::Abs(a) => vals[*a].abs(),
        TapeOp::Sqrt(a) => vals[*a].sqrt(),
        TapeOp::Exp(a) => vals[*a].exp(),
        TapeOp::Log(a) => vals[*a].ln(),
        TapeOp::Log10(a) => vals[*a].log10(),
        TapeOp::Sin(a) => vals[*a].sin(),
        TapeOp::Cos(a) => vals[*a].cos(),
        TapeOp::Tan(a) => vals[*a].tan(),
        TapeOp::Atan(a) => vals[*a].atan(),
        TapeOp::Acos(a) => vals[*a].acos(),
        TapeOp::Sinh(a) => vals[*a].sinh(),
        TapeOp::Cosh(a) => vals[*a].cosh(),
        TapeOp::Tanh(a) => vals[*a].tanh(),
        TapeOp::Asin(a) => vals[*a].asin(),
        TapeOp::Acosh(a) => vals[*a].acosh(),
        TapeOp::Asinh(a) => vals[*a].asinh(),
        TapeOp::Atanh(a) => vals[*a].atanh(),
        TapeOp::Erf(a) => erf(vals[*a]),
        TapeOp::XLogX(a) => xlogx(vals[*a]),
        TapeOp::CEntropy(a, b) => centropy(vals[*a], vals[*b]),
        TapeOp::Atan2(a, b) => vals[*a].atan2(vals[*b]),
        TapeOp::Cmp(_, _, _)
        | TapeOp::And(_, _)
        | TapeOp::Or(_, _)
        | TapeOp::Not(_)
        | TapeOp::Select(_, _, _)
        | TapeOp::Min(_, _)
        | TapeOp::Max(_, _) => panic!(
            "GlobalTape free-function kernels do not implement conditional / logical \
             / min-max TapeOps; use the Tape (build_with_externals) interpreter path \
             instead."
        ),
        TapeOp::Funcall(fc) => {
            let FuncallData { lib, name, args } = fc.as_ref();
            let call_args = funcall_to_ext_args(args, vals);
            let res = lib
                .eval(name, &call_args, false, false)
                .unwrap_or_else(|e| panic!("external function '{name}' eval failed: {e}"));
            res.value
        }
    }
}

#[inline]
fn rev_step(op: &TapeOp, i: usize, vals: &[f64], adj: &mut [f64], a: f64, grad: &mut [f64]) {
    match op {
        TapeOp::Const(_) => {}
        TapeOp::Var(j) => {
            grad[*j] += a;
        }
        TapeOp::Add(l, r) => {
            adj[*l] += a;
            adj[*r] += a;
        }
        TapeOp::Sub(l, r) => {
            adj[*l] += a;
            adj[*r] -= a;
        }
        TapeOp::Mul(l, r) => {
            adj[*l] += a * vals[*r];
            adj[*r] += a * vals[*l];
        }
        TapeOp::Div(l, r) => {
            // Kahan form -- see the identical arm in the reverse sweep above.
            let rv = vals[*r];
            adj[*l] += a / rv;
            adj[*r] -= a * vals[i] / rv;
        }
        TapeOp::Pow(l, r) => {
            let lv = vals[*l];
            let rv = vals[*r];
            if rv != 0.0 {
                adj[*l] += a * rv * lv.powf(rv - 1.0);
            }
            if lv > 0.0 {
                adj[*r] += a * vals[i] * lv.ln();
            }
        }
        TapeOp::Neg(j) => {
            adj[*j] -= a;
        }
        TapeOp::Abs(j) => {
            if vals[*j] >= 0.0 {
                adj[*j] += a;
            } else {
                adj[*j] -= a;
            }
        }
        TapeOp::Sqrt(j) => {
            let sv = vals[i];
            if sv > 0.0 {
                adj[*j] += a * 0.5 / sv;
            }
        }
        TapeOp::Exp(j) => {
            adj[*j] += a * vals[i];
        }
        TapeOp::Log(j) => {
            adj[*j] += a / vals[*j];
        }
        TapeOp::Log10(j) => {
            adj[*j] += a / (vals[*j] * std::f64::consts::LN_10);
        }
        TapeOp::Sin(j) => {
            adj[*j] += a * vals[*j].cos();
        }
        TapeOp::Cos(j) => {
            adj[*j] -= a * vals[*j].sin();
        }
        TapeOp::Tan(j) => {
            let t = vals[i];
            adj[*j] += a * (1.0 + t * t);
        }
        TapeOp::Atan(j) => {
            let u = vals[*j];
            adj[*j] += a / (1.0 + u * u);
        }
        TapeOp::Acos(j) => {
            let u = vals[*j];
            adj[*j] -= a / (1.0 - u * u).sqrt();
        }
        TapeOp::Sinh(j) => {
            adj[*j] += a * vals[*j].cosh();
        }
        TapeOp::Cosh(j) => {
            adj[*j] += a * vals[*j].sinh();
        }
        TapeOp::Tanh(j) => {
            let t = vals[i];
            adj[*j] += a * (1.0 - t * t);
        }
        TapeOp::Asin(j) => {
            let u = vals[*j];
            adj[*j] += a / (1.0 - u * u).sqrt();
        }
        TapeOp::Acosh(j) => {
            let u = vals[*j];
            adj[*j] += a / (u * u - 1.0).sqrt();
        }
        TapeOp::Asinh(j) => {
            let u = vals[*j];
            adj[*j] += a / (u * u + 1.0).sqrt();
        }
        TapeOp::Atanh(j) => {
            let u = vals[*j];
            adj[*j] += a / (1.0 - u * u);
        }
        TapeOp::Erf(j) => {
            adj[*j] += a * erf_d1(vals[*j]);
        }
        TapeOp::XLogX(j) => {
            adj[*j] += a * xlogx_d1(vals[*j]);
        }
        TapeOp::CEntropy(l, r) => {
            adj[*l] += a * centropy_da(vals[*l], vals[*r]);
            adj[*r] += a * centropy_db(vals[*l], vals[*r]);
        }
        TapeOp::Atan2(l, r) => {
            let y = vals[*l];
            let x = vals[*r];
            let d = y * y + x * x;
            adj[*l] += a * (x / d);
            adj[*r] += a * (-y / d);
        }
        TapeOp::Cmp(_, _, _)
        | TapeOp::And(_, _)
        | TapeOp::Or(_, _)
        | TapeOp::Not(_)
        | TapeOp::Select(_, _, _)
        | TapeOp::Min(_, _)
        | TapeOp::Max(_, _) => panic!(
            "GlobalTape free-function kernels do not implement conditional / logical \
             / min-max TapeOps; use the Tape (build_with_externals) interpreter path \
             instead."
        ),
        TapeOp::Funcall(fc) => {
            let FuncallData { lib, name, args } = fc.as_ref();
            let call_args = funcall_to_ext_args(args, vals);
            let res = lib
                .eval(name, &call_args, true, false)
                .unwrap_or_else(|e| panic!("external function '{name}' reverse eval failed: {e}"));
            let derivs = res.derivs.expect("want_derivs=true returns derivs");
            let mut k = 0usize;
            for arg in args {
                if let TapeFuncallArg::Tape(idx) = arg {
                    adj[*idx] += a * derivs[k];
                    k += 1;
                }
            }
            let _ = i;
            let _ = grad;
        }
    }
}

/// Directional forward-tangent step: [`fwd_tan_step`] with a dense
/// seed vector instead of a single seed variable. The per-op chain
/// rule is identical (and identical to the inline arms of
/// [`Tape::hessian_directional`], which the hybrid Hessian path must
/// agree with); only the `Var` arm differs — it reads `seed[k]`
/// rather than testing `k == seed_var` — which is what lets one pass
/// carry a whole Hessian color.
#[inline]
fn fwd_dir_step(op: &TapeOp, seed: &[f64], vals: &[f64], dot: &[f64], i: usize) -> f64 {
    match op {
        TapeOp::Const(_) => 0.0,
        TapeOp::Var(k) => seed[*k],
        TapeOp::Add(a, b) => dot[*a] + dot[*b],
        TapeOp::Sub(a, b) => dot[*a] - dot[*b],
        TapeOp::Mul(a, b) => dot[*a] * vals[*b] + vals[*a] * dot[*b],
        TapeOp::Div(a, b) => {
            // Kahan form -- see the identical arm in `forward_tangent` for why the
            // squared denominator loses representable tangents at extreme |b|.
            (dot[*a] - vals[i] * dot[*b]) / vals[*b]
        }
        TapeOp::Pow(a, b) => {
            let u = vals[*a];
            let r = vals[*b];
            let du = dot[*a];
            let dr = dot[*b];
            let mut result = 0.0;
            // Match the reverse-mode gradient's guard (`rv != 0.0` only): at base
            // u == 0 the slope is still well defined for r >= 1 (and a genuine
            // ±inf for r < 1), so it must not be silently dropped, or the forward
            // tangent disagrees with the reverse gradient.
            if r != 0.0 {
                result += r * u.powf(r - 1.0) * du;
            }
            if u > 0.0 {
                result += vals[i] * u.ln() * dr;
            }
            result
        }
        TapeOp::Neg(a) => -dot[*a],
        TapeOp::Abs(a) => {
            if vals[*a] >= 0.0 {
                dot[*a]
            } else {
                -dot[*a]
            }
        }
        TapeOp::Sqrt(a) => {
            let sv = vals[i];
            if sv > 0.0 { dot[*a] * 0.5 / sv } else { 0.0 }
        }
        TapeOp::Exp(a) => vals[i] * dot[*a],
        TapeOp::Log(a) => dot[*a] / vals[*a],
        TapeOp::Log10(a) => dot[*a] / (vals[*a] * std::f64::consts::LN_10),
        TapeOp::Sin(a) => vals[*a].cos() * dot[*a],
        TapeOp::Cos(a) => -vals[*a].sin() * dot[*a],
        TapeOp::Tan(a) => {
            let t = vals[i];
            (1.0 + t * t) * dot[*a]
        }
        TapeOp::Atan(a) => {
            let u = vals[*a];
            dot[*a] / (1.0 + u * u)
        }
        TapeOp::Acos(a) => {
            let u = vals[*a];
            -dot[*a] / (1.0 - u * u).sqrt()
        }
        TapeOp::Sinh(a) => dot[*a] * vals[*a].cosh(),
        TapeOp::Cosh(a) => dot[*a] * vals[*a].sinh(),
        TapeOp::Tanh(a) => {
            let t = vals[i];
            (1.0 - t * t) * dot[*a]
        }
        TapeOp::Asin(a) => {
            let u = vals[*a];
            dot[*a] / (1.0 - u * u).sqrt()
        }
        TapeOp::Acosh(a) => {
            let u = vals[*a];
            dot[*a] / (u * u - 1.0).sqrt()
        }
        TapeOp::Asinh(a) => {
            let u = vals[*a];
            dot[*a] / (u * u + 1.0).sqrt()
        }
        TapeOp::Atanh(a) => {
            let u = vals[*a];
            dot[*a] / (1.0 - u * u)
        }
        TapeOp::Erf(a) => erf_d1(vals[*a]) * dot[*a],
        TapeOp::XLogX(a) => xlogx_d1(vals[*a]) * dot[*a],
        TapeOp::CEntropy(a, b) => {
            centropy_da(vals[*a], vals[*b]) * dot[*a] + centropy_db(vals[*a], vals[*b]) * dot[*b]
        }
        TapeOp::Atan2(a, b) => {
            let y = vals[*a];
            let x = vals[*b];
            let d = y * y + x * x;
            (x * dot[*a] - y * dot[*b]) / d
        }
        TapeOp::Cmp(_, _, _)
        | TapeOp::And(_, _)
        | TapeOp::Or(_, _)
        | TapeOp::Not(_)
        | TapeOp::Select(_, _, _)
        | TapeOp::Min(_, _)
        | TapeOp::Max(_, _) => panic!(
            "GlobalTape free-function kernels do not implement conditional / logical \
             / min-max TapeOps; use the Tape (build_with_externals) interpreter path \
             instead."
        ),
        TapeOp::Funcall(fc) => {
            let FuncallData { lib, name, args } = fc.as_ref();
            let call_args = funcall_to_ext_args(args, vals);
            let res = lib
                .eval(name, &call_args, true, false)
                .unwrap_or_else(|e| panic!("external function '{name}' tangent eval failed: {e}"));
            let derivs = res.derivs.expect("want_derivs=true returns derivs");
            let mut acc = 0.0;
            let mut k = 0usize;
            for arg in args {
                if let TapeFuncallArg::Tape(idx) = arg {
                    acc += derivs[k] * dot[*idx];
                    k += 1;
                }
            }
            let _ = seed;
            acc
        }
    }
}

/// Directional reverse-over-tangent step: [`ror_step`] with the
/// hash-map scatter replaced by the dense accumulation
/// [`Tape::hessian_directional`] uses — at `Var(k)` the emitted
/// second-order contribution is `out[k] += weight * wd`, landing in
/// the coloring's per-color `compressed` buffer with no hashing. All
/// other arms are the exact arithmetic of `hessian_directional`'s
/// reverse sweep, so the local part of the hybrid Hessian is
/// bit-identical to the flat tape's.
#[allow(clippy::too_many_arguments)]
#[inline]
fn ror_dir_step(
    op: &TapeOp,
    i: usize,
    vals: &[f64],
    dot: &[f64],
    adj: &mut [f64],
    adj_dot: &mut [f64],
    w: f64,
    wd: f64,
    weight: f64,
    out: &mut [f64],
) {
    match op {
        TapeOp::Const(_) => {}
        TapeOp::Var(k) => {
            if wd != 0.0 {
                out[*k] += weight * wd;
            }
        }
        TapeOp::Add(a, b) => {
            adj[*a] += w;
            adj[*b] += w;
            adj_dot[*a] += wd;
            adj_dot[*b] += wd;
        }
        TapeOp::Sub(a, b) => {
            adj[*a] += w;
            adj[*b] -= w;
            adj_dot[*a] += wd;
            adj_dot[*b] -= wd;
        }
        TapeOp::Mul(a, b) => {
            adj[*a] += w * vals[*b];
            adj[*b] += w * vals[*a];
            adj_dot[*a] += wd * vals[*b] + w * dot[*b];
            adj_dot[*b] += wd * vals[*a] + w * dot[*a];
        }
        TapeOp::Div(a, b) => {
            // Kahan form -- see the identical arm in the dense
            // reverse-over-forward sweep for the derivation.
            let vb = vals[*b];
            let q = vals[i];
            let qd = dot[i];
            adj[*a] += w / vb;
            adj_dot[*a] += wd / vb - w * (dot[*b] / vb) / vb;
            adj[*b] -= w * q / vb;
            adj_dot[*b] += -(wd * q) / vb + (w / vb) * (-qd + q * (dot[*b] / vb));
        }
        TapeOp::Pow(a, b) => {
            let u = vals[*a];
            let r = vals[*b];
            let du = dot[*a];
            let dr = dot[*b];
            if r != 0.0 {
                if u != 0.0 {
                    let p_a = r * u.powf(r - 1.0);
                    adj[*a] += w * p_a;
                    let mut dp_a = dr * u.powf(r - 1.0);
                    if u > 0.0 {
                        dp_a += r * u.powf(r - 1.0) * ((r - 1.0) * du / u + dr * u.ln());
                    } else {
                        dp_a += r * (r - 1.0) * u.powf(r - 2.0) * du;
                    }
                    adj_dot[*a] += wd * p_a + w * dp_a;
                } else if r >= 2.0 {
                    let p_a = 0.0;
                    adj[*a] += w * p_a;
                    let dp_a = if r == 2.0 {
                        2.0 * du
                    } else {
                        r * (r - 1.0) * (0.0_f64).powf(r - 2.0) * du
                    };
                    adj_dot[*a] += wd * p_a + w * dp_a;
                }
            }
            if u > 0.0 {
                let ln_u = u.ln();
                let p_b = vals[i] * ln_u;
                adj[*b] += w * p_b;
                let dur = vals[i] * (r * du / u + dr * ln_u);
                let dp_b = dur * ln_u + vals[i] * du / u;
                adj_dot[*b] += wd * p_b + w * dp_b;
            }
        }
        TapeOp::Neg(a) => {
            adj[*a] -= w;
            adj_dot[*a] -= wd;
        }
        TapeOp::Abs(a) => {
            let s = if vals[*a] >= 0.0 { 1.0 } else { -1.0 };
            adj[*a] += w * s;
            adj_dot[*a] += wd * s;
        }
        TapeOp::Sqrt(a) => {
            let sv = vals[i];
            if sv > 0.0 {
                let fp = 0.5 / sv;
                let fpp = -0.25 / (vals[*a] * sv);
                adj[*a] += w * fp;
                adj_dot[*a] += wd * fp + w * fpp * dot[*a];
            }
        }
        TapeOp::Exp(a) => {
            let ev = vals[i];
            adj[*a] += w * ev;
            adj_dot[*a] += wd * ev + w * ev * dot[*a];
        }
        TapeOp::Log(a) => {
            let u = vals[*a];
            adj[*a] += w / u;
            adj_dot[*a] += wd / u + w * (-1.0 / (u * u)) * dot[*a];
        }
        TapeOp::Log10(a) => {
            let u = vals[*a];
            let c = std::f64::consts::LN_10;
            adj[*a] += w / (u * c);
            adj_dot[*a] += wd / (u * c) + w * (-1.0 / (u * u * c)) * dot[*a];
        }
        TapeOp::Sin(a) => {
            let u = vals[*a];
            let cu = u.cos();
            adj[*a] += w * cu;
            adj_dot[*a] += wd * cu + w * (-u.sin()) * dot[*a];
        }
        TapeOp::Cos(a) => {
            let u = vals[*a];
            let su = u.sin();
            adj[*a] -= w * su;
            adj_dot[*a] += wd * (-su) + w * (-u.cos()) * dot[*a];
        }
        TapeOp::Tan(a) => {
            let t = vals[i];
            let gp = 1.0 + t * t;
            let gpp = 2.0 * t * gp;
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Atan(a) => {
            let u = vals[*a];
            let d = 1.0 + u * u;
            let gp = 1.0 / d;
            let gpp = -2.0 * u / (d * d);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Acos(a) => {
            let u = vals[*a];
            let s = 1.0 - u * u;
            let r = s.sqrt();
            let gp = -1.0 / r;
            let gpp = -u / (s * r);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Sinh(a) => {
            let u = vals[*a];
            let gp = u.cosh();
            let gpp = vals[i]; // sinh(u)
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Cosh(a) => {
            let u = vals[*a];
            let gp = u.sinh();
            let gpp = vals[i]; // cosh(u)
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Tanh(a) => {
            let t = vals[i];
            let gp = 1.0 - t * t;
            let gpp = -2.0 * t * gp;
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Asin(a) => {
            let u = vals[*a];
            let s = 1.0 - u * u;
            let r = s.sqrt();
            let gp = 1.0 / r;
            let gpp = u / (s * r);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Acosh(a) => {
            let u = vals[*a];
            let s = u * u - 1.0;
            let r = s.sqrt();
            let gp = 1.0 / r;
            let gpp = -u / (s * r);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Asinh(a) => {
            let u = vals[*a];
            let s = u * u + 1.0;
            let r = s.sqrt();
            let gp = 1.0 / r;
            let gpp = -u / (s * r);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Atanh(a) => {
            let u = vals[*a];
            let d = 1.0 - u * u;
            let gp = 1.0 / d;
            let gpp = 2.0 * u / (d * d);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::Erf(a) => {
            let u = vals[*a];
            let gp = erf_d1(u);
            let gpp = erf_d2(u);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::XLogX(a) => {
            // See the identical arm in `hessian_accumulate`: gpp is 1/u, never
            // the unrepresentable -1/u² of `ln''`.
            let u = vals[*a];
            let gp = xlogx_d1(u);
            let gpp = xlogx_d2(u);
            adj[*a] += w * gp;
            adj_dot[*a] += wd * gp + w * gpp * dot[*a];
        }
        TapeOp::CEntropy(a, b) => {
            let ua = vals[*a];
            let ub = vals[*b];
            let fa = centropy_da(ua, ub);
            let fb = centropy_db(ua, ub);
            let faa = centropy_daa(ua);
            let fab = centropy_dab(ub);
            let fbb = centropy_dbb(ua, ub);
            adj[*a] += w * fa;
            adj[*b] += w * fb;
            adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
            adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
        }
        TapeOp::Atan2(a, b) => {
            let y = vals[*a];
            let x = vals[*b];
            let d = y * y + x * x;
            let d2 = d * d;
            let fa = x / d;
            let fb = -y / d;
            let faa = -2.0 * x * y / d2;
            let fab = (y * y - x * x) / d2;
            let fbb = 2.0 * x * y / d2;
            adj[*a] += w * fa;
            adj[*b] += w * fb;
            adj_dot[*a] += wd * fa + w * (faa * dot[*a] + fab * dot[*b]);
            adj_dot[*b] += wd * fb + w * (fab * dot[*a] + fbb * dot[*b]);
        }
        TapeOp::Cmp(_, _, _)
        | TapeOp::And(_, _)
        | TapeOp::Or(_, _)
        | TapeOp::Not(_)
        | TapeOp::Select(_, _, _)
        | TapeOp::Min(_, _)
        | TapeOp::Max(_, _) => panic!(
            "GlobalTape free-function kernels do not implement conditional / logical \
             / min-max TapeOps; use the Tape (build_with_externals) interpreter path \
             instead."
        ),
        TapeOp::Funcall(fc) => {
            let FuncallData { lib, name, args } = fc.as_ref();
            let call_args = funcall_to_ext_args(args, vals);
            let res = lib.eval(name, &call_args, true, true).unwrap_or_else(|e| {
                panic!("external function '{name}' 2nd-order eval failed: {e}")
            });
            let derivs = res.derivs.expect("want_derivs=true returns derivs");
            let hes = res.hessian.expect("want_hes=true returns hessian");
            let real_tape: Vec<usize> = args
                .iter()
                .filter_map(|a| match a {
                    TapeFuncallArg::Tape(t) => Some(*t),
                    TapeFuncallArg::Str(_) => None,
                })
                .collect();
            for (k, &tk) in real_tape.iter().enumerate() {
                adj[tk] += w * derivs[k];
                let mut second_term = 0.0;
                for (l, &tl) in real_tape.iter().enumerate() {
                    let (lo, hi) = if k <= l { (k, l) } else { (l, k) };
                    let h_kl = hes[lo + hi * (hi + 1) / 2];
                    second_term += h_kl * dot[tl];
                }
                adj_dot[tk] += wd * derivs[k] + w * second_term;
            }
            let _ = out;
            let _ = weight;
            let _ = i;
        }
    }
}

/// Per-op Hessian-sparsity propagation. Same algorithm as
/// `Tape::hessian_sparsity` but as a free function so `GlobalTape`
/// can call it over its shared `ops` slice.
fn hessian_sparsity_impl(ops: &[TapeOp]) -> BTreeSet<(usize, usize)> {
    let n = ops.len();
    let mut var_sets: Vec<BTreeSet<usize>> = Vec::with_capacity(n);
    let mut pairs: BTreeSet<(usize, usize)> = BTreeSet::new();

    let emit_cross =
        |s1: &BTreeSet<usize>, s2: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
            for &v1 in s1 {
                for &v2 in s2 {
                    let (r, c) = if v1 >= v2 { (v1, v2) } else { (v2, v1) };
                    pairs.insert((r, c));
                }
            }
        };
    let emit_self = |s: &BTreeSet<usize>, pairs: &mut BTreeSet<(usize, usize)>| {
        let vars: Vec<usize> = s.iter().copied().collect();
        for (ai, &vi) in vars.iter().enumerate() {
            for &vj in &vars[..=ai] {
                let (r, c) = if vi >= vj { (vi, vj) } else { (vj, vi) };
                pairs.insert((r, c));
            }
        }
    };

    for op in ops {
        let vset = match op {
            TapeOp::Const(_) => BTreeSet::new(),
            TapeOp::Var(j) => {
                let mut s = BTreeSet::new();
                s.insert(*j);
                s
            }
            TapeOp::Add(a, b) | TapeOp::Sub(a, b) => {
                var_sets[*a].union(&var_sets[*b]).copied().collect()
            }
            TapeOp::Neg(a) | TapeOp::Abs(a) => var_sets[*a].clone(),
            TapeOp::Mul(a, b) => {
                emit_cross(&var_sets[*a], &var_sets[*b], &mut pairs);
                var_sets[*a].union(&var_sets[*b]).copied().collect()
            }
            TapeOp::Div(a, b) => {
                emit_cross(&var_sets[*a], &var_sets[*b], &mut pairs);
                emit_self(&var_sets[*b], &mut pairs);
                var_sets[*a].union(&var_sets[*b]).copied().collect()
            }
            TapeOp::Pow(a, b) | TapeOp::Atan2(a, b) | TapeOp::CEntropy(a, b) => {
                let combined: BTreeSet<usize> =
                    var_sets[*a].union(&var_sets[*b]).copied().collect();
                emit_self(&combined, &mut pairs);
                combined
            }
            TapeOp::Sqrt(a)
            | TapeOp::Exp(a)
            | TapeOp::Log(a)
            | TapeOp::Log10(a)
            | TapeOp::Sin(a)
            | TapeOp::Cos(a)
            | TapeOp::Tan(a)
            | TapeOp::Atan(a)
            | TapeOp::Acos(a)
            | TapeOp::Sinh(a)
            | TapeOp::Cosh(a)
            | TapeOp::Tanh(a)
            | TapeOp::Asin(a)
            | TapeOp::Acosh(a)
            | TapeOp::Asinh(a)
            | TapeOp::Erf(a)
            | TapeOp::XLogX(a)
            | TapeOp::Atanh(a) => {
                emit_self(&var_sets[*a], &mut pairs);
                var_sets[*a].clone()
            }
            TapeOp::Funcall(fc) => {
                let args = &fc.args;
                let mut combined: BTreeSet<usize> = BTreeSet::new();
                for arg in args {
                    if let TapeFuncallArg::Tape(t) = arg {
                        for &vv in &var_sets[*t] {
                            combined.insert(vv);
                        }
                    }
                }
                emit_self(&combined, &mut pairs);
                combined
            }
            TapeOp::Cmp(_, _, _) | TapeOp::And(_, _) | TapeOp::Or(_, _) | TapeOp::Not(_) => {
                // Comparisons / logical ops have identically-zero derivative, so
                // they contribute no Hessian structure.
                BTreeSet::new()
            }
            TapeOp::Select(_, t, e) => {
                // Either branch may be active; the structural superset is the
                // union of both branches' variable sets.
                var_sets[*t].union(&var_sets[*e]).copied().collect()
            }
            TapeOp::Min(a, b) | TapeOp::Max(a, b) => {
                // min/max are piecewise linear: zero second derivative (no
                // pairs); dependence set is the union of both operands.
                var_sets[*a].union(&var_sets[*b]).copied().collect()
            }
        };
        var_sets.push(vset);
    }
    pairs
}

#[cfg(test)]
mod tests {
    use super::*;

    fn cnst(c: f64) -> Expr {
        Expr::Const(c)
    }
    fn var(i: usize) -> Expr {
        Expr::Var(i)
    }
    fn add(a: Expr, b: Expr) -> Expr {
        Expr::Binary(BinOp::Add, Box::new(a), Box::new(b))
    }
    fn mul(a: Expr, b: Expr) -> Expr {
        Expr::Binary(BinOp::Mul, Box::new(a), Box::new(b))
    }
    fn pow(a: Expr, b: Expr) -> Expr {
        Expr::Binary(BinOp::Pow, Box::new(a), Box::new(b))
    }
    fn div(a: Expr, b: Expr) -> Expr {
        Expr::Binary(BinOp::Div, Box::new(a), Box::new(b))
    }
    fn unary(op: UnaryOp, a: Expr) -> Expr {
        Expr::Unary(op, Box::new(a))
    }
    fn cmp(op: CmpOp, a: Expr, b: Expr) -> Expr {
        Expr::Compare(op, Box::new(a), Box::new(b))
    }
    fn cond(c: Expr, t: Expr, e: Expr) -> Expr {
        Expr::Cond {
            cond: Box::new(c),
            then_: Box::new(t),
            else_: Box::new(e),
        }
    }

    #[test]
    fn polynomial_eval_and_grad() {
        // f = 3*x0^2 + 2*x1
        let e = add(
            mul(cnst(3.0), pow(var(0), cnst(2.0))),
            mul(cnst(2.0), var(1)),
        );
        let t = Tape::build(&e);
        assert!((t.eval(&[2.0, 3.0]) - 18.0).abs() < 1e-12);
        let mut g = vec![0.0; 2];
        t.gradient_seed(&[2.0, 3.0], 1.0, &mut g);
        // df/dx0 = 6*x0 = 12, df/dx1 = 2
        assert!((g[0] - 12.0).abs() < 1e-12);
        assert!((g[1] - 2.0).abs() < 1e-12);
    }

    #[test]
    fn cse_shared_body_evaluated_once() {
        // body = x0 + x1, shared via Arc. f = body^2 + body.
        let body = Arc::new(add(var(0), var(1)));
        let e = add(
            pow(Expr::Cse(body.clone()), cnst(2.0)),
            Expr::Cse(body.clone()),
        );
        let t = Tape::build(&e);
        // body should appear once in the tape: count Add(Var(0),Var(1)) ops
        let n_body_adds = t
            .ops
            .iter()
            .filter(|op| {
                matches!(op, TapeOp::Add(a, b) if {
                    matches!(t.ops[*a], TapeOp::Var(0)) && matches!(t.ops[*b], TapeOp::Var(1))
                })
            })
            .count();
        assert_eq!(n_body_adds, 1, "CSE body should be emitted exactly once");

        // f(1, 2) = 9 + 3 = 12
        assert!((t.eval(&[1.0, 2.0]) - 12.0).abs() < 1e-12);
        let mut g = vec![0.0; 2];
        t.gradient_seed(&[1.0, 2.0], 1.0, &mut g);
        // df/dx0 = 2*(x0+x1) + 1 = 7, same for x1
        assert!((g[0] - 7.0).abs() < 1e-12);
        assert!((g[1] - 7.0).abs() < 1e-12);
    }

    fn fd_check(tape: &Tape, x: &[f64], n: usize, tol: f64) {
        let vars = tape.variables();
        let mut hess_map: HashMap<(usize, usize), usize> = HashMap::new();
        let mut pairs = Vec::new();
        for (ai, &vi) in vars.iter().enumerate() {
            for &vj in &vars[..=ai] {
                let (r, c) = if vi >= vj { (vi, vj) } else { (vj, vi) };
                hess_map.entry((r, c)).or_insert_with(|| {
                    let p = pairs.len();
                    pairs.push((r, c));
                    p
                });
            }
        }
        let nnz = pairs.len();
        let mut ad = vec![0.0; nnz];
        tape.hessian_accumulate(x, 1.0, &hess_map, &mut ad);

        let mut fd = vec![0.0; nnz];
        let mut xp = x.to_vec();
        let mut gp = vec![0.0; n];
        let mut gm = vec![0.0; n];
        for &j in &vars {
            let h = (1e-7_f64).max(x[j].abs() * 1e-7);
            xp[j] = x[j] + h;
            gp.iter_mut().for_each(|v| *v = 0.0);
            tape.gradient_seed(&xp, 1.0, &mut gp);
            xp[j] = x[j] - h;
            gm.iter_mut().for_each(|v| *v = 0.0);
            tape.gradient_seed(&xp, 1.0, &mut gm);
            xp[j] = x[j];
            for &i in &vars {
                if i >= j {
                    if let Some(&pos) = hess_map.get(&(i, j)) {
                        fd[pos] = (gp[i] - gm[i]) / (2.0 * h);
                    }
                }
            }
        }
        for (k, &(r, c)) in pairs.iter().enumerate() {
            let scale = fd[k].abs().max(1.0);
            assert!(
                (ad[k] - fd[k]).abs() / scale < tol,
                "H[{},{}]: AD={:.6e} FD={:.6e}",
                r,
                c,
                ad[k],
                fd[k]
            );
        }
    }

    #[test]
    fn hessian_quadratic_matches_fd() {
        // f = 3 x0^2 + 2 x0 x1 + x1^2
        let e = add(
            add(
                mul(cnst(3.0), pow(var(0), cnst(2.0))),
                mul(cnst(2.0), mul(var(0), var(1))),
            ),
            pow(var(1), cnst(2.0)),
        );
        let t = Tape::build(&e);
        fd_check(&t, &[2.0, 3.0], 2, 1e-5);
    }

    #[test]
    fn hessian_transcendental_matches_fd() {
        // f = exp(x0) + sin(x1) + log(x0) + sqrt(x1) + x0*x1
        let e = Expr::Sum(vec![
            unary(UnaryOp::Exp, var(0)),
            unary(UnaryOp::Sin, var(1)),
            unary(UnaryOp::Log, var(0)),
            unary(UnaryOp::Sqrt, var(1)),
            mul(var(0), var(1)),
        ]);
        let t = Tape::build(&e);
        fd_check(&t, &[1.5, 2.0], 2, 1e-5);
    }

    #[test]
    fn inverse_trig_grad_and_hessian_match_fd() {
        // f = tan(x0) + atan(x1) + acos(x2) + x0*x1
        // Point chosen so every op is in its smooth domain:
        // tan away from pi/2, acos arg in (-1, 1).
        let e = Expr::Sum(vec![
            unary(UnaryOp::Tan, var(0)),
            unary(UnaryOp::Atan, var(1)),
            unary(UnaryOp::Acos, var(2)),
            mul(var(0), var(1)),
        ]);
        let t = Tape::build(&e);
        let x = [0.5, 1.3, 0.3];

        // Gradient vs central finite difference of the value. This
        // pins the first derivatives independently of the Hessian
        // (fd_check only ties the Hessian to the AD gradient).
        let mut g = vec![0.0; 3];
        t.gradient_seed(&x, 1.0, &mut g);
        for j in 0..3 {
            let h = (1e-7_f64).max(x[j].abs() * 1e-7);
            let mut xp = x;
            let mut xm = x;
            xp[j] += h;
            xm[j] -= h;
            let fd = (t.eval(&xp) - t.eval(&xm)) / (2.0 * h);
            let scale = fd.abs().max(1.0);
            assert!(
                (g[j] - fd).abs() / scale < 1e-5,
                "grad[{j}]: AD={:.6e} FD={:.6e}",
                g[j],
                fd
            );
        }

        // Hessian (forward-over-reverse) vs FD of the gradient.
        fd_check(&t, &x, 3, 1e-5);
    }

    /// Shared helper: check AD gradient vs central FD of the value at
    /// `x`, then the Hessian via `fd_check`.
    fn grad_and_hess_match_fd(e: &Expr, x: &[f64], tol: f64) {
        let n = x.len();
        let t = Tape::build(e);
        let mut g = vec![0.0; n];
        t.gradient_seed(x, 1.0, &mut g);
        for j in 0..n {
            let h = (1e-7_f64).max(x[j].abs() * 1e-7);
            let mut xp = x.to_vec();
            let mut xm = x.to_vec();
            xp[j] += h;
            xm[j] -= h;
            let fd = (t.eval(&xp) - t.eval(&xm)) / (2.0 * h);
            let scale = fd.abs().max(1.0);
            assert!(
                (g[j] - fd).abs() / scale < tol,
                "grad[{j}]: AD={:.6e} FD={:.6e}",
                g[j],
                fd
            );
        }
        fd_check(&t, x, n, tol);
    }

    #[test]
    fn hyperbolic_grad_and_hessian_match_fd() {
        // f = sinh(x0) + cosh(x1) + tanh(x2) + asinh(x3) + x0*x1 + x2*x3
        // sinh/cosh/tanh/asinh are smooth on all of R.
        let e = Expr::Sum(vec![
            unary(UnaryOp::Sinh, var(0)),
            unary(UnaryOp::Cosh, var(1)),
            unary(UnaryOp::Tanh, var(2)),
            unary(UnaryOp::Asinh, var(3)),
            mul(var(0), var(1)),
            mul(var(2), var(3)),
        ]);
        grad_and_hess_match_fd(&e, &[0.5, 0.7, 0.3, 1.1], 1e-5);
    }

    #[test]
    fn restricted_inverse_grad_and_hessian_match_fd() {
        // f = asin(x0) + acosh(x1) + atanh(x2) + x0*x2
        // Point chosen in each op's smooth domain:
        // asin/atanh need |arg| < 1; acosh needs arg > 1.
        let e = Expr::Sum(vec![
            unary(UnaryOp::Asin, var(0)),
            unary(UnaryOp::Acosh, var(1)),
            unary(UnaryOp::Atanh, var(2)),
            mul(var(0), var(2)),
        ]);
        grad_and_hess_match_fd(&e, &[0.4, 1.8, 0.3], 1e-5);
    }

    #[test]
    fn erf_value_matches_reference() {
        // Reference values from the standard erf (musl / C99), to full
        // double precision. Pinning these guards the `libm` delegation:
        // a swap to a series approximation would fail here long before it
        // showed up as a mysteriously loose KKT residual.
        let t = Tape::build(&unary(UnaryOp::Erf, var(0)));
        for (x, want) in [
            (0.0, 0.0),
            (0.5, 0.520_499_877_813_046_5),
            (1.0, 0.842_700_792_949_714_9),
            (-1.0, -0.842_700_792_949_714_9),
            (2.0, 0.995_322_265_018_952_7),
            (3.0, 0.999_977_909_503_001_4),
        ] {
            let got = t.eval(&[x]);
            assert!(
                (got - want).abs() < 1e-15,
                "erf({x}): got {got:.17e}, want {want:.17e}"
            );
        }
        // Odd and saturating: the two properties a wrong implementation
        // most often breaks.
        assert!((t.eval(&[-0.3]) + t.eval(&[0.3])).abs() < 1e-16);
        assert!((t.eval(&[10.0]) - 1.0).abs() < 1e-15);
    }

    #[test]
    fn erf_second_derivative_stays_finite_at_extreme_magnitudes() {
        // `erf_d1` underflows to 0 around |u| > 27, and `-2.0 * u`
        // overflows to ±inf past f64::MAX/2. Written as `(-2u)·erf'(u)`
        // the two meet as `inf * 0.0 = NaN`, where the true limit is 0.
        // Parenthesizing as `-2·(u·erf'(u))` keeps every finite input
        // finite. Nothing in a well-posed model reaches 1e308 — `.nl`'s
        // unbounded sentinel is 1e19 — but the Python binding now takes an
        // unguarded `x`, and a NaN Hessian entry poisons a whole factorization.
        for u in [1e19, 1e150, 1e300, f64::MAX, f64::MAX / 2.0] {
            for signed in [u, -u] {
                let d2 = erf_d2(signed);
                assert!(
                    d2.is_finite(),
                    "erf_d2({signed:e}) = {d2} — must be finite (the limit is 0)"
                );
            }
        }
        // Still correct where it matters: -2u·erf'(u) at a normal point.
        let u = 0.7;
        let want = -2.0 * u * (2.0 / std::f64::consts::PI.sqrt()) * (-u * u).exp();
        assert!((erf_d2(u) - want).abs() < 1e-15, "{}", erf_d2(u));
    }

    #[test]
    fn quotient_rule_keeps_representable_derivatives_at_extreme_denominators() {
        // The quotient rule used to form `b*b` explicitly. That overflows for
        // |b| > 1.3e154 and underflows below 1.5e-154, and what it destroys is a
        // derivative that is itself perfectly representable: at a = b = 1e300,
        // ∂(a/b)/∂b = -a/b² = -1e-300, but `b*b` is inf so the squared form
        // returned -0.0. A silently ZEROED gradient entry, not a NaN anyone
        // would catch downstream.
        //
        // Reachable in practice: discopt lowers GAMS `centropy(x, y)` to
        // `x·log(x/y)`, and a 600-digit audit measured a wrong y-derivative at
        // (1e300, 1e300) through exactly this path.
        let t = Tape::build(&div(var(0), var(1)));
        let mut g = vec![0.0; 2];
        for b in [1e300_f64, 1e200, 1e160, -1e300, 1e-160, 1e-300] {
            // At a = b: ∂/∂a = 1/b and ∂/∂b = -a/b² = -1/b, both representable
            // for every b here even though b² is not.
            // `gradient_seed` ACCUMULATES; without this reset the previous
            // iteration's entry survives and the assert compares against stale
            // data (it did, and caught itself: 1e-160 leaked into the b=-1e300 row).
            g.iter_mut().for_each(|v| *v = 0.0);
            t.gradient_seed(&[b, b], 1.0, &mut g);
            assert_eq!(g[0], 1.0 / b, "d(a/b)/da at a=b={b:e}");
            assert_eq!(
                g[1],
                -1.0 / b,
                "d(a/b)/db at a=b={b:e} — b² is not representable"
            );
        }

        // Second order, where the rule squared and CUBED the denominator.
        // b = 1e155 makes b² = 1e310 overflow while the cross partial
        // ∂²(a/b)/∂a∂b = -1/b² = -1e-310 is still (subnormally) representable.
        let b = 1e155_f64;
        let mut hess_map: HashMap<(usize, usize), usize> = HashMap::new();
        hess_map.insert((0, 0), 0);
        hess_map.insert((1, 0), 1);
        hess_map.insert((1, 1), 2);
        let mut ad = vec![0.0; 3];
        t.hessian_accumulate(&[b, b], 1.0, &hess_map, &mut ad);
        // Compute the truth as -(1/b)/b, NOT -1/(b*b) -- the latter is the very
        // expression under test and evaluates to -0.0 here, which would have made
        // this assertion compare the bug against itself and pass vacuously.
        let want = -(1.0 / b) / b;
        assert!(
            want != 0.0 && want.is_finite(),
            "test premise: -1/b² must be representable, got {want:e}"
        );
        assert!(
            (ad[1] - want).abs() <= 1e-10 * want.abs(),
            "d²(a/b)/da db at a=b={b:e}: got {:e}, want {want:e}",
            ad[1]
        );
    }

    #[test]
    fn erf_grad_and_hessian_match_fd() {
        // f = erf(x0) + erf(2*x1) + x0*x1. The inner `2*x1` makes the
        // chain rule non-trivial, so a missing factor in the tangent or
        // adjoint arm shows up rather than cancelling.
        let e = Expr::Sum(vec![
            unary(UnaryOp::Erf, var(0)),
            unary(UnaryOp::Erf, mul(cnst(2.0), var(1))),
            mul(var(0), var(1)),
        ]);
        grad_and_hess_match_fd(&e, &[0.4, -0.7], 1e-5);
    }

    #[test]
    fn erf_directional_hessian_matches_accumulated() {
        // `hessian_directional` (the HVP / coloring sweep) and
        // `hessian_accumulate` (the sparse-entry sweep) are separate
        // match arms over the same op — the easiest place for an erf
        // second derivative to be right in one and wrong in the other.
        let e = Expr::Sum(vec![
            unary(UnaryOp::Erf, var(0)),
            unary(UnaryOp::Erf, mul(var(0), var(1))),
        ]);
        let tape = Tape::build(&e);
        let x = [0.6, -0.9];
        let n = x.len();

        let pairs: Vec<(usize, usize)> = tape.hessian_sparsity().into_iter().collect();
        let hess_map: HashMap<(usize, usize), usize> =
            pairs.iter().enumerate().map(|(k, p)| (*p, k)).collect();
        let mut acc = vec![0.0; pairs.len()];
        tape.hessian_accumulate(&x, 1.0, &hess_map, &mut acc);

        // One directional product per unit seed reproduces column j.
        let ops = tape.ops.len();
        let mut vals = vec![0.0; ops];
        tape.forward_into(&x, &mut vals);
        for j in 0..n {
            let mut seed = vec![0.0; n];
            seed[j] = 1.0;
            let mut col = vec![0.0; n];
            let (mut dot, mut adj, mut adj_dot) = (vec![0.0; ops], vec![0.0; ops], vec![0.0; ops]);
            tape.hessian_directional(
                &vals,
                &seed,
                1.0,
                &mut col,
                &mut dot,
                &mut adj,
                &mut adj_dot,
            );
            for i in 0..n {
                let (r, c) = if i >= j { (i, j) } else { (j, i) };
                let want = hess_map.get(&(r, c)).map_or(0.0, |&k| acc[k]);
                assert!(
                    (col[i] - want).abs() < 1e-12,
                    "H[{i},{j}]: directional={:.6e} accumulated={want:.6e}",
                    col[i]
                );
            }
        }
    }

    fn centropy_expr(a: Expr, b: Expr) -> Expr {
        Expr::Binary(BinOp::CEntropy, Box::new(a), Box::new(b))
    }

    /// Dense 2×2 Hessian via `hessian_accumulate`, as `(h00, h10, h11)`.
    fn hess2(e: &Expr, x: &[f64; 2]) -> (f64, f64, f64) {
        let tape = Tape::build(e);
        let mut hess_map: HashMap<(usize, usize), usize> = HashMap::new();
        hess_map.insert((0, 0), 0);
        hess_map.insert((1, 0), 1);
        hess_map.insert((1, 1), 2);
        let mut ad = vec![0.0; 3];
        tape.hessian_accumulate(x, 1.0, &hess_map, &mut ad);
        (ad[0], ad[1], ad[2])
    }

    #[test]
    fn xlogx_and_centropy_grad_and_hessian_match_fd() {
        // f = xlogx(x0) + centropy(x0, x1) + x0*x1. The trailing product
        // makes the cross partial nonzero, so a dropped ∂²/∂a∂b term shows
        // up instead of hiding in an all-zero off-diagonal.
        let e = Expr::Sum(vec![
            unary(UnaryOp::XLogX, var(0)),
            centropy_expr(var(0), var(1)),
            mul(var(0), var(1)),
        ]);
        grad_and_hess_match_fd(&e, &[1.7, 0.6], 1e-5);
    }

    #[test]
    fn centropy_directional_hessian_matches_accumulated() {
        // `hessian_directional` and `hessian_accumulate` are separate match
        // arms over the same op — the easiest place for a second derivative
        // to be right in one sweep and wrong in the other.
        let e = Expr::Sum(vec![
            centropy_expr(var(0), var(1)),
            unary(UnaryOp::XLogX, mul(var(0), var(1))),
        ]);
        let tape = Tape::build(&e);
        let x = [1.3, 0.8];
        let n = x.len();

        let pairs: Vec<(usize, usize)> = tape.hessian_sparsity().into_iter().collect();
        let hess_map: HashMap<(usize, usize), usize> =
            pairs.iter().enumerate().map(|(k, p)| (*p, k)).collect();
        let mut acc = vec![0.0; pairs.len()];
        tape.hessian_accumulate(&x, 1.0, &hess_map, &mut acc);

        let ops = tape.ops.len();
        let mut vals = vec![0.0; ops];
        tape.forward_into(&x, &mut vals);
        let mut compared = 0usize;
        for j in 0..n {
            let mut seed = vec![0.0; n];
            seed[j] = 1.0;
            let mut col = vec![0.0; n];
            let (mut dot, mut adj, mut adj_dot) = (vec![0.0; ops], vec![0.0; ops], vec![0.0; ops]);
            tape.hessian_directional(
                &vals,
                &seed,
                1.0,
                &mut col,
                &mut dot,
                &mut adj,
                &mut adj_dot,
            );
            for i in 0..n {
                let (r, c) = if i >= j { (i, j) } else { (j, i) };
                let want = hess_map.get(&(r, c)).map_or(0.0, |&k| acc[k]);
                assert!(
                    (col[i] - want).abs() < 1e-12,
                    "H[{i},{j}]: directional={:.6e} accumulated={want:.6e}",
                    col[i]
                );
                compared += 1;
            }
        }
        // §6: prove the probe fired. A tape whose sparsity came back empty
        // would sail through the loop above comparing 0.0 against 0.0.
        assert_eq!(compared, 4, "expected a full 2x2 comparison");
        assert!(
            acc.iter().any(|v| v.abs() > 1e-12),
            "every accumulated Hessian entry was zero — the arms under test never ran"
        );
    }

    #[test]
    fn fused_entropy_ops_reach_derivatives_the_chain_rule_cannot() {
        // The reason these ops exist. `(a·ln a)'' = 1/a` is finite for every
        // positive a down to 1e-308, but EVERY decomposition of it routes
        // through `ln''(a) = -1/a²`, which leaves f64 range below a ≈ 1e-154.
        // The composite is in range while the factor it is built from is not,
        // so this is a structural limit of the chain rule, not a sloppy rule.
        let a = 1e-299_f64;
        let want = 1.0 / a; // 1e299 — an ordinary number.
        assert!(want.is_finite(), "test premise: 1/a must be representable");

        let (fused, _, _) = hess2(&unary(UnaryOp::XLogX, var(0)), &[a, 0.0]);
        assert!(
            (fused - want).abs() <= 1e-12 * want,
            "xlogx''({a:e}): got {fused:e}, want {want:e}"
        );

        // The control: the same function written as `a * log(a)`. Asserting it
        // is BROKEN is what proves the test is in the regime the fusion exists
        // for — without this, a passing `fused` assertion could just mean the
        // point was never extreme enough to matter.
        let (decomposed, _, _) = hess2(&mul(var(0), unary(UnaryOp::Log, var(0))), &[a, 0.0]);
        assert!(
            !decomposed.is_finite(),
            "test premise: a*log(a) is supposed to lose this second derivative, \
             but it returned {decomposed:e} — if the Log/Mul arms now reach it, \
             re-derive whether the fused op is still needed"
        );

        // Same story for centropy's ∂²/∂a², which is also 1/a.
        let (fused_aa, _, _) = hess2(&centropy_expr(var(0), var(1)), &[a, 1.0]);
        assert!(
            (fused_aa - want).abs() <= 1e-12 * want,
            "centropy ∂²/∂a² at a={a:e}: got {fused_aa:e}, want {want:e}"
        );
    }

    #[test]
    fn centropy_second_derivative_in_b_survives_a_squared_denominator() {
        // ∂²/∂b² = a/b². At (1e300, 1e200) the answer is 1e-100 while b² is
        // 1e400 — out of range. Evaluating it as (a/b)/b keeps it.
        let (a, b) = (1e300_f64, 1e200_f64);
        let want = (a / b) / b;
        // The squared form does not blow up loudly — `b*b` is inf and
        // `a/inf` is a silent 0.0, i.e. a ZEROED second derivative. Assert
        // that shape explicitly so the premise says what actually goes wrong.
        assert!(
            want.is_finite() && want != 0.0 && a / (b * b) == 0.0,
            "test premise: a/b² representable ({want:e}) but a/(b*b) collapses"
        );
        let (_, _, h11) = hess2(&centropy_expr(var(0), var(1)), &[a, b]);
        assert!(
            (h11 - want).abs() <= 1e-12 * want,
            "centropy ∂²/∂b² at ({a:e}, {b:e}): got {h11:e}, want {want:e}"
        );
    }

    #[test]
    fn centropy_value_survives_an_out_of_range_ratio_and_the_zero_limit() {
        let t = |e: &Expr, x: &[f64]| Tape::build(e).eval(x);
        let ce = centropy_expr(var(0), var(1));

        // a/b = 1e600 overflows, but ln of it is an ordinary 1381.55, so the
        // product is a perfectly representable 1.38e303. `ln_ratio` falls back
        // to ln(a) - ln(b) exactly here.
        let want = 1e300 * (1e300_f64.ln() - 1e-300_f64.ln());
        let got = t(&ce, &[1e300, 1e-300]);
        assert!(
            got.is_finite() && (got - want).abs() <= 1e-12 * want,
            "centropy(1e300, 1e-300): got {got:e}, want {want:e}"
        );
        // The naive form is what this replaces.
        assert!(
            !t(
                &mul(var(0), unary(UnaryOp::Log, div(var(0), var(1)))),
                &[1e300, 1e-300]
            )
            .is_finite(),
            "test premise: a*log(a/b) is supposed to overflow here"
        );

        // Near a = b, where forming the ratio first throws away the digits
        // that carry the answer. At a = 1e10+1, b = 1e10 the quotient rounds
        // to 1 + 1e-10 with ~1e-16 absolute slop — 1e-6 RELATIVE slop in the
        // part that survives ln — while (a - b) is exactly 1.0 by Sterbenz.
        // Taylor: a·ln(1+1e-10) = 1 + 5e-11 - 1.7e-21, so 1.00000000005 is
        // the correctly-rounded f64 (the neglected term is ~1e-5 ulp).
        let (a, b) = (1e10 + 1.0, 1e10_f64);
        assert_eq!(a - b, 1.0, "test premise: a - b must be exact here");
        let got = t(&ce, &[a, b]);
        assert!(
            (got - 1.00000000005).abs() <= 2.3e-16,
            "centropy(1e10+1, 1e10): got {got:.17e}, want 1.00000000005"
        );
        // The naive form is off by 8e-8 here — eight digits, not one ulp.
        let naive = t(
            &mul(var(0), unary(UnaryOp::Log, div(var(0), var(1)))),
            &[a, b],
        );
        assert!(
            (naive - 1.00000000005).abs() > 1e-9,
            "test premise: a*log(a/b) is supposed to lose digits here, got {naive:.17e}"
        );

        // 0·ln(0/b) = 0 by the limit, not NaN.
        assert_eq!(t(&ce, &[0.0, 2.0]), 0.0);
        assert_eq!(t(&unary(UnaryOp::XLogX, var(0)), &[0.0]), 0.0);
    }

    #[test]
    fn cond_does_not_leak_a_non_finite_from_its_inactive_branch() {
        // Frontends clamp `entropy`/`centropy` with a Cond so the x -> 0+ limit
        // stays finite: `select(x < 1e-300, x*ln(1e-300), xlogx(x))`. The tape's
        // forward sweep evaluates EVERY slot, so at x = -1 the inactive
        // `xlogx(-1)` arm is NaN and its derivative arms are NaN too. Whether
        // that NaN reaches the caller is the difference between a usable clamp
        // and a poisoned Hessian row, and it is decided independently in three
        // sweeps — so check all three, not just the gradient.
        let cond = Expr::Compare(CmpOp::Lt, Box::new(var(0)), Box::new(cnst(1e-300)));
        let e = Expr::Cond {
            cond: Box::new(cond),
            then_: Box::new(mul(var(0), cnst(1e-300f64.ln()))),
            else_: Box::new(unary(UnaryOp::XLogX, var(0))),
        };
        let t = Tape::build(&e);
        let x = [-1.0_f64];

        // Premise: the inactive arm really is NaN at this point, so the test is
        // exercising the leak it claims to rule out.
        assert!(
            Tape::build(&unary(UnaryOp::XLogX, var(0)))
                .eval(&x)
                .is_nan(),
            "test premise: xlogx(-1) must be NaN"
        );

        assert_eq!(t.eval(&x), -1e-300_f64.ln());

        let mut g = vec![0.0; 1];
        t.gradient_seed(&x, 1.0, &mut g);
        assert_eq!(g[0], 1e-300_f64.ln(), "gradient leaked the inactive branch");

        let mut hess_map: HashMap<(usize, usize), usize> = HashMap::new();
        hess_map.insert((0, 0), 0);
        let mut acc = vec![0.0; 1];
        t.hessian_accumulate(&x, 1.0, &hess_map, &mut acc);
        assert_eq!(acc[0], 0.0, "hessian_accumulate leaked the inactive branch");

        let ops = t.ops.len();
        let mut vals = vec![0.0; ops];
        t.forward_into(&x, &mut vals);
        let mut col = vec![0.0; 1];
        let (mut dot, mut adj, mut adj_dot) = (vec![0.0; ops], vec![0.0; ops], vec![0.0; ops]);
        t.hessian_directional(
            &vals,
            &[1.0],
            1.0,
            &mut col,
            &mut dot,
            &mut adj,
            &mut adj_dot,
        );
        assert_eq!(
            col[0], 0.0,
            "hessian_directional leaked the inactive branch"
        );
    }

    #[test]
    fn atan2_grad_and_hessian_match_fd() {
        // f = atan2(x0, x1) + x0*x1, away from the origin.
        let atan2 = |a: Expr, b: Expr| Expr::Binary(BinOp::Atan2, Box::new(a), Box::new(b));
        let e = Expr::Sum(vec![atan2(var(0), var(1)), mul(var(0), var(1))]);
        grad_and_hess_match_fd(&e, &[1.2, 0.7], 1e-5);
    }

    #[test]
    fn minmax_grad_and_hessian_match_fd() {
        // f = min(x0, x1, x2) + max(x1, x2) + x0*x2
        // Point chosen so each list has a UNIQUE strictly-active
        // operand, so the subgradient equals the FD slope (the ±h
        // probes never cross a kink):
        //   min(0.5, 3.0, 2.0) = 0.5  -> active x0
        //   max(3.0, 2.0)      = 3.0  -> active x1
        let e = Expr::Sum(vec![
            Expr::MinList(vec![var(0), var(1), var(2)]),
            Expr::MaxList(vec![var(1), var(2)]),
            mul(var(0), var(2)),
        ]);
        grad_and_hess_match_fd(&e, &[0.5, 3.0, 2.0], 1e-5);
    }

    #[test]
    fn minmax_value_and_active_operand() {
        // Spot-check the value and that the gradient routes entirely
        // through the active operand (zero second derivative).
        let e = Expr::Sum(vec![
            Expr::MinList(vec![var(0), var(1)]),
            Expr::MaxList(vec![var(0), var(1)]),
        ]);
        let t = Tape::build(&e);
        // min(x0,x1) + max(x0,x1) == x0 + x1 for any inputs.
        let x = [1.3, -0.4];
        assert!((t.eval(&x) - (x[0] + x[1])).abs() < 1e-12);
        let mut g = vec![0.0; 2];
        t.gradient_seed(&x, 1.0, &mut g);
        // min active = x1 (smaller), max active = x0 (larger):
        // d/dx0 = 1 (from max), d/dx1 = 1 (from min).
        assert!((g[0] - 1.0).abs() < 1e-12, "g0={}", g[0]);
        assert!((g[1] - 1.0).abs() < 1e-12, "g1={}", g[1]);
    }

    #[test]
    fn hessian_division_matches_fd() {
        // f = x0/x1 + cos(x0)
        let e = add(div(var(0), var(1)), unary(UnaryOp::Cos, var(0)));
        let t = Tape::build(&e);
        fd_check(&t, &[0.5, 1.2], 2, 1e-5);
    }

    #[test]
    fn conditional_value_grad_hessian_active_branch() {
        // f = if x0 >= 1 then x0*x1 else x1^2
        // The if-then-else differentiates only the active branch; the
        // condition (a comparison) contributes no derivative.
        let e = cond(
            cmp(CmpOp::Ge, var(0), cnst(1.0)),
            mul(var(0), var(1)),
            pow(var(1), cnst(2.0)),
        );
        let t = Tape::build(&e);

        // x0 = 2 (>= 1) -> "then" branch x0*x1 is active.
        let x = [2.0, 5.0];
        assert!((t.eval(&x) - 10.0).abs() < 1e-12);
        let mut g = vec![0.0; 2];
        t.gradient_seed(&x, 1.0, &mut g);
        // d(x0*x1) = (x1, x0) = (5, 2)
        assert!((g[0] - 5.0).abs() < 1e-10);
        assert!((g[1] - 2.0).abs() < 1e-10);
        // H[0,1] = 1, diagonals 0. (Stay clear of the x0 = 1 kink.)
        fd_check(&t, &x, 2, 1e-5);

        // x0 = 0 (< 1) -> "else" branch x1^2 is active; x0 drops out.
        let x2 = [0.0, 5.0];
        assert!((t.eval(&x2) - 25.0).abs() < 1e-12);
        let mut g2 = vec![0.0; 2];
        t.gradient_seed(&x2, 1.0, &mut g2);
        assert!(g2[0].abs() < 1e-10);
        assert!((g2[1] - 10.0).abs() < 1e-10);
        fd_check(&t, &x2, 2, 1e-5);
    }

    #[test]
    fn comparison_and_logical_have_zero_derivative() {
        // f = (x0 < x1) + (x0 > 0 && x1 > 0) + !(x0 == x1)
        // Every term is piecewise-constant in the variables, so the
        // gradient must be identically zero away from the kinks.
        let lt = cmp(CmpOp::Lt, var(0), var(1));
        let and = Expr::And(
            Box::new(cmp(CmpOp::Gt, var(0), cnst(0.0))),
            Box::new(cmp(CmpOp::Gt, var(1), cnst(0.0))),
        );
        let notc = Expr::Not(Box::new(cmp(CmpOp::Eq, var(0), var(1))));
        let e = add(add(lt, and), notc);
        let t = Tape::build(&e);

        let x = [1.0, 2.0];
        // 1 (1<2) + 1 (both > 0) + 1 (1 != 2) = 3
        assert!((t.eval(&x) - 3.0).abs() < 1e-12);
        let mut g = vec![0.0; 2];
        t.gradient_seed(&x, 1.0, &mut g);
        assert!(g[0].abs() < 1e-12, "d/dx0 should be 0, got {}", g[0]);
        assert!(g[1].abs() < 1e-12, "d/dx1 should be 0, got {}", g[1]);
    }

    #[test]
    fn logical_or_value() {
        // f = (x0 > 0 || x1 > 0)
        let e = Expr::Or(
            Box::new(cmp(CmpOp::Gt, var(0), cnst(0.0))),
            Box::new(cmp(CmpOp::Gt, var(1), cnst(0.0))),
        );
        let t = Tape::build(&e);
        assert!((t.eval(&[-1.0, 3.0]) - 1.0).abs() < 1e-12);
        assert!((t.eval(&[-1.0, -3.0]) - 0.0).abs() < 1e-12);
    }

    /// `hessian_directional` (one forward-over-reverse pass with
    /// a seed vector) recovers `H · e_j` for each unit-vector seed,
    /// matching column `j` of the dense Hessian computed by
    /// `hessian_accumulate`.
    fn directional_matches_accumulate(tape: &Tape, x: &[f64], n: usize) {
        let vars = tape.variables();
        let mut hess_map: HashMap<(usize, usize), usize> = HashMap::new();
        let mut pairs = Vec::new();
        for (ai, &vi) in vars.iter().enumerate() {
            for &vj in &vars[..=ai] {
                let (r, c) = if vi >= vj { (vi, vj) } else { (vj, vi) };
                hess_map.entry((r, c)).or_insert_with(|| {
                    let p = pairs.len();
                    pairs.push((r, c));
                    p
                });
            }
        }
        let nnz = pairs.len();
        let mut ad = vec![0.0; nnz];
        tape.hessian_accumulate(x, 1.0, &hess_map, &mut ad);

        let nops = tape.ops.len();
        let mut vals = vec![0.0; nops];
        tape.forward_into(x, &mut vals);
        let mut dot = vec![0.0; nops];
        let mut adj = vec![0.0; nops];
        let mut adj_dot = vec![0.0; nops];

        for &j in &vars {
            let mut seed = vec![0.0; n];
            seed[j] = 1.0;
            let mut col = vec![0.0; n];
            tape.hessian_directional(
                &vals,
                &seed,
                1.0,
                &mut col,
                &mut dot,
                &mut adj,
                &mut adj_dot,
            );
            for &i in &vars {
                let (r, c) = if i >= j { (i, j) } else { (j, i) };
                let expect = ad[hess_map[&(r, c)]];
                assert!(
                    (col[i] - expect).abs() < 1e-10,
                    "directional H[{i},{j}] = {} vs accumulate {}",
                    col[i],
                    expect
                );
            }
        }
    }

    #[test]
    fn directional_quadratic_matches_accumulate() {
        // f = 3 x0^2 + 2 x0 x1 + x1^2
        let e = add(
            add(
                mul(cnst(3.0), pow(var(0), cnst(2.0))),
                mul(mul(cnst(2.0), var(0)), var(1)),
            ),
            pow(var(1), cnst(2.0)),
        );
        let t = Tape::build(&e);
        directional_matches_accumulate(&t, &[0.5, -0.3], 2);
    }

    #[test]
    fn directional_transcendental_matches_accumulate() {
        let e = Expr::Sum(vec![
            unary(UnaryOp::Exp, var(0)),
            unary(UnaryOp::Sin, var(1)),
            unary(UnaryOp::Log, var(0)),
            unary(UnaryOp::Sqrt, var(1)),
            mul(var(0), var(1)),
        ]);
        let t = Tape::build(&e);
        directional_matches_accumulate(&t, &[1.5, 2.0], 2);
    }

    #[test]
    fn directional_with_division_matches_accumulate() {
        let e = add(div(var(0), var(1)), unary(UnaryOp::Cos, var(0)));
        let t = Tape::build(&e);
        directional_matches_accumulate(&t, &[0.5, 1.2], 2);
    }

    #[test]
    fn hessian_sparsity_separable() {
        // f = sin(x0) + x1*x2; couplings: (0,0) from sin, (2,1) from x1*x2
        let e = add(unary(UnaryOp::Sin, var(0)), mul(var(1), var(2)));
        let t = Tape::build(&e);
        let s = t.hessian_sparsity();
        assert!(s.contains(&(0, 0)));
        assert!(s.contains(&(2, 1)));
        assert!(!s.contains(&(1, 0)));
        assert!(!s.contains(&(2, 0)));
    }

    fn count_op<F: Fn(&TapeOp) -> bool>(t: &Tape, pred: F) -> usize {
        t.ops.iter().filter(|o| pred(o)).count()
    }

    #[test]
    fn pow_zero_const_folds_to_one() {
        // x^0 → 1 (no Pow, no reference to x in the tape)
        let e = pow(var(0), cnst(0.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Var(_))), 0);
        assert!((t.eval(&[7.0]) - 1.0).abs() < 1e-12);
    }

    #[test]
    fn pow_one_passes_through() {
        // x^1 → x (no Pow, no Const introduced for the exponent)
        let e = pow(var(0), cnst(1.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Const(_))), 0);
        assert!((t.eval(&[3.5]) - 3.5).abs() < 1e-12);
    }

    #[test]
    fn pow_half_lowers_to_sqrt() {
        let e = pow(var(0), cnst(0.5));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Sqrt(_))), 1);
        assert!((t.eval(&[16.0]) - 4.0).abs() < 1e-12);
    }

    #[test]
    fn pow_two_lowers_to_single_mul() {
        let e = pow(var(0), cnst(2.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Mul(..))), 1);
        assert!((t.eval(&[3.0]) - 9.0).abs() < 1e-12);
    }

    #[test]
    fn pow_three_lowers_to_two_muls() {
        let e = pow(var(0), cnst(3.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Mul(..))), 2);
        assert!((t.eval(&[2.0]) - 8.0).abs() < 1e-12);
    }

    #[test]
    fn pow_eight_lowers_to_three_muls() {
        // Binary expansion: x → x² → x⁴ → x⁸ (3 squarings)
        let e = pow(var(0), cnst(8.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Mul(..))), 3);
        assert!((t.eval(&[2.0]) - 256.0).abs() < 1e-12);
    }

    #[test]
    fn pow_negative_two_lowers_to_div() {
        // x^-2 → 1 / (x*x)
        let e = pow(var(0), cnst(-2.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Div(..))), 1);
        assert!((t.eval(&[4.0]) - (1.0 / 16.0)).abs() < 1e-12);
    }

    #[test]
    fn pow_large_const_stays_generic() {
        // x^9 stays as Pow — beyond the cutoff, generic is cheaper.
        let e = pow(var(0), cnst(9.0));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 1);
    }

    #[test]
    fn pow_non_integer_const_stays_generic() {
        // x^1.5 stays as Pow until half-integer handling is added.
        let e = pow(var(0), cnst(1.5));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 1);
    }

    #[test]
    fn pow_const_through_cse_const() {
        // Exponent wrapped in Cse — peek_const should still see it.
        let two = Arc::new(cnst(2.0));
        let e = Expr::Binary(BinOp::Pow, Box::new(var(0)), Box::new(Expr::Cse(two)));
        let t = Tape::build(&e);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Pow(..))), 0);
        assert_eq!(count_op(&t, |o| matches!(o, TapeOp::Mul(..))), 1);
    }

    #[test]
    fn hessian_pow_three_matches_fd() {
        // f = 5 * x0^3 + x0 * x1 — exercises the lowered cubic + cross term.
        let e = add(mul(cnst(5.0), pow(var(0), cnst(3.0))), mul(var(0), var(1)));
        let t = Tape::build(&e);
        fd_check(&t, &[1.7, 0.8], 2, 1e-5);
    }

    #[test]
    fn hessian_pow_negative_matches_fd() {
        // f = 1/x0^2 + x1^2 — exercises lowered x^-2 and x^2.
        let e = add(pow(var(0), cnst(-2.0)), pow(var(1), cnst(2.0)));
        let t = Tape::build(&e);
        fd_check(&t, &[1.3, 2.4], 2, 1e-5);
    }

    #[test]
    fn hessian_pow_half_matches_fd() {
        // f = sqrt(x0) + x0*x1 (via Pow(_, 0.5) → Sqrt)
        let e = add(pow(var(0), cnst(0.5)), mul(var(0), var(1)));
        let t = Tape::build(&e);
        fd_check(&t, &[2.5, 1.1], 2, 1e-5);
    }

    #[test]
    fn hessian_sparsity_through_cse() {
        // body = x0+x1 (CSE). f = body^2 + body.
        // d²/dx² of body^2 couples (0,0), (1,0), (1,1).
        let body = Arc::new(add(var(0), var(1)));
        let e = add(
            pow(Expr::Cse(body.clone()), cnst(2.0)),
            Expr::Cse(body.clone()),
        );
        let t = Tape::build(&e);
        let s = t.hessian_sparsity();
        assert!(s.contains(&(0, 0)));
        assert!(s.contains(&(1, 0)));
        assert!(s.contains(&(1, 1)));
        assert_eq!(s.len(), 3);
    }

    #[test]
    fn pow_forward_tangent_matches_reverse_gradient_at_base_zero() {
        // Code review L29: `Pow` first-order tangent disagreed with the
        // reverse-mode gradient at base 0. f = x0 ^ x1 keeps a genuine `Pow`
        // op (variable exponent is not lowered to a Mul/Sqrt chain). At the
        // `.nl` default start x0 = 0, the base derivative d/dx0 (x0^1) = 1 is
        // well defined; reverse mode has always computed it, but the forward
        // tangent used to guard on `u != 0` and drop it, so Jacobian-vector
        // products silently disagreed with the gradient at x = 0. After the
        // fix both arms must agree.
        let e = pow(var(0), var(1));
        let t = Tape::build(&e);
        // Guard: the op must survive as a real Pow (not lowered away), else
        // this test would no longer exercise the fixed branch.
        assert!(
            t.ops.iter().any(|op| matches!(op, TapeOp::Pow(_, _))),
            "expected a Pow op in the tape; got {:?}",
            t.ops
        );
        let x = [0.0, 1.0];
        let n = t.ops.len();

        // Reverse-mode gradient w.r.t. x0.
        let mut grad = vec![0.0; 2];
        t.gradient_seed(&x, 1.0, &mut grad);

        // Forward tangent seeded on x0: dot[output] = df/dx0.
        let vals = t.forward(&x);
        let mut dot = vec![0.0; n];
        t.forward_tangent(&vals, 0, &mut dot);
        let fwd_dfx0 = dot[n - 1];

        assert!(
            (grad[0] - 1.0).abs() < 1e-12,
            "reverse gradient df/dx0 at base 0 should be 1, got {}",
            grad[0]
        );
        assert!(
            (fwd_dfx0 - grad[0]).abs() < 1e-12,
            "forward tangent df/dx0 = {fwd_dfx0} must match reverse gradient {} at base 0",
            grad[0]
        );
    }

    #[test]
    #[should_panic(expected = "external function calls are not supported on the")]
    fn hybrid_promoted_cse_with_funcall_reports_clear_message() {
        // Code review L34: `HybridTape::build_multi` builds a promoted CSE
        // (one shared across ≥2 summands) via `build_recursive` with an empty
        // resolver. A funcall inside that promoted body used to panic with the
        // misleading "unresolved AMPL funcall id 0" — implying a resolution
        // failure — instead of the real reason: funcalls are unsupported on
        // the hybrid path. Here the funcall body is shared across two roots so
        // it is promoted; assert the clear hybrid-unsupported message fires.
        let body = Arc::new(Expr::Funcall {
            id: 0,
            args: vec![FuncallArg::Real(var(0))],
        });
        let exprs = vec![
            add(Expr::Cse(body.clone()), cnst(1.0)),
            add(Expr::Cse(body.clone()), cnst(2.0)),
        ];
        HybridTape::build_multi(&exprs);
    }

    /// The directional hybrid Hessian (issue #557) against the flat
    /// [`Tape::hessian_directional`] it replaces: three summands sharing one
    /// transcendental CSE body, each with its own multiplier, swept with a
    /// dense two-variable seed. The per-summand *local* contributions are the
    /// exact arithmetic of the flat tape; the shared-prelude contribution
    /// folds the multipliers into the boundary adjoints and runs one unit-
    /// weight prelude sweep, which reassociates the floating-point products —
    /// mathematically identical by linearity of reverse-over-tangent in the
    /// adjoint seeds, so the comparison is at near-machine tolerance rather
    /// than bitwise.
    #[test]
    fn directional_hybrid_hessian_matches_flat_directional() {
        // body = exp(0.5*(x0 + x1)), shared by all three summands:
        //   s0 = body^2,  s1 = body * x2,  s2 = sin(body) + x2^2.
        let body = Arc::new(unary(UnaryOp::Exp, mul(cnst(0.5), add(var(0), var(1)))));
        let exprs = vec![
            pow(Expr::Cse(body.clone()), cnst(2.0)),
            mul(Expr::Cse(body.clone()), var(2)),
            add(
                unary(UnaryOp::Sin, Expr::Cse(body.clone())),
                pow(var(2), cnst(2.0)),
            ),
        ];
        let weights = [1.25, -0.75, 2.5];
        let x = [0.3, -0.1, 0.7];
        let n = 3;

        let hybrid = HybridTape::build_multi(&exprs);
        assert!(
            hybrid.n_prelude_ops() > 0,
            "a CSE shared by 3 roots must be promoted into the prelude"
        );

        // Flat reference: independent tapes, one directional pass each.
        let seeds = [[1.0, 0.0, 1.0], [0.0, 1.0, 0.0]];
        for seed in &seeds {
            let mut flat_out = vec![0.0; n];
            for (e, &wt) in exprs.iter().zip(&weights) {
                let t = Tape::build(e);
                let vals = t.forward(&x);
                let m = t.ops.len();
                let (mut dot, mut adj, mut adj_dot) = (vec![0.0; m], vec![0.0; m], vec![0.0; m]);
                t.hessian_directional(
                    &vals,
                    seed,
                    wt,
                    &mut flat_out,
                    &mut dot,
                    &mut adj,
                    &mut adj_dot,
                );
            }

            let mut hyb_out = vec![0.0; n];
            let np = hybrid.n_prelude_ops();
            let ml = hybrid.max_summand_ops();
            let mut prelude_vals = vec![0.0; np];
            let mut prelude_dot = vec![0.0; np];
            let mut prelude_adj = vec![0.0; np];
            let mut prelude_adj_dot = vec![0.0; np];
            let (mut local_dot, mut local_adj, mut local_adj_dot) =
                (vec![0.0; ml], vec![0.0; ml], vec![0.0; ml]);
            // The per-color prelude reach the caller is required to pass:
            // the union of the color's summands' `prelude_reach`, ascending.
            // Built here rather than passing `0..np` so the test exercises the
            // narrowed walk `eval_h` actually performs — a reach that wrongly
            // omitted a slot would leave its tangent stale and show up below.
            let creach: Vec<u32> = {
                let mut u: BTreeSet<u32> = BTreeSet::new();
                for s in &hybrid.summands {
                    u.extend(s.prelude_reach.iter().map(|&p| p as u32));
                }
                u.into_iter().collect()
            };
            hybrid.forward_prelude(&x, &mut prelude_vals);
            hybrid.prelude_tangent(&prelude_vals, seed, &creach, &mut prelude_dot);
            for (s, &wt) in hybrid.summands.iter().zip(&weights) {
                let mut local_vals = vec![0.0; s.ops.len()];
                hybrid.forward_summand(s, &x, &prelude_vals, &mut local_vals);
                hybrid.hessian_summand_directional(
                    s,
                    &local_vals,
                    &prelude_dot,
                    seed,
                    wt,
                    &mut hyb_out,
                    &mut local_dot,
                    &mut local_adj,
                    &mut local_adj_dot,
                    &mut prelude_adj,
                    &mut prelude_adj_dot,
                );
            }
            hybrid.prelude_reverse_directional(
                &prelude_vals,
                &prelude_dot,
                &creach,
                &mut hyb_out,
                &mut prelude_adj,
                &mut prelude_adj_dot,
            );

            for k in 0..n {
                let scale = flat_out[k].abs().max(1.0);
                assert!(
                    (hyb_out[k] - flat_out[k]).abs() <= 1e-13 * scale,
                    "seed {seed:?} entry {k}: hybrid {} vs flat {}",
                    hyb_out[k],
                    flat_out[k]
                );
            }
            assert!(
                hyb_out.iter().any(|v| *v != 0.0),
                "an all-zero H·s is no comparison"
            );
            // The accumulators must come back all-zero, ready for the
            // next color (`prelude_reverse_directional`'s contract).
            assert!(prelude_adj.iter().all(|v| *v == 0.0));
            assert!(prelude_adj_dot.iter().all(|v| *v == 0.0));
        }
    }
}