basin 0.4.0

use crate::core::math::{
    AddDiagonalVectorInPlace, ComponentDivAssign, ComponentMaxAssign, ComponentMulAssign, Dot,
    FloorZerosInPlace, GramMatrix, LinearSolveSpd, MatDiagonal, MatTransposeVec, NegInPlace,
    NormInfinity, NormSquared, ScaleInPlace, ScaledAdd,
};
use crate::core::problem::{Jacobian, Residual};
use crate::core::solver::Solver;
use crate::core::state::BasicState;
use crate::core::termination::TerminationReason;

/// Levenberg-Marquardt solver for nonlinear least-squares problems
/// `min ½‖r(x)‖²`, with Marquardt diagonal scaling and the Nielsen
/// 1999 smooth μ-update.
///
/// Each iteration solves the damped normal equations
/// `(JᵀJ + μ·D) h = −Jᵀr` via Cholesky, then adapts the damping
/// parameter μ from the gain ratio
/// `ρ = (F(x) − F(x+h)) / (L(0) − L(h))` (Nielsen eq. 2.2). On a
/// successful step (ρ > 0) μ is reduced via the smooth cubic
/// `μ ← μ · max(1/3, 1 − (2ρ−1)³)`; on a failed step (ρ ≤ 0) μ grows
/// geometrically `μ ← μ·ν, ν ← 2ν` with ν initialized to 2 — Nielsen
/// shows this avoids the discontinuities of the classical
/// multiply/divide threshold rule and lands roughly 25 % fewer
/// iterations on average. See Nielsen, *Damping Parameter in
/// Marquardt's Method* (IMM-REP-1999-05) for the derivation and
/// Madsen, Nielsen, Tingleff (2004), *Methods for Non-Linear Least
/// Squares Problems*, §3.2.
///
/// **Marquardt scaling (`μ·D`, not `μI`).** The damping matrix is the
/// diagonal of the Gram, `D = diag(JᵀJ)` — the per-parameter curvature
/// — rather than the identity. This makes the trust region ellipsoidal
/// in the metric of the columns of `J`, so the algorithm is invariant
/// to diagonal rescaling of the parameters (Marquardt 1963; Moré 1978,
/// *The Levenberg-Marquardt Algorithm: Implementation and Theory*).
/// Isotropic `μI` damping over-damps well-scaled directions and
/// under-damps poorly-scaled ones when the columns of `J` have very
/// different norms (e.g. parameters in a mixed log/linear/angle
/// encoding), which biases the step and can pull the iterate into a
/// worse basin. `D` is maintained as a **monotone running max**
/// `D_k = max(D_{k−1}, diag(J(x_k)ᵀJ(x_k)))` so a column whose
/// curvature momentarily drops keeps the damping floor it earned
/// earlier (Moré 1978; the same safeguard MINPACK applies to its
/// column-norm scaling). Columns that are exactly zero at `x₀` (a
/// parameter with no first-order effect on any residual) would make
/// `μ·D` vanish there and the Gram singular; following MINPACK, their
/// scale is floored to `1` at `init` (see [`FloorZerosInPlace`]), so a
/// fully-insensitive parameter stays put rather than failing Cholesky.
///
/// Initial damping is `μ₀ = τ` — dimensionless, because the
/// per-parameter magnitude now lives in `D` (the initial per-column
/// damping is `τ·diag(J(x₀)ᵀJ(x₀))`). τ is the *relative* trust
/// parameter; use a smaller value (e.g. `1e-6`) when `x₀` is believed
/// close to the optimum, larger (e.g. `1.0`) when far. Default
/// `τ = 10⁻³` matches Nielsen's "moderate trust" recommendation.
///
/// **Cholesky-on-(JᵀJ + μ·D) vs QR-on-stacked-system.** The damping
/// makes the SPD path strictly better-conditioned than pure
/// Gauss-Newton's `JᵀJ` — `μ·D` regularizes the rank deficiency that
/// makes GN fail. We stay on the SPD path because that's the only one
/// the [`linalg`](crate::core::math) tier exposes today, and the
/// regularization is sufficient for unconstrained LM.
/// QR-on-stacked-system (`[J; √(μD)]`) is more robust to ill-conditioned
/// `J` near rank deficiency but adds a second factorization route to
/// the linalg surface; deferred until S6 (TRF), where rank-deficient
/// Jacobians and box constraints make QR materially better.
///
/// # Failure modes
///
/// - **Cholesky failure under bumped μ.** When the initial damping is
///   too small to make `JᵀJ + μ·D` SPD (effectively never, for any
///   sensible `JᵀJ` and finite μ), the inner damping loop bumps μ via
///   `μ := μ·ν, ν := 2ν` and retries. Default
///   [`max_inner_attempts`](Self::max_inner_attempts) is 50 — far more
///   than enough; in practice the first attempt succeeds. If the cap
///   is exhausted (μ overflowing to `inf`), the solver returns
///   [`TerminationReason::SolverFailed`]. Note that bumping μ cannot
///   rescue a coordinate whose `D` entry is zero (`μ·0 = 0`); the
///   `init` zero-column floor exists precisely to keep `D > 0`.
/// - **Divergence on highly nonlinear / poorly initialized problems.**
///   The damping itself prevents divergent steps (failed steps are
///   rejected via the gain-ratio test), so divergence manifests as
///   μ growing without bound. Catch this with
///   [`MaxIter`](crate::core::termination::MaxIter) on the executor.
///
/// # Termination
///
/// Beyond the framework criteria
/// ([`MaxIter`](crate::core::termination::MaxIter),
/// [`CostTolerance`](crate::core::termination::CostTolerance),
/// [`ParamTolerance`](crate::core::termination::ParamTolerance), …),
/// the solver emits [`TerminationReason::SolverConverged`] when any of
/// four MINPACK-style tests is satisfied — the same independent
/// `info`-code structure MINPACK uses, so converging on whichever fires
/// first:
///
/// - **`tol_grad`** — absolute first-order optimality (Madsen et al.
///   eq. 3.3a): `‖Jᵀr‖_∞ ≤ tol_grad`. Default `1e-8`; `0.0` disables.
/// - **`tol_grad_rel`** — relative first-order optimality, MINPACK
///   `gtol` (Moré 1978): the cosine of the angle between the residual
///   `r` and every column of `J`,
///   `max_j |gⱼ| / (‖J·,ⱼ‖ · ‖r‖) ≤ tol_grad_rel`. This measure is
///   dimensionless — invariant to scaling the residuals — so a single
///   tolerance is portable across problems whose residuals carry
///   different normalizations (where the absolute `‖Jᵀr‖_∞` is too
///   tight for some and too loose for others). Default `0.0`
///   (disabled); set e.g. `1e-8` for parity. The per-column norms
///   `‖J·,ⱼ‖ = √diag(JᵀJ)ⱼ` reuse the Marquardt scaling diagonal the
///   solver already forms.
/// - **`ftol`** — relative cost reduction, MINPACK `ftol` (Moré 1978):
///   `|actred| ≤ ftol·F  ∧  prered ≤ ftol·F  ∧  ρ ≤ 2`, with the actual
///   and *predicted* per-iteration reductions in `F = ½‖r‖²`. The
///   `prered` clause is what the framework's
///   [`RelativeCostTolerance`](crate::core::termination::RelativeCostTolerance)
///   cannot express — it gates on the LM model, so the solver iterates
///   through temporary settling points (small actual gain, large
///   predicted gain) instead of stopping short. Default `0.0`
///   (disabled). See [`ftol`](Self::ftol).
/// - **`xtol`** — relative step, MINPACK `xtol` (Moré 1978):
///   `‖h‖ ≤ xtol·‖x‖`. Default `0.0` (disabled). See [`xtol`](Self::xtol).
///
/// The two gradient tests run before the step is computed (a step at a
/// stationary point is wasted); `ftol`/`xtol` run after, since they need
/// the attempted step and its predicted/actual reduction.
///
/// LM deliberately leaves `state.gradient = None` — the framework's
/// [`GradientTolerance`](crate::core::termination::GradientTolerance)
/// uses the L2 squared norm and is the wrong metric for NLLS, where
/// the canonical first-order test is the ∞-norm of `Jᵀr`. Same choice
/// as [`GaussNewton`](super::GaussNewton).
///
/// # Backends
///
/// LA-heavy: nalgebra (`DVector<f64>` / `DMatrix<f64>`) and faer
/// (`Col<f64>` / `Mat<f64>`) at the dense tier; nalgebra-sparse
/// (`DVector<f64>` / `CscMatrix<f64>`) and faer-sparse (`Col<f64>` /
/// `SparseColMat<usize, f64>`) at the sparse tier. `Vec<f64>` and
/// `ndarray::Array1<f64>` produce a compile-time error per tenet 5.
/// The sparse damping path requires the diagonal of `JᵀJ` to be in the
/// CSC pattern (always true when `J` has no zero columns); see
/// [`AddDiagonalVectorInPlace`] and [`MatDiagonal`].
///
/// # State convention
///
/// `state.cost` carries the LM convention `½‖r‖²`, derived from the
/// residual the solver evaluates itself. The bound on `P` is
/// [`Residual`] + [`Jacobian`], not
/// [`CostFunction`](crate::core::problem::CostFunction); problems
/// whose user-facing `cost()` uses an unscaled `Σ rᵢ²` form will see
/// `state.cost()` differ from `problem.cost(state.param())` by a
/// factor of two. Both go to zero at the optimum, so cost-based
/// termination criteria are unaffected.
pub struct LevenbergMarquardt<V, M> {
    tol_grad: f64,
    tol_grad_rel: f64,
    ftol: f64,
    xtol: f64,
    tau: f64,
    max_inner_attempts: u32,

    // Runtime state, populated by `init` and mutated by `next_iter`
    // through `&mut self`.
    mu: Option<f64>,
    nu: f64,

    // Marquardt scaling diagonal `D` — the monotone running max of
    // `diag(JᵀJ)` (Moré 1978). Seeded at `init` from `diag(J(x₀)ᵀJ(x₀))`
    // with zero columns floored to 1, then maxed against each
    // iteration's Gram diagonal. The damped system is `(JᵀJ + μ·D) h =
    // −Jᵀr`.
    diag: Option<V>,

    // Cross-iteration caches keyed on "did the iterate move?". A
    // rejected step leaves `x` unchanged, so everything derived from it
    // — the residual `r`, the Gram `A = JᵀJ`, and the gradient
    // `g = Jᵀr` — is still valid on the next outer iteration and is
    // reused. Madsen-Nielsen Algorithm 3.16 recomputes `A` and `g` only
    // *after* an accepted step (line 13), never on a reject; caching
    // them here makes basin's executor-driven loop (one `next_iter` per
    // damping attempt) match that, instead of reforming the
    // dominant-cost Gram from an unchanged `J` on every rejected step
    // (issue #10).
    //
    // - `r_cache`: refreshed to `r(x_trial)` on accept, to the unchanged
    //   `r` on reject; the trial residual is computed in the gain-ratio
    //   test, so accepting never re-evaluates it.
    // - `gram_cache` / `jtr_cache`: set together on reject (the iterate
    //   held still), cleared together on accept (the new `J` — and so
    //   `A`, `g` — must be recomputed at the moved iterate). When both
    //   are absent the step re-evaluates `J` once and rebuilds them.
    r_cache: Option<V>,
    gram_cache: Option<M>,
    jtr_cache: Option<V>,
}

impl<V, M> Default for LevenbergMarquardt<V, M> {
    fn default() -> Self {
        Self::new()
    }
}

impl<V, M> LevenbergMarquardt<V, M> {
    /// Levenberg-Marquardt with Nielsen's defaults: `tol_grad = 1e-8`,
    /// `tol_grad_rel = 0.0` (disabled), `ftol = 0.0` (disabled),
    /// `xtol = 0.0` (disabled), `tau = 1e-3`, `max_inner_attempts = 50`.
    pub fn new() -> Self {
        Self {
            tol_grad: 1e-8,
            tol_grad_rel: 0.0,
            ftol: 0.0,
            xtol: 0.0,
            tau: 1e-3,
            max_inner_attempts: 50,
            mu: None,
            nu: 2.0,
            diag: None,
            r_cache: None,
            gram_cache: None,
            jtr_cache: None,
        }
    }

    /// Absolute first-order optimality tolerance: emit
    /// [`TerminationReason::SolverConverged`] when `‖Jᵀr‖_∞ ≤ tol`
    /// (Madsen et al. eq. 3.3a). Set to `0.0` to disable the check and
    /// rely solely on [`tol_grad_rel`](Self::tol_grad_rel) and/or
    /// framework termination criteria. Default `1e-8`.
    pub fn tol_grad(mut self, tol: f64) -> Self {
        assert!(tol >= 0.0, "tol_grad must be ≥ 0");
        self.tol_grad = tol;
        self
    }

    /// Relative (scale-invariant) first-order optimality tolerance —
    /// the MINPACK `gtol` test (Moré 1978): emit
    /// [`TerminationReason::SolverConverged`] when the cosine of the
    /// angle between the residual `r` and every Jacobian column is at
    /// most `tol`, i.e. `max_j |gⱼ| / (‖J·,ⱼ‖ · ‖r‖) ≤ tol` with
    /// `g = Jᵀr`. Being a dimensionless cosine, it is invariant to
    /// scaling of the residuals, so one tolerance ports across problems
    /// with different residual normalizations — unlike the absolute
    /// [`tol_grad`](Self::tol_grad). Set to `0.0` to disable. Default
    /// `0.0` (disabled); use e.g. `1e-8` for MINPACK `gtol` parity.
    ///
    /// Both gradient tests can be active at once; the solver converges
    /// when *either* fires (matching MINPACK, which checks `ftol`,
    /// `xtol`, and `gtol` independently).
    pub fn tol_grad_rel(mut self, tol: f64) -> Self {
        assert!(tol >= 0.0, "tol_grad_rel must be ≥ 0");
        self.tol_grad_rel = tol;
        self
    }

    /// Relative cost-reduction tolerance — the MINPACK `ftol` test
    /// (Moré 1978): emit [`TerminationReason::SolverConverged`] when both
    /// the *actual* and the *predicted* reduction in `½‖r‖²` over an
    /// iteration are at most `tol` relative to the current cost, and the
    /// gain ratio is sane:
    ///
    /// ```text
    /// |actred| ≤ tol·F   AND   prered ≤ tol·F   AND   ρ ≤ 2
    /// ```
    ///
    /// with `actred = F(x) − F(x+h)`, `prered = L(0) − L(h)` the model's
    /// predicted reduction, `F = ½‖r‖²`, and `ρ = actred/prered`.
    ///
    /// The `prered` clause is the load-bearing difference from the
    /// framework's [`RelativeCostTolerance`] — which sees only the
    /// achieved reduction between consecutive costs and has no access to
    /// the LM model. At a *temporary settling point* a single step's
    /// actual gain can be small while the model still predicts substantial
    /// progress; gating on `prered` keeps LM iterating through such points
    /// to the true minimum, where a plain achieved-reduction test would
    /// stop short. This is exactly MINPACK's behavior and the reason
    /// `ftol` belongs on the solver rather than in the termination layer.
    ///
    /// Set to `0.0` to disable. Default `0.0` (disabled); use e.g. `1e-8`
    /// for MINPACK `ftol` parity. Converges when *any* enabled test fires
    /// (see [`tol_grad`](Self::tol_grad)).
    ///
    /// [`RelativeCostTolerance`]: crate::core::termination::RelativeCostTolerance
    pub fn ftol(mut self, tol: f64) -> Self {
        assert!(tol >= 0.0, "ftol must be ≥ 0");
        self.ftol = tol;
        self
    }

    /// Relative step tolerance — the MINPACK `xtol` test (Moré 1978):
    /// emit [`TerminationReason::SolverConverged`] when the accepted (or
    /// attempted) step is negligible relative to the iterate,
    /// `‖h‖ ≤ tol·‖x‖`. Nielsen's smooth μ-update carries no explicit
    /// trust radius `δ`, so the step norm is the natural analog of
    /// MINPACK's `delta ≤ xtol·xnorm`. Set to `0.0` to disable. Default
    /// `0.0` (disabled); use e.g. `1e-8` for MINPACK `xtol` parity.
    /// Converges when *any* enabled test fires (see
    /// [`tol_grad`](Self::tol_grad)).
    pub fn xtol(mut self, tol: f64) -> Self {
        assert!(tol >= 0.0, "xtol must be ≥ 0");
        self.xtol = tol;
        self
    }

    /// Relative initial damping `τ`: `μ₀ = τ`, giving an initial
    /// per-column damping of `τ·diag(J(x₀)ᵀJ(x₀))` under Marquardt
    /// scaling. Use a smaller value (e.g. `1e-6`) when `x₀` is believed
    /// close to the optimum; a larger value (e.g. `1.0`) when far from
    /// it. Default `1e-3` (Nielsen's "moderate trust").
    pub fn tau(mut self, tau: f64) -> Self {
        assert!(tau > 0.0, "tau must be > 0");
        self.tau = tau;
        self
    }

    /// Maximum number of damping bumps inside a single outer iteration
    /// before giving up with [`TerminationReason::SolverFailed`]. Each
    /// bump multiplies μ by ν (initially 2) and doubles ν. With the
    /// default 50, μ grows by a factor of `2^50 ≈ 10¹⁵` before bailing
    /// — effectively unreachable in practice. Default `50`.
    pub fn max_inner_attempts(mut self, n: u32) -> Self {
        assert!(n > 0, "max_inner_attempts must be > 0");
        self.max_inner_attempts = n;
        self
    }
}

impl<P, V, M> Solver<P, BasicState<V>> for LevenbergMarquardt<V, M>
where
    P: Residual<Param = V, Output = V> + Jacobian<Param = V, Output = M>,
    V: ScaledAdd<f64>
        + NormSquared
        + NormInfinity
        + NegInPlace
        + Dot
        + ScaleInPlace
        + ComponentMulAssign
        + ComponentDivAssign
        + ComponentMaxAssign
        + FloorZerosInPlace
        + Clone,
    M: GramMatrix
        + MatTransposeVec<V>
        + LinearSolveSpd<V>
        + AddDiagonalVectorInPlace<V>
        + MatDiagonal<V>
        + Clone,
{
    fn init(&mut self, problem: &P, mut state: BasicState<V>) -> BasicState<V> {
        // Seed cost so iter-0 termination criteria see a populated
        // state. Also evaluate J(x₀) once to seed the Marquardt scaling
        // diagonal `D`. The Gram `A₀`, gradient `g₀`, and residual `r`
        // are stashed into the caches so the first `next_iter` reuses
        // them — no redundant evaluation (or re-formed Gram) at the
        // init/iter-0 boundary.
        let r = problem.residual(&state.param);
        let j = problem.jacobian(&state.param);
        state.cost = Some(0.5 * r.norm_squared());
        state.cost_evals += 1;
        state.gradient_evals += 1;

        // A₀ = J(x₀)ᵀJ(x₀); its diagonal is D₀, the per-parameter
        // curvature. A column that's exactly zero at x₀ contributes 0
        // there, which would make `μ·D` vanish on that coordinate and
        // the Gram singular; following MINPACK we floor those to 1 so an
        // insensitive parameter simply doesn't move. The running max in
        // `next_iter` then keeps `D` monotone.
        let a = j.gram();
        let mut d = a.diagonal();
        d.floor_zeros_in_place(1.0);
        self.diag = Some(d);

        // μ₀ = τ. Dimensionless: the per-parameter magnitude lives in
        // `D`, so the initial per-column damping is `τ·diag(J(x₀)ᵀJ(x₀))`.
        self.mu = Some(self.tau);
        self.nu = 2.0;
        self.jtr_cache = Some(j.mat_transpose_vec(&r));
        self.gram_cache = Some(a);
        self.r_cache = Some(r);
        state
    }

    fn next_iter(
        &mut self,
        problem: &P,
        mut state: BasicState<V>,
    ) -> (BasicState<V>, Option<TerminationReason>) {
        // `r` is at the current `state.param` after init (initial point)
        // or the previous iteration's bookkeeping (post-accept: r at the
        // new iterate; post-reject: unchanged). Only count an eval on a
        // cache miss, so `cost_evals` grows with *actual* invocations.
        let r = match self.r_cache.take() {
            Some(r) => r,
            None => {
                state.cost_evals += 1;
                problem.residual(&state.param)
            }
        };

        // A = JᵀJ (Gram) and g = Jᵀr (the gradient of ½‖r‖²). On a
        // rejected step the iterate didn't move, so A and g are unchanged
        // and reused from the caches; Madsen-Nielsen Algorithm 3.16
        // recomputes them only after an *accepted* step. When the caches
        // are cold (post-accept or — but for init's seeding — first
        // iteration) re-evaluate J once and rebuild them. This is the
        // issue-#10 fix: a rejected step no longer reforms the
        // dominant-cost Gram from an unchanged J.
        let (a, g) = match (self.gram_cache.take(), self.jtr_cache.take()) {
            (Some(a), Some(g)) => (a, g),
            _ => {
                state.gradient_evals += 1;
                let j = problem.jacobian(&state.param);
                (j.gram(), j.mat_transpose_vec(&r))
            }
        };
        // The current per-column curvatures `diag(JᵀJ)ⱼ = ‖J·,ⱼ‖²` feed
        // both the damping and the relative gradient test.
        let diag_cur = a.diagonal();

        // First-order optimality — converge on *either* test, matching
        // MINPACK's independent checks:
        //   * absolute   ‖Jᵀr‖_∞ ≤ tol_grad           (Madsen et al. 3.3a)
        //   * relative   max_j |gⱼ|/(‖J·,ⱼ‖·‖r‖) ≤ tol_grad_rel  (MINPACK gtol)
        // The relative measure is the cosine of the angle between r and
        // each Jacobian column. Squaring avoids a sqrt: it's
        // `max_j gⱼ²/diag(JᵀJ)ⱼ ≤ tol_grad_rel²·‖r‖²`. A zero column has
        // `diag(JᵀJ)ⱼ = 0` and `gⱼ = 0`; flooring the denominator to 1
        // makes that term `0/1 = 0` rather than `0/0 = NaN`, which is
        // MINPACK's "skip zero columns" behavior.
        let abs_converged = self.tol_grad > 0.0 && g.norm_infinity() <= self.tol_grad;
        let rel_converged = self.tol_grad_rel > 0.0 && {
            let mut cos_sq = g.clone();
            cos_sq.component_mul_assign(&g);
            let mut denom = diag_cur.clone();
            denom.floor_zeros_in_place(1.0);
            cos_sq.component_div_assign(&denom);
            cos_sq.norm_infinity() <= self.tol_grad_rel * self.tol_grad_rel * r.norm_squared()
        };
        if abs_converged || rel_converged {
            // Restore the caches so a subsequent `run()` (e.g. via
            // `InnerExecutor`) doesn't see corrupted state — though
            // in practice `init` resets them on each reuse. The iterate
            // didn't move, so A and g are still valid.
            self.r_cache = Some(r);
            self.gram_cache = Some(a);
            self.jtr_cache = Some(g);
            return (state, Some(TerminationReason::SolverConverged));
        }

        let mut neg_g = g.clone();
        neg_g.neg_in_place();

        // Marquardt scaling: maintain `D` as the monotone running max of
        // `diag(JᵀJ)` (Moré 1978). `D` was floored away from zero at
        // `init`, and the max only grows entries, so it stays strictly
        // positive — the damped Gram below is SPD by construction.
        let mut d = self
            .diag
            .take()
            .expect("diag not set: Solver::init must run before next_iter");
        d.component_max_assign(&diag_cur);

        let mut mu = self
            .mu
            .expect("mu not set: Solver::init must run before next_iter");
        let mut nu = self.nu;

        // Inner damping loop: bump μ on Cholesky failure. In practice
        // the first attempt succeeds — a properly damped (JᵀJ + μ·D) is
        // SPD by construction. The retry path matters only for
        // pathological cases where the initial μ is too small to
        // overcome arithmetic roundoff.
        let h;
        let mut attempts: u32 = 0;
        loop {
            let mut a_damped = a.clone();
            // damping = μ·D, added to the Gram diagonal.
            let mut damping = d.clone();
            damping.scale_in_place(mu);
            a_damped.add_diagonal_vector_in_place(&damping);
            match a_damped.solve_spd(&neg_g) {
                Ok(step) => {
                    h = step;
                    break;
                }
                Err(_) => {
                    attempts += 1;
                    if attempts >= self.max_inner_attempts || !mu.is_finite() {
                        self.mu = Some(mu);
                        self.nu = nu;
                        self.diag = Some(d);
                        // State unchanged; r, A and g are all still valid
                        // at the current iterate.
                        self.r_cache = Some(r);
                        self.gram_cache = Some(a);
                        self.jtr_cache = Some(g);
                        return (state, Some(TerminationReason::SolverFailed));
                    }
                    mu *= nu;
                    nu *= 2.0;
                }
            }
        }

        // L(0) − L(h) = ½ hᵀ(μ·D·h − g) = ½(μ·hᵀD h − hᵀg) (Nielsen eq.
        // 2.3, with the scaling diagonal D folded into the quadratic
        // term — `μI` is the D = I special case). Both terms make the
        // predicted reduction positive: μ·hᵀD h > 0 since D > 0, and
        // −hᵀg > 0 since h is a descent direction. Form hᵀD h as
        // h·(D ⊙ h) to avoid materializing μ·D·h − g.
        let mut dh = d.clone();
        dh.component_mul_assign(&h);
        let l_diff = 0.5 * (mu * h.dot(&dh) - h.dot(&g));

        // Trial step.
        let mut x_trial = state.param.clone();
        x_trial.scaled_add(1.0, &h);
        let r_trial = problem.residual(&x_trial);
        state.cost_evals += 1;
        let f_trial = 0.5 * r_trial.norm_squared();

        let prev_cost = state
            .cost
            .expect("cost not set: Solver::init must run before next_iter");
        let actual_diff = prev_cost - f_trial;
        let rho = if l_diff > 0.0 {
            actual_diff / l_diff
        } else {
            0.0
        };

        if rho > 0.0 {
            // Accept. Update x and cost; adapt μ via Nielsen eq. 2.5
            // with β=2, γ=3, p=3. The trial residual is at the new
            // iterate — stash it; the iterate moved, so the Gram and
            // gradient are stale — clear them so the next iteration
            // re-evaluates J(x_trial) and rebuilds A and g.
            state.param = x_trial;
            state.cost = Some(f_trial);
            let factor = 1.0 - (2.0 * rho - 1.0).powi(3);
            mu *= factor.max(1.0 / 3.0);
            nu = 2.0;
            self.r_cache = Some(r_trial);
            self.gram_cache = None;
            self.jtr_cache = None;
        } else {
            // Reject. Keep state; bump μ geometrically and double ν so
            // consecutive failures escalate damping faster. The iterate
            // held still, so r, A and g all remain valid — cache A and g
            // so the next attempt re-solves with a new μ instead of
            // reforming the Gram (Madsen-Nielsen Alg. 3.16; issue #10).
            mu *= nu;
            nu *= 2.0;
            self.r_cache = Some(r);
            self.gram_cache = Some(a);
            self.jtr_cache = Some(g);
        }

        self.mu = Some(mu);
        self.nu = nu;
        // `d` is unchanged by accept/reject (the running max happens
        // once, above) — persist it for the next iteration.
        self.diag = Some(d);

        // MINPACK ftol / xtol convergence (Moré 1978), checked after the
        // accept/reject decision so a converging-but-productive final step
        // is committed to `state` before we stop. Both default to 0.0
        // (disabled); converge on *either*, matching MINPACK's
        // independent `info` codes.
        //
        //   * ftol  |actred| ≤ ftol·F  AND  prered ≤ ftol·F  AND  ρ ≤ 2.
        //     Neither the achieved nor the *predicted* reduction is
        //     meaningful. The `prered ≤ ftol·F` clause is load-bearing: it
        //     separates a true plateau from a temporary settling point
        //     where one step's actual gain is small but the model still
        //     predicts progress — there `prered` is large, so we keep
        //     iterating. `|actred|` (not `actred`) mirrors MINPACK's
        //     `dabs(actred)`: a step that *raised* the cost only counts as
        //     converged if the increase is itself below tolerance, so a
        //     large-jump rejected step keeps the solver going.
        //   * xtol  ‖h‖ ≤ xtol·‖x‖ — the step is negligible relative to
        //     the iterate. Squared on both sides to avoid a sqrt.
        let ftol_converged = self.ftol > 0.0
            && actual_diff.abs() <= self.ftol * prev_cost
            && l_diff <= self.ftol * prev_cost
            && rho <= 2.0;
        let xtol_converged = self.xtol > 0.0
            && h.norm_squared() <= self.xtol * self.xtol * state.param.norm_squared();
        if ftol_converged || xtol_converged {
            return (state, Some(TerminationReason::SolverConverged));
        }

        (state, None)
    }
}