sva-engine 0.8.1

// Concern: where each node can be nonzero, exactly, in whole samples of its own clock | Non-concern: where a reader asks for it, where a sound ends | IO: (NodeId) -> Extent, a state's start

use std::cell::{Cell, RefCell};
use std::collections::{BTreeSet, HashMap};

use sva_formula::{Body, C64, Fold, NodeId, Unary, exp_zero_at};
use sva_samples::{Extent, Grid, Round};

use crate::cast::Cast;
use crate::refs::{PerNode, written_form};
use crate::schedule;
use crate::time::{Affine, Lattice, Q};
use crate::typing::{Step, SumSlot, Typing, Value, When};

/// Where each node can be nonzero, in samples of its own grid: outside its support a node is
/// zero. A node is its own clock: a read's shift moves it by whole samples.
pub(crate) struct Supports<'a> {
    tys: &'a Typing,
    base: Option<&'a Memo>,
    held: RefCell<HashMap<NodeId, Extent>>,
    open: RefCell<BTreeSet<NodeId>>,
    reaches: RefCell<HashMap<Reach, Option<f64>>>,
    steady: PerNode<bool>,
    priming: Cell<bool>,
    /// While a node is probed, each support it asked unfound.
    probing: RefCell<Option<Vec<NodeId>>>,
}

/// A node over an interval, or every one inside `STEADY`.
type Reach = (NodeId, Option<(u64, u64)>);

/// Past any grid's instants.
const STEADY: f64 = 1e20;

const SHIFTED: f64 = 1e80;

/// Past `STEADY` plus `1e19` refs' shifts of `SHIFTED`.
const REACHED: f64 = 1e100;

/// Each support found.
#[derive(Default)]
pub(crate) struct Memo(HashMap<NodeId, Extent>);

impl Memo {
    pub(crate) fn extend(&mut self, more: Memo) {
        self.0.extend(more.0);
    }

    /// Each support found of `freed`, nodes let go, forgotten.
    pub(crate) fn forget(&mut self, freed: &[NodeId]) {
        for id in freed {
            self.0.remove(id);
        }
    }
}

impl<'a> Supports<'a> {
    pub(crate) fn new(tys: &'a Typing) -> Supports<'a> {
        Supports::over(tys, None)
    }

    pub(crate) fn over(tys: &'a Typing, base: Option<&'a Memo>) -> Self {
        Supports {
            tys,
            base,
            held: RefCell::default(),
            open: RefCell::default(),
            reaches: RefCell::default(),
            steady: PerNode::new(),
            priming: Cell::new(false),
            probing: RefCell::default(),
        }
    }

    pub(crate) fn into_memo(self) -> Memo {
        Memo(self.held.into_inner())
    }

    fn found(&self, id: NodeId) -> Option<Extent> {
        let held = self.held.borrow().get(&id).copied();
        held.or_else(|| self.base?.0.get(&id).copied())
    }

    fn grid(&self, id: NodeId) -> Grid {
        self.tys.grid(id)
    }

    pub(crate) fn of(&self, id: NodeId) -> Extent {
        if let Some(missing) = self.probing.borrow_mut().as_mut() {
            return match self.found(id) {
                Some(support) => support,
                None => {
                    missing.push(id);
                    Extent::EVERYWHERE
                }
            };
        }
        if let Some(support) = self.found(id) {
            return support;
        }
        if !self.priming.replace(true) {
            self.prime(id);
            self.priming.set(false);
        }
        self.fresh_of(id)
    }

    /// Finds what finding `root` asks for, readers after what they read, so it recurses one
    /// ref deep.
    fn prime(&self, root: NodeId) {
        let mut seen = BTreeSet::new();
        let mut open = vec![(root, false)];
        while let Some((id, asked)) = open.pop() {
            if self.found(id).is_some() || self.open.borrow().contains(&id) {
                continue;
            }
            match asked {
                true if id != root => {
                    self.fresh_of(id);
                }
                true => {}
                false if seen.insert(id) => {
                    open.push((id, true));
                    open.extend(self.asks(id).into_iter().rev().map(|n| (n, false)));
                }
                false => {}
            }
        }
    }

    /// Probed: nothing is found.
    fn asks(&self, id: NodeId) -> Vec<NodeId> {
        *self.probing.borrow_mut() = Some(Vec::new());
        self.exact(id);
        self.probing.borrow_mut().take().unwrap_or_default()
    }

    fn exact(&self, id: NodeId) -> Extent {
        match self.looping(id) {
            true => self.looped(id),
            false => self.fresh(id),
        }
    }

    fn looping(&self, id: NodeId) -> bool {
        schedule::holds_self(self.tys, id, &mut BTreeSet::new())
    }

    fn fresh_of(&self, id: NodeId) -> Extent {
        if !self.open.borrow_mut().insert(id) {
            return Extent::EVERYWHERE;
        }
        let support = self.retired(id).fold(self.exact(id), Extent::hull);
        self.open.borrow_mut().remove(&id);
        self.held.borrow_mut().insert(id, support);
        support
    }

    /// Where a stream's note sum was nonzero through the terms it retired: a reader carrying
    /// state from their past rings on as though they still sounded.
    fn retired(&self, id: NodeId) -> impl Iterator<Item = Extent> {
        let slots = self.tys.sum_slots(id).unwrap_or_default().iter();
        slots.filter_map(|slot| match slot {
            SumSlot::Retired(support) => Some(*support),
            SumSlot::Node(_) => None,
        })
    }

    /// A closed form a node wrote inside its own body, as a value of its own on `grid`.
    pub(crate) fn formula(&self, body: &Body, grid: Grid) -> Extent {
        self.body(body, grid)
    }

    fn fresh(&self, id: NodeId) -> Extent {
        let grid = self.grid(id);
        match self.tys.value(id) {
            Value::ClosedForm(form) => match form.var {
                sva_formula::Var::T => self.body(&form.body, grid),
                sva_formula::Var::F => self.spectrum(&form.body),
            },
            Value::Cast(Cast::Fourier | Cast::IFourier, _) => Extent::EVERYWHERE,
            Value::Cast(_, source) => self.of(*source),
            Value::Op { name, args } => self.operation(name, args, grid, &|arg| self.of(arg)),
            Value::SelfAt { .. } => Extent::NOWHERE,
            Value::Noise(_) => Extent::EVERYWHERE,
            Value::Read {
                source,
                at: When::Step(step),
                ..
            } => match reach(self.tys, step, grid) {
                Some(reach) => reached(self.of(*source), reach),
                None => Extent::EVERYWHERE,
            },
            Value::Read { source, at, .. } => match landed(at, grid) {
                Some(map) => map.preimage(self.of(*source)),
                None => Extent::EVERYWHERE,
            },
            Value::Filter { x, .. } => stateful(self.of(*x)),
            Value::Solver { .. } => Extent::from(0),
        }
    }

    /// A loop starts where its input does and rings on, as far as a crop around it allows.
    fn looped(&self, id: NodeId) -> Extent {
        let rings = stateful(self.with_past(id, Extent::NOWHERE));
        match rings.is_empty() {
            true => Extent::NOWHERE,
            false => self.with_past(id, rings).intersect(rings),
        }
    }

    /// The loop `id` with its own past nonzero over `past`.
    fn with_past(&self, id: NodeId, past: Extent) -> Extent {
        match self.tys.value(id) {
            Value::SelfAt { .. } => past,
            Value::Op { name, args } => {
                self.operation(
                    name,
                    args,
                    self.grid(id),
                    &|arg| match schedule::holds_self(self.tys, arg, &mut BTreeSet::new()) {
                        true => self.with_past(arg, past),
                        false => self.of(arg),
                    },
                )
            }
            Value::Filter { x, .. } => stateful(self.with_past(*x, past)),
            _ => self.of(id),
        }
    }

    fn operation(
        &self,
        name: &str,
        args: &[NodeId],
        grid: Grid,
        of: &dyn Fn(NodeId) -> Extent,
    ) -> Extent {
        let number = |at: usize| {
            args.get(at)
                .and_then(|a| crate::lower::number_of(self.tys, *a))
        };
        match name {
            "+" | "-" | "join" => args
                .iter()
                .fold(Extent::NOWHERE, |held, a| held.hull(of(*a))),
            "*" => args
                .iter()
                .fold(Extent::EVERYWHERE, |held, a| held.intersect(of(*a))),
            "/" if number(1).is_some_and(|d| d != 0.0) => of(args[0]),
            "pow" if number(1).is_some_and(|n| n > 0.0) => of(args[0]),
            "crop" => match (number(1), number(2)) {
                (Some(l), Some(r)) => of(args[0]).intersect(window(grid, l, r)),
                _ => of(args[0]),
            },
            "ch" => of(args[0]),
            other => match Unary::from_name(other) {
                Some(op) if keeps_zero(op) => of(args[0]),
                _ => Extent::EVERYWHERE,
            },
        }
    }

    /// A form in `t`, where a factor's zero is a zero in time.
    fn body(&self, body: &Body, grid: Grid) -> Extent {
        let each = |parts: &[sva_formula::Part]| -> Vec<Extent> {
            parts.iter().map(|p| self.body(&p.body, grid)).collect()
        };
        let found = match body {
            Body::Const(c) if *c == C64::ZERO => Extent::NOWHERE,
            Body::Node(id) => self.of(*id),
            Body::Add(parts) | Body::Join(parts) => {
                each(parts).into_iter().fold(Extent::NOWHERE, Extent::hull)
            }
            Body::Mul(parts) => {
                let held = each(parts)
                    .into_iter()
                    .fold(Extent::EVERYWHERE, Extent::intersect);
                held.intersect(self.underflows(&factors(body), grid))
            }
            Body::Apply(Unary::Exp, _) => self.underflows(&[body], grid),
            Body::Div(num, den) if matches!(*den.body, Body::Const(c) if c != C64::ZERO) => {
                self.body(&num.body, grid)
            }
            Body::Pow(base, n) if *n > 0 => self.body(&base.body, grid),
            Body::Apply(op, arg) if keeps_zero(*op) => self.body(&arg.body, grid),
            Body::Shift { by, of } => match (&*of.body, placed(*by, grid)) {
                (Body::Node(id), Some(map)) => map.preimage(self.of(*id)),
                _ => moved(self.body(&of.body, grid), by * grid.sr()),
            },
            Body::Crop { of, l, r, .. } => {
                self.body(&of.body, grid)
                    .intersect(window(grid, l.value(), r.value()))
            }
            Body::Channel(of, _) => self.body(&of.body, grid),
            _ => Extent::EVERYWHERE,
        };
        found.intersect(saturated(body, grid))
    }

    /// Where a form in `f` is nonzero in `t`: a product there is a convolution here, a shift a
    /// modulation, a constant an impulse at 0; any other shape in `f` spreads everywhere.
    fn spectrum(&self, body: &Body) -> Extent {
        let of = |part: &sva_formula::Part| self.spectrum(&part.body);
        match body {
            Body::Const(c) if *c == C64::ZERO => Extent::NOWHERE,
            Body::Const(_) => Extent::new(0, 1),
            Body::Node(id) => self.of(*id),
            Body::Add(parts) | Body::Join(parts) => {
                parts.iter().map(of).fold(Extent::NOWHERE, Extent::hull)
            }
            Body::Mul(parts) => parts.iter().map(of).fold(Extent::new(0, 1), spread),
            Body::Div(num, den) if matches!(*den.body, Body::Const(c) if c != C64::ZERO) => of(num),
            Body::Pow(base, n) if *n > 0 => spread_times(of(base), i64::from(*n)),
            Body::Shift { of: inner, .. } | Body::Channel(inner, _) => of(inner),
            _ => Extent::EVERYWHERE,
        }
    }

    /// Where the node `owner`'s machine run must start so that every state it holds starts
    /// where its input does: a filter's input, a solver at t = 0, a loop at its own support.
    pub(crate) fn state_start(&self, id: NodeId, owner: NodeId) -> Option<i64> {
        let earliest = |a: Option<i64>, b: Option<i64>| match (a, b) {
            (Some(a), Some(b)) => Some(a.min(b)),
            (a, b) => a.or(b),
        };
        match self.tys.value(id) {
            Value::Filter {
                x, cutoff, q, gain, ..
            } => [*x, *cutoff, *q, *gain]
                .into_iter()
                .fold(starting(self.of(id)), |held, arg| {
                    earliest(held, self.state_start(arg, owner))
                }),
            Value::Solver { .. } => Some(0),
            Value::SelfAt { .. } => starting(self.of(owner)),
            Value::Op { args, .. } => args.iter().fold(None, |held, arg| {
                earliest(held, self.state_start(*arg, owner))
            }),
            _ => None,
        }
    }
}

impl Supports<'_> {
    /// Where a product of exponentials is exactly zero: their exponents add, until every
    /// factor's `exp` or the product of them underflows past the engine's own zero. Every
    /// other factor is bounded there, so zero times it stays zero; the margin covers each
    /// evaluator's own rounding of the exponent, the subnormals included.
    fn underflows(&self, factors: &[&Body], grid: Grid) -> Extent {
        let (mut exponent, mut sizes) = ([0.0f64; 3], [0.0f64; 3]);
        let (mut exps, mut others) = (Vec::new(), Vec::new());
        for factor in factors {
            match exponential(factor) {
                Some(p) => {
                    for k in 0..3 {
                        exponent[k] += p[k];
                        sizes[k] += p[k].abs();
                    }
                    exps.push(*factor);
                }
                None => others.push(*factor),
            }
        }
        let count = exps.len() as f64;
        if count == 0.0 || !exponent.iter().all(|c| c.is_finite()) {
            return Extent::EVERYWHERE;
        }
        let farthest = i64::MAX as f64 / grid.sr();
        let at = |t: f64| exponent[0] + exponent[1] * t + exponent[2] * t * t;
        let margin =
            |t: f64| 8.0 + 2.0 * count + 1e-9 * (sizes[0] + sizes[1] * t.abs() + sizes[2] * t * t);
        let side = |from: f64, to: f64| -> Option<f64> {
            let zero =
                |t: f64, bound: f64| at(t) + bound.max(1.0).ln() + margin(t) <= exp_zero_at();
            let mut edge = crossing(from, to, |t| zero(t, 1.0))?;
            for _ in 0..2 {
                let (lo, hi) = (edge.min(to), edge.max(to));
                let bound = others.iter().try_fold(1.0f64, |held, f| {
                    Some(held * self.bound(f, lo, hi)?.max(1.0))
                })?;
                edge = crossing(from, to, |t| zero(t, bound))?;
            }
            let (lo, hi) = (edge.min(to), edge.max(to));
            let every = exps.iter().chain(&others);
            let mut reach = every.map(|f| self.bound(f, lo, hi).map(|b| b.max(1.0)));
            let product = reach.try_fold(1.0f64, |held, b| Some(held * b?))?;
            (product < 1e300).then_some(edge)
        };
        let vertex = match exponent[2] {
            a if a < 0.0 => -exponent[1] / (2.0 * a),
            _ if exponent[1] < 0.0 => -farthest,
            _ => return Extent::EVERYWHERE,
        };
        let end = side(vertex, farthest).map_or(i64::MAX, |t| {
            (grid.count(t).ceil() as i64).saturating_add(1)
        });
        let start = match exponent[2] < 0.0 {
            true => side(vertex, -farthest).map_or(i64::MIN, |t| {
                (grid.count(t).floor() as i64).saturating_sub(1)
            }),
            false => i64::MIN,
        };
        match start < end {
            true => Extent::new(start, end),
            false => Extent::NOWHERE,
        }
    }

    /// The largest magnitude `body` reaches over `[lo, hi]` seconds, where one is known.
    fn bound(&self, body: &Body, lo: f64, hi: f64) -> Option<f64> {
        let of = |b: &Body| self.bound(b, lo, hi);
        let held = match body {
            Body::Const(c) => c.re.abs() + c.im.abs(),
            Body::Line => lo.abs().max(hi.abs()),
            Body::Add(parts) => parts.iter().try_fold(0.0, |h, p| Some(h + of(&p.body)?))?,
            Body::Join(parts) | Body::Fold(_, parts) => parts
                .iter()
                .try_fold(0.0f64, |h, p| Some(h.max(of(&p.body)?)))?,
            Body::Mul(parts) => parts
                .iter()
                .try_fold(1.0, |h, p| Some(h * of(&p.body)?.max(1.0)))?,
            Body::Div(num, den) => match &*den.body {
                Body::Const(c) if !c.is_zero() => of(&num.body)? / (c.re.abs() + c.im.abs()) * 2.0,
                _ => return None,
            },
            Body::Pow(base, n) if *n >= 0 => of(&base.body)?.max(1.0).powi(*n),
            Body::Apply(Unary::Sin | Unary::Cos, arg) => {
                let [c0, c1, c2] = real_polynomial(&arg.body)?;
                let t = lo.abs().max(hi.abs());
                let reach = c0.abs() + c1.abs() * t + c2.abs() * t * t;
                reach.is_finite().then_some(1.0)?
            }
            Body::Apply(Unary::Tanh | Unary::Sat | Unary::Step, arg) => {
                of(&arg.body).map(|_| 1.0)?
            }
            Body::Apply(Unary::Abs, arg) => of(&arg.body)?,
            Body::Apply(Unary::Exp, arg) => {
                let [c0, c1, c2] = real_polynomial(&arg.body)?;
                let at = |t: f64| c0 + c1 * t + c2 * t * t;
                let mut top = at(lo).max(at(hi));
                if c2 != 0.0 {
                    let vertex = -c1 / (2.0 * c2);
                    if lo < vertex && vertex < hi {
                        top = top.max(at(vertex));
                    }
                }
                (top + 1.0).exp()
            }
            Body::Crop { of: inner, .. } | Body::Channel(inner, _) => of(&inner.body)?,
            Body::Shift { by, of: inner } => self.bound(&inner.body, lo - by, hi - by)?,
            Body::Node(id) => {
                let steady = lo.abs().max(hi.abs()) <= STEADY && self.steady(*id);
                let key = (*id, (!steady).then(|| (lo.to_bits(), hi.to_bits())));
                let known = self.reaches.borrow().get(&key).copied();
                let found = known.unwrap_or_else(|| {
                    let found = match self.tys.value(*id) {
                        Value::ClosedForm(form)
                            if crate::refs::reads_through(self.tys, body, form.var) =>
                        {
                            self.bound(&form.body, lo, hi)
                        }
                        _ => None,
                    };
                    self.reaches.borrow_mut().insert(key, found);
                    found
                });
                found?
            }
            _ => return None,
        };
        (held < 1e300).then_some(held * 1.0001)
    }

    /// Whether a node bounds alike over every interval in `STEADY`.
    fn steady(&self, id: NodeId) -> bool {
        self.steady.of(id, || match self.tys.value(id) {
            Value::ClosedForm(form)
                if crate::refs::reads_through(self.tys, &Body::Node(id), form.var) =>
            {
                self.unmoved(&form.body)
            }
            _ => true,
        })
    }

    /// As `bound` reads it: an argument no polynomial reads bounds nothing.
    fn unmoved(&self, body: &Body) -> bool {
        match body {
            Body::Line => false,
            Body::Node(id) => self.steady(*id),
            Body::Apply(Unary::Sin | Unary::Cos, arg) => match real_polynomial(&arg.body) {
                None => true,
                Some([c0, c1, c2]) => {
                    (c0.abs() + c1.abs() * REACHED + c2.abs() * REACHED * REACHED).is_finite()
                }
            },
            Body::Apply(Unary::Exp, arg) => match real_polynomial(&arg.body) {
                None => true,
                Some([_, c1, c2]) => c1 == 0.0 && c2 == 0.0,
            },
            Body::Shift { by, of } => by.abs() <= SHIFTED && self.unmoved(&of.body),
            other => sva_formula::closed_form::children(other)
                .iter()
                .all(|p| self.unmoved(&p.body)),
        }
    }
}

fn factors(body: &Body) -> Vec<&Body> {
    match body {
        Body::Mul(parts) => parts.iter().flat_map(|p| factors(&p.body)).collect(),
        other => vec![other],
    }
}

/// `exp` of a polynomial in `t`, as its exponent's real coefficients.
fn exponential(body: &Body) -> Option<[f64; 3]> {
    match body {
        Body::Apply(Unary::Exp, arg) => {
            let [c0, c1, c2] = sva_formula::affine::polynomial(&arg.body)?
                .iter()
                .map(|c| c.exact())
                .collect::<Option<Vec<_>>>()?
                .into_iter()
                .chain(std::iter::repeat(C64::ZERO))
                .take(3)
                .map(|c| c.re)
                .collect::<Vec<_>>()[..]
            else {
                return None;
            };
            Some([c0, c1, c2])
        }
        _ => None,
    }
}

fn real_polynomial(body: &Body) -> Option<[f64; 3]> {
    let held = sva_formula::affine::polynomial(body)?;
    let mut out = [0.0; 3];
    for (slot, c) in out.iter_mut().zip(held) {
        let c = c.exact()?;
        if c.im != 0.0 || !c.re.is_finite() {
            return None;
        }
        *slot = c.re;
    }
    Some(out)
}

/// The first `t` from `from` towards `to` a predicate false then true holds at, to within
/// the doubles' own spacing.
fn crossing(from: f64, to: f64, holds: impl Fn(f64) -> bool) -> Option<f64> {
    if !holds(to) {
        return None;
    }
    let (mut no, mut yes) = (from, to);
    for _ in 0..200 {
        let mid = no + (yes - no) / 2.0;
        if mid == no || mid == yes {
            break;
        }
        match holds(mid) {
            true => yes = mid,
            false => no = mid,
        }
    }
    Some(yes)
}

/// A form reading `t` only through clamps of one monotone line each is constant on each side
/// where every clamp holds its bound; where that constant is exactly zero, so is the form,
/// from the first sample every clamp holds it.
fn saturated(body: &Body, grid: Grid) -> Extent {
    let mut chains = Vec::new();
    if !clamped(body, &mut chains) || chains.is_empty() {
        return Extent::EVERYWHERE;
    }
    let form = sva_samples::Evaluator::of(body, 0);
    let at = |n: i64| form.at(sva_samples::At::Sample(grid, n)).ok();
    let zero = |n: i64| at(n).is_some_and(|x| x.re == 0.0 && x.im == 0.0);
    let reach = 1i64 << 62;
    let side = |later: bool| -> Option<i64> {
        if !zero(if later { reach } else { -reach }) {
            return None;
        }
        let held = chains.iter().map(|c| c.held(grid, later));
        let held = held.collect::<Option<Vec<i64>>>()?.into_iter();
        let n = match later {
            true => held.max(),
            false => held.min(),
        };
        n.filter(|n| zero(*n))
    };
    let (end, start) = (side(true), side(false));
    let start = start.map_or(i64::MIN, |n| n.saturating_add(1));
    let end = end.unwrap_or(i64::MAX);
    match start < end {
        true => Extent::new(start, end),
        false => Extent::NOWHERE,
    }
}

/// A `min` or `max` of constants and one operand, itself a clamp or a line monotone in `t`.
struct Clamp<'b> {
    body: &'b Body,
    rising: bool,
}

/// `false` where `body` reads `t` other than through a clamp.
fn clamped<'b>(body: &'b Body, chains: &mut Vec<Clamp<'b>>) -> bool {
    match body {
        Body::Const(_) => true,
        Body::Fold(Fold::Min | Fold::Max, _) => match rising(body) {
            Some(rising) => {
                chains.push(Clamp { body, rising });
                true
            }
            None => false,
        },
        Body::Add(parts) | Body::Mul(parts) => parts.iter().all(|p| clamped(&p.body, chains)),
        Body::Div(num, den) => clamped(&num.body, chains) && clamped(&den.body, chains),
        Body::Pow(base, _) | Body::Apply(_, base) => clamped(&base.body, chains),
        _ => false,
    }
}

fn rising(body: &Body) -> Option<bool> {
    let (_, operand) = clamp_of(body)?;
    match operand {
        Body::Fold(Fold::Min | Fold::Max, _) => rising(operand),
        line => {
            let (slope, _) = sva_formula::affine::exact_affine(line)?;
            let moving = real_polynomial(line).is_some() && monotone(line, &mut false);
            (moving && slope.im == 0.0 && slope.re != 0.0).then_some(slope.re > 0.0)
        }
    }
}

fn clamp_of(body: &Body) -> Option<(Fold, &Body)> {
    let Body::Fold(fold @ (Fold::Min | Fold::Max), parts) = body else {
        return None;
    };
    let mut operands = parts
        .iter()
        .filter(|p| !matches!(*p.body, Body::Const(c) if c.im == 0.0));
    let operand = operands.next()?;
    operands.next().is_none().then_some((*fold, &*operand.body))
}

impl Clamp<'_> {
    /// The value it holds once its line passes every bound it meets going `up`.
    fn bound(body: &Body, up: bool) -> Option<f64> {
        let (fold, operand) = clamp_of(body)?;
        let Body::Fold(_, parts) = body else {
            return None;
        };
        let constants = parts.iter().filter_map(|p| match *p.body {
            Body::Const(c) if c.im == 0.0 => Some(c.re),
            _ => None,
        });
        let inner = match operand {
            Body::Fold(..) => Clamp::bound(operand, up),
            _ => None,
        };
        match (fold, inner, up) {
            (Fold::Min, None, true) => Some(constants.fold(f64::INFINITY, f64::min)),
            (Fold::Max, None, false) => Some(constants.fold(f64::NEG_INFINITY, f64::max)),
            (Fold::Min, Some(x), _) => Some(constants.fold(x, f64::min)),
            (Fold::Max, Some(x), _) => Some(constants.fold(x, f64::max)),
            _ => None,
        }
    }

    /// Towards `later` time, the first sample from which it holds its bound, or towards earlier
    /// time the last up to which it does.
    fn held(&self, grid: Grid, later: bool) -> Option<i64> {
        let bound = Clamp::bound(self.body, later == self.rising)?;
        let form = sva_samples::Evaluator::of(self.body, 0);
        let at = |n: i64| {
            let x = form.at(sva_samples::At::Sample(grid, n));
            x.is_ok_and(|x| x.im == 0.0 && x.re == bound)
        };
        let reach = 1i64 << 62;
        let (inside, outside) = match later {
            true => (reach, -reach),
            false => (-reach, reach),
        };
        if !at(inside) {
            return None;
        }
        if at(outside) {
            return Some(outside);
        }
        let (mut no, mut yes) = (outside, inside);
        while (i128::from(yes) - i128::from(no)).abs() > 1 {
            let mid = ((i128::from(no) + i128::from(yes)) / 2) as i64;
            match at(mid) {
                true => yes = mid,
                false => no = mid,
            }
        }
        let step = if later { 1 } else { -1 };
        (0..4)
            .map(|k| yes + k * step)
            .find(|n| self.firmly(grid, *n, later, bound))
    }

    /// Held at `n` with its line moved back towards the bound by sixteen times what any
    /// evaluator of it errs: `k` roundings over terms no larger than its absolute form `m`
    /// err at most `k*2^-52*m`, and an evaluator rounds at most its operations and eight more.
    fn firmly(&self, grid: Grid, n: i64, later: bool, bound: f64) -> bool {
        let mut line = self.body;
        while let Some((_, operand)) = clamp_of(line) {
            line = operand;
        }
        let t = grid.instant(n);
        let (Some((a, _)), Some((m, ops))) =
            (sva_formula::affine::exact_affine(line), absolute(line, t))
        else {
            return false;
        };
        let margin = f64::from(ops + 8) * 2f64.powi(-48) * m / a.re.abs();
        let moved = if later { t - margin } else { t + margin };
        let x = sva_samples::Evaluator::of(self.body, 0).at(sva_samples::At::Free(moved));
        x.is_ok_and(|x| x.im == 0.0 && x.re == bound) && margin.is_finite()
    }
}

/// A line's value with every constant and `t` at its magnitude, and the operations it takes.
fn absolute(body: &Body, t: f64) -> Option<(f64, u32)> {
    let each = |parts: &[sva_formula::Part]| {
        parts
            .iter()
            .try_fold((Vec::new(), 0u32), |(mut held, ops), p| {
                let (m, k) = absolute(&p.body, t)?;
                held.push(m);
                Some((held, ops + k + 1))
            })
    };
    Some(match body {
        Body::Const(c) => (c.abs(), 0),
        Body::Line => (t.abs(), 0),
        Body::Add(parts) => {
            let (held, ops) = each(parts)?;
            (held.iter().sum(), ops)
        }
        Body::Mul(parts) => {
            let (held, ops) = each(parts)?;
            (held.iter().product(), ops)
        }
        Body::Div(num, den) => {
            let (m, k) = absolute(&num.body, t)?;
            let Body::Const(c) = *den.body else {
                return None;
            };
            (m / c.abs(), k + 1)
        }
        Body::Shift { by, of } => {
            let (m, k) = absolute(&of.body, t.abs() + by.abs())?;
            (m, k + 1)
        }
        _ => return None,
    })
}

/// Built of steps each rising or falling with `t` alone, so its rounded value does too.
fn monotone(body: &Body, moving: &mut bool) -> bool {
    match body {
        Body::Const(_) => true,
        Body::Line => !std::mem::replace(moving, true),
        Body::Add(parts) | Body::Mul(parts) => parts.iter().all(|p| monotone(&p.body, moving)),
        Body::Div(num, den) => matches!(*den.body, Body::Const(_)) && monotone(&num.body, moving),
        Body::Shift { of, .. } => monotone(&of.body, moving),
        _ => false,
    }
}

fn starting(support: Extent) -> Option<i64> {
    (!support.is_empty()).then_some(support.start)
}

/// A stateful node starts where its input does, t = 0 where that has no start, and rings on.
fn stateful(input: Extent) -> Extent {
    match input.is_empty() {
        true => Extent::NOWHERE,
        false if input.start == i64::MIN => Extent::from(0),
        false => Extent::from(input.start),
    }
}

/// Where `n` convolutions of a value nonzero over `a` with itself can be.
fn spread_times(a: Extent, n: i64) -> Extent {
    if a.is_empty() {
        return Extent::NOWHERE;
    }
    let start = match a.start {
        i64::MIN => i64::MIN,
        x => x.saturating_mul(n),
    };
    let end = match a.end {
        i64::MAX => i64::MAX,
        x => (x - 1).saturating_mul(n).saturating_add(1),
    };
    Extent::new(start, end.max(start))
}

/// Where a convolution of values nonzero over `a` and `b` can be.
fn spread(a: Extent, b: Extent) -> Extent {
    if a.is_empty() || b.is_empty() {
        return Extent::NOWHERE;
    }
    let start = match (a.start, b.start) {
        (i64::MIN, _) | (_, i64::MIN) => i64::MIN,
        (x, y) => x.saturating_add(y),
    };
    let end = match (a.end, b.end) {
        (i64::MAX, _) | (_, i64::MAX) => i64::MAX,
        (x, y) => x.saturating_add(y - 1),
    };
    Extent::new(start, end.max(start))
}

/// `sin`, `tanh`, `abs`, `sqrt` and `sat` hold zero at zero; the rest move it.
fn keeps_zero(op: Unary) -> bool {
    matches!(
        op,
        Unary::Sin | Unary::Tanh | Unary::Abs | Unary::Sqrt | Unary::Sat
    )
}

/// The samples whose instants lie in a crop's `[l, r)`, by the grid's one edge rule.
pub(crate) fn window(grid: Grid, l: f64, r: f64) -> Extent {
    if l.is_nan() || r.is_nan() {
        return Extent::EVERYWHERE;
    }
    let (start, end) = (grid.first_at(l), grid.first_at(r));
    match start < end {
        true => Extent::new(start, end),
        false => Extent::NOWHERE,
    }
}

/// The whole-sample read a shift of a node is, `by` seconds rounded to the nearest sample of
/// `grid`, ties to even.
pub(crate) fn placed(by: f64, grid: Grid) -> Option<sva_samples::Map> {
    grid.snapped(shifted(by)?)
}

pub(crate) fn shifted(by: f64) -> Option<Affine> {
    Some(Affine {
        scale: Q::ONE,
        shift: Q::decimal(by)?.neg(),
    })
}

/// Which sample of its source each sample on `grid` reads: a time lands on the nearest sample
/// of the grid it steps its source on, ties to even; `None` where the instant moves.
pub(crate) fn landed(at: &When, grid: Grid) -> Option<sva_samples::Map> {
    match at {
        When::At(time) => grid.snapped(*time),
        When::Index(index) => index.map(grid),
        When::Moving(_) | When::Step(_) => None,
    }
}

/// Every offset from the sample being written that `step` lands at, where constants, maps
/// and clamps by constants bound it; the machine checks each read lands inside it.
pub(crate) fn reach(tys: &Typing, step: &Step, grid: Grid) -> Option<(i64, i64)> {
    match lines(tys, step, grid)? {
        (1, least, most) => Some((least, most)),
        _ => None,
    }
}

/// `(slope, least, most)`: `k - slope*n` lies in `[least, most]`.
fn lines(tys: &Typing, step: &Step, grid: Grid) -> Option<(i64, i64, i64)> {
    let each = |parts: &[Step]| {
        parts
            .iter()
            .map(|p| lines(tys, p, grid))
            .collect::<Option<Vec<_>>>()
    };
    match step {
        Step::Index(index) => {
            let map = index.map(grid)?;
            let slope = i64::try_from(map.a / map.d).ok()?;
            (map.a % map.d == 0).then(|| (slope, map.least(), map.lead()))
        }
        Step::Nearest(time, round) => {
            let (slope, lo, hi) = offset(tys, *time)?;
            let sr = grid.sr();
            let whole = |v: f64| (v.abs() < 2f64.powi(62)).then_some(v as i64);
            // A step each side holds the time's rounding; a non-positive offset reads no later step.
            let most = whole((hi * sr).ceil() + 1.0)?;
            let most = match slope == 1 && hi <= 0.0 && *round != Round::Ceil {
                true => most.min(0),
                false => most,
            };
            Some((slope, whole((lo * sr).floor() - 1.0)?, most))
        }
        Step::Add(parts) => {
            each(parts)?
                .into_iter()
                .try_fold((0i64, 0i64, 0i64), |(s, l, m), (s2, l2, m2)| {
                    Some((s.checked_add(s2)?, l.checked_add(l2)?, m.checked_add(m2)?))
                })
        }
        Step::Neg(part) => {
            let (s, l, m) = lines(tys, part, grid)?;
            Some((s.checked_neg()?, m.checked_neg()?, l.checked_neg()?))
        }
        Step::Mul(parts) => each(parts)?
            .into_iter()
            .try_fold((0i64, 1i64, 1i64), |held, next| {
                let (c, (s, l, m)) = match (held, next) {
                    ((0, c, d), other) | (other, (0, c, d)) if c == d => (c, other),
                    _ => return None,
                };
                let (a, b) = (l.checked_mul(c)?, m.checked_mul(c)?);
                Some((s.checked_mul(c)?, a.min(b), a.max(b)))
            }),
    }
}

/// How many `t` a time holds, none or one, and `[lo, hi]` holding the rest, as the machine sums it.
fn offset(tys: &Typing, time: NodeId) -> Option<(i64, f64, f64)> {
    let Value::ClosedForm(form) = tys.value(time) else {
        return None;
    };
    if !crate::refs::reads_through(tys, &form.body, form.var) {
        return None;
    }
    let offsets = Offsets {
        tys,
        addends: PerNode::new(),
        spans: PerNode::new(),
    };
    let mut held = Addends::default();
    offsets.addends(&form.body, &mut held);
    let Addends { lines, lo, hi } = held;
    let slope = i64::try_from(lines).ok().filter(|n| *n <= 1)?;
    (lo.is_finite() && hi.is_finite()).then_some((slope, lo, hi))
}

/// How many bare `t` a sum holds and the span of the rest, added from `+0` in written order.
#[derive(Clone, Copy, Default)]
struct Addends {
    lines: usize,
    lo: f64,
    hi: f64,
}

struct Offsets<'a> {
    tys: &'a Typing,
    addends: PerNode<Addends>,
    spans: PerNode<(f64, f64)>,
}

impl Offsets<'_> {
    fn addends(&self, body: &Body, held: &mut Addends) {
        match body {
            Body::Add(parts) => parts.iter().for_each(|p| self.addends(&p.body, held)),
            Body::Line => held.lines += 1,
            Body::Node(id) => {
                let node = self.addends.of(*id, || {
                    let mut found = Addends::default();
                    self.addends(written_form(self.tys, *id), &mut found);
                    found
                });
                held.lines += node.lines;
                (held.lo, held.hi) = (held.lo + node.lo, held.hi + node.hi);
            }
            other => {
                let (l, h) = self.span(other);
                (held.lo, held.hi) = (held.lo + l, held.hi + h);
            }
        }
    }

    fn span(&self, body: &Body) -> (f64, f64) {
        let each = |parts: &[sva_formula::Part]| {
            parts.iter().map(|p| self.span(&p.body)).collect::<Vec<_>>()
        };
        match body {
            Body::Node(id) => self
                .spans
                .of(*id, || self.span(written_form(self.tys, *id))),
            Body::Const(c) if c.im == 0.0 => (c.re, c.re),
            Body::Add(parts) => each(parts)
                .into_iter()
                .fold((0.0, 0.0), |(lo, hi), (l, h)| (lo + l, hi + h)),
            Body::Mul(parts) => {
                each(parts)
                    .into_iter()
                    .fold((1.0, 1.0), |(lo, hi), (l, h)| match (lo == hi, l == h) {
                        (true, _) => scale((l, h), lo),
                        (_, true) => scale((lo, hi), l),
                        _ => (f64::NEG_INFINITY, f64::INFINITY),
                    })
            }
            Body::Fold(Fold::Min, parts) => each(parts)
                .into_iter()
                .fold((f64::INFINITY, f64::INFINITY), |(lo, hi), (l, h)| {
                    (lo.min(l), hi.min(h))
                }),
            Body::Fold(Fold::Max, parts) => each(parts).into_iter().fold(
                (f64::NEG_INFINITY, f64::NEG_INFINITY),
                |(lo, hi), (l, h)| (lo.max(l), hi.max(h)),
            ),
            Body::Apply(Unary::Sin | Unary::Cos | Unary::Tanh | Unary::Sat, _) => (-1.0, 1.0),
            _ => (f64::NEG_INFINITY, f64::INFINITY),
        }
    }
}

fn scale((lo, hi): (f64, f64), c: f64) -> (f64, f64) {
    match c {
        0.0 => (0.0, 0.0),
        c if c > 0.0 => (lo * c, hi * c),
        c => (hi * c, lo * c),
    }
}

/// Every sample whose offsets in `reach` land inside `support`. A form on a moved grid is
/// read by rows of the form moved, whose edges round a step either way.
fn reached(support: Extent, (least, most): (i64, i64)) -> Extent {
    if support.is_empty() || support == Extent::EVERYWHERE {
        return support;
    }
    Extent::new(
        support.start.saturating_sub(most),
        support.end.saturating_sub(least),
    )
}

/// Moved by a count of samples that need not be whole, widened to the samples either side.
fn moved(support: Extent, count: f64) -> Extent {
    if support.is_empty() || support == Extent::EVERYWHERE {
        return support;
    }
    if count.fract() == 0.0 {
        return support.shifted(count as i64);
    }
    let (early, late) = (count.floor() as i64 - 1, count.ceil() as i64 + 1);
    let start = support.shifted(early).start;
    let end = support.shifted(late).end;
    Extent::new(start, end)
}

#[cfg(test)]
mod tests {
    use super::{spread, spread_times};
    use sva_samples::Extent;

    /// Values nonzero over samples 0..=9 and 5..=6 convolve to 5..=15; three copies of -3..=1
    /// to -9..=3; an endless side stays endless and a silent one silences the product.
    #[test]
    fn a_convolution_is_nonzero_over_the_sum_of_its_supports() {
        assert_eq!(
            spread(Extent::new(0, 10), Extent::new(5, 7)),
            Extent::new(5, 16)
        );
        assert_eq!(spread_times(Extent::new(-3, 2), 3), Extent::new(-9, 4));
        assert_eq!(
            spread_times(Extent::new(-3, 2), 3),
            spread(
                spread(Extent::new(-3, 2), Extent::new(-3, 2)),
                Extent::new(-3, 2)
            )
        );
        let endless = spread(Extent::new(0, 10), Extent::new(4, i64::MAX));
        assert_eq!(endless, Extent::new(4, i64::MAX));
        assert!(spread(Extent::NOWHERE, Extent::EVERYWHERE).is_empty());
    }
}