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//! Step-size controllers for [`crate::AdaptivePI`] (Wave 4, ADR-0044).
//!
//! ## Controllers
//!
//! - [`ClassicalPI<F>`] — Söderlind 2002 "PI.4.7" law. **NORMATIVE default** per
//! math.md §11.1.bis. Bit-identical accepted-τ trajectory to v1.0.0.
//! - [`H211bFilter<F>`] — Söderlind 2003 digital low-pass filter (ADVISORY,
//! opt-in only via `.with_controller`). NOT in math.md.
//!
//! Both are zero-alloc (`core` + `libm` only) and safe (`deny(unsafe_code)`).
use crate::float::SemiflowFloat;
// ---------------------------------------------------------------------------
// Trait
// ---------------------------------------------------------------------------
/// Step-size law for [`crate::AdaptivePI`].
///
/// The controller owns its own mutable state (e.g. previous error norm) and
/// returns the next-τ *multiplier* per call. Clamping to `[min_ratio,
/// max_ratio]` is performed by [`crate::AdaptivePI`], not here.
///
/// `propose_accept` and `propose_reject` are separate so that digital-filter
/// controllers update their history only on accepted steps (Söderlind 2003 §3).
pub trait StepController<F: SemiflowFloat> {
/// Called after a substep is **accepted**.
///
/// Returns the multiplier `r` such that `next_τ = clamp(prev_τ · r)`.
/// The controller MUST update its internal state on this call.
///
/// # Arguments
/// - `err_norm` — Richardson error norm of the accepted step.
/// - `tol` — mixed abs/rel tolerance at the current state.
/// - `safety` — safety factor from `AdaptivePI` (e.g. `0.9`).
/// - `p_order` — consistency order of the inner Chernoff function.
fn propose_accept(&mut self, err_norm: F, tol: F, safety: F, p_order: u32) -> F;
/// Called after a substep is **rejected**.
///
/// Returns the shrink multiplier. Controllers MAY or MAY NOT update their
/// internal state on reject — `ClassicalPI` and `H211bFilter` do NOT.
fn propose_reject(&mut self, err_norm: F, tol: F, safety: F, p_order: u32) -> F;
/// Reset internal state to initial conditions.
///
/// Called at the top of `evolve_adaptive` so consecutive evolutions on the
/// same integrator are independent. Must be idempotent.
fn reset(&mut self);
}
// ---------------------------------------------------------------------------
// ClassicalPI<F>
// ---------------------------------------------------------------------------
/// Söderlind 2002 "PI.4.7" step-size law (NORMATIVE default).
///
/// Gains: `α = 0.7/p`, `β = 0.4/p` (math.md §11.1.bis). Bit-identical to the
/// v1.0.0 inlined `pi_step_factor` / `reject_step_factor` helpers (ADR-0044).
///
/// The FP association `safety * e * e_prev` (left-to-right) is NORMATIVE and
/// enforced by `tests/adaptive_classical_bit_equal.rs`.
#[derive(Clone, Debug)]
pub struct ClassicalPI<F: SemiflowFloat> {
/// Söderlind PI.4.7 P-gain exponent `α = 0.7 / p`.
pub alpha: F,
/// Söderlind PI.4.7 I-gain exponent `β = 0.4 / p`.
pub beta: F,
/// Previous-step error norm (I-term memory). Seeded to `F::one()`.
err_prev: F,
}
impl<F: SemiflowFloat> ClassicalPI<F> {
/// Construct with §11.1.bis gains for the given inner order `p`.
#[must_use]
pub fn with_order(p: u32) -> Self {
let pf = F::from(f64::from(p)).unwrap_or(F::one());
Self {
alpha: F::from(0.7).unwrap_or(F::one()) / pf,
beta: F::from(0.4).unwrap_or(F::one()) / pf,
err_prev: F::one(),
}
}
/// Direct constructor for tests and advanced users.
#[must_use]
pub fn new(alpha: F, beta: F) -> Self {
Self {
alpha,
beta,
err_prev: F::one(),
}
}
/// Return the current alpha (P-gain exponent).
#[inline]
pub fn alpha(&self) -> F {
self.alpha
}
/// Return the current beta (I-gain exponent).
#[inline]
pub fn beta(&self) -> F {
self.beta
}
}
impl<F: SemiflowFloat> Default for ClassicalPI<F> {
/// Defaults assume `p = 2` (matches `DiffusionChernoff::order()` post-D1).
fn default() -> Self {
Self::with_order(2)
}
}
impl<F: SemiflowFloat> StepController<F> for ClassicalPI<F> {
fn propose_accept(&mut self, err_norm: F, tol: F, safety: F, _p: u32) -> F {
// NORMATIVE FP association: `(safety * e) * e_prev` — left-to-right.
// Changing this order breaks bit-equality with v1.0.0 (ADR-0044 §Risk 1).
let floor = F::from(1e-300).unwrap_or(F::min_positive_value());
let safe_err = err_norm.max(floor);
let e = (tol / safe_err).libm_powf(self.alpha);
let e_prev = (self.err_prev / safe_err).libm_powf(self.beta);
let factor = safety * e * e_prev; // NORMATIVE: left-to-right
self.err_prev = err_norm; // I-term update on accept
factor
}
fn propose_reject(&mut self, err_norm: F, tol: F, safety: F, _p: u32) -> F {
// I-term only; err_prev NOT updated on reject (matches v1.0.0).
let floor = F::from(1e-300).unwrap_or(F::min_positive_value());
let safe_err = err_norm.max(floor);
safety * (tol / safe_err).libm_powf(self.alpha)
}
fn reset(&mut self) {
self.err_prev = F::one();
}
}
// ---------------------------------------------------------------------------
// H211bFilter<F>
// ---------------------------------------------------------------------------
/// Söderlind 2003 H211b digital low-pass filter (ADVISORY, opt-in only).
///
/// Reduces step-size variance on stiff/oscillatory error norms at negligible
/// accuracy cost. **NOT NORMATIVE** — documented at ADR-0044 + contract scope
/// only. NOT in math.md §11. Use only via:
/// `AdaptivePI::new(func).with_controller(H211bFilter::default())`.
///
/// Parameters: `b = 4`, `c = 1` (H211b convention, Söderlind 2003 Table 1).
/// Per-step exponent: `1/(b·p) = 1/(4p)`. Multiplier feedback: `−c/b = −1/4`.
#[derive(Clone, Debug)]
pub struct H211bFilter<F: SemiflowFloat> {
/// Previous error norm `err_{n-1}`. Seeded to `F::one()`.
err_prev: F,
/// Previous accepted multiplier `r_{n-1}`. Seeded to `F::one()`.
r_prev: F,
}
impl<F: SemiflowFloat> H211bFilter<F> {
/// Construct with neutral initial state (all seeds = 1).
#[must_use]
pub fn new() -> Self {
Self {
err_prev: F::one(),
r_prev: F::one(),
}
}
}
impl<F: SemiflowFloat> Default for H211bFilter<F> {
fn default() -> Self {
Self::new()
}
}
impl<F: SemiflowFloat> StepController<F> for H211bFilter<F> {
fn propose_accept(&mut self, err_norm: F, tol: F, safety: F, p_order: u32) -> F {
// H211b: ρ_n = (tol/e_n)^{1/(4p)} × (tol/e_{n-1})^{1/(4p)} × r_{n-1}^{-1/4}
// r_n = safety · ρ_n
let p = F::from(f64::from(p_order)).unwrap_or(F::one());
let b4p = F::from(4.0).unwrap_or(F::one()) * p; // b·p with b=4
let exp_e = F::one() / b4p; // 1/(4p)
let exp_r = F::from(-0.25).unwrap_or(-F::one() / F::from(4.0).unwrap_or(F::one()));
let floor = F::from(1e-300).unwrap_or(F::min_positive_value());
let safe_e = err_norm.max(floor);
let safe_ep = self.err_prev.max(floor);
let term_e = (tol / safe_e).libm_powf(exp_e);
let term_ep = (tol / safe_ep).libm_powf(exp_e);
let term_r = self.r_prev.libm_powf(exp_r);
let factor = safety * term_e * term_ep * term_r; // left-to-right
// Update state on accept only:
self.err_prev = err_norm;
self.r_prev = factor;
factor
}
fn propose_reject(&mut self, err_norm: F, tol: F, safety: F, p_order: u32) -> F {
// On reject: fall back to I-term shrink (classical-style).
// H211b literature is silent on the reject branch; I-term keeps shrink predictable.
// State (err_prev, r_prev) NOT updated on reject.
let p = F::from(f64::from(p_order)).unwrap_or(F::one());
let alpha = F::from(0.7).unwrap_or(F::one()) / p;
let floor = F::from(1e-300).unwrap_or(F::min_positive_value());
let safe_e = err_norm.max(floor);
safety * (tol / safe_e).libm_powf(alpha)
}
fn reset(&mut self) {
self.err_prev = F::one();
self.r_prev = F::one();
}
}