rlevo-evolution 0.3.1

//! Differential Evolution.
//!
//! Classical DE over `Tensor<B, 2>` populations with all common
//! mutation/crossover variants enumerated in [`DeVariant`].
//!
//! # Variants
//!
//! | Variant | Mutation formula |
//! |---|---|
//! | [`DeVariant::Rand1Bin`], [`DeVariant::Rand1Exp`] | `v = x_{r1} + F · (x_{r2} − x_{r3})` |
//! | [`DeVariant::Best1Bin`] | `v = x_{best} + F · (x_{r2} − x_{r3})` |
//! | [`DeVariant::CurrentToBest1Bin`] | `v = x_i + F · (x_{best} − x_i) + F · (x_{r1} − x_{r2})` |
//! | [`DeVariant::Rand2Bin`] | `v = x_{r1} + F · (x_{r2} − x_{r3}) + F · (x_{r4} − x_{r5})` |
//!
//! The suffix `Bin`/`Exp` selects between binomial and exponential
//! crossover. All index draws reject repeated and self-referential
//! indices.
//!
//! # Hot path
//!
//! A fused `CubeCL` kernel for trial-vector construction is tracked as
//! follow-up work (see [`crate::ops::kernels`]). Until then this module
//! uses host-sampled indices and composes the update from primitive
//! tensor ops.
//!
//! # Reference
//!
//! - Storn & Price (1997), *Differential Evolution — A Simple and
//!   Efficient Heuristic for Global Optimization over Continuous
//!   Spaces*.

use std::marker::PhantomData;

use burn::tensor::{Int, Tensor, TensorData, backend::Backend};
use rand::{Rng, RngExt};

use rlevo_core::bounds::Bounds;
use rlevo_core::config::{self, ConfigError, Validate};

use crate::ops::selection::argmax_host;
use crate::rng::{SeedPurpose, seed_stream};
use crate::strategy::{Strategy, StrategyMetrics};

/// Mutation + crossover variant for differential evolution.
///
/// # Convergence caveats
///
/// Not every variant converges to machine precision on every landscape
/// within the same budget. On unimodal landscapes like Sphere,
/// [`Best1Bin`](DeVariant::Best1Bin) and
/// [`CurrentToBest1Bin`](DeVariant::CurrentToBest1Bin) tend to
/// **converge prematurely**: the population collapses around the
/// current best before the differential search has fully explored, and
/// the per-generation variance `F · (x_{r2} − x_{r3})` shrinks to zero.
/// Classical DE literature documents this as the core trade-off of
/// best-biased variants. The crate's integration tests therefore only
/// require strong *reduction* from the random baseline for those
/// variants, not optimality — see
/// `algorithms::de::tests::all_variants_converge_on_sphere_d10` for the
/// per-variant tolerance choice.
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum DeVariant {
    /// `x_{r1} + F · (x_{r2} − x_{r3})`, binomial crossover. Balanced
    /// exploration / exploitation; reaches machine precision on Sphere
    /// within a few hundred generations.
    Rand1Bin,
    /// `x_{best} + F · (x_{r2} − x_{r3})`, binomial crossover.
    ///
    /// Strong exploitation — the mutation base is always the current
    /// best, so the population concentrates quickly. Prone to
    /// **premature convergence** on landscapes where the current best
    /// is far from the global optimum; on Sphere-D10 with 500 gens this
    /// variant stalls around `best_fitness ≈ 1` while `Rand1Bin` reaches
    /// `< 1e-20`.
    Best1Bin,
    /// `x_i + F · (x_{best} − x_i) + F · (x_{r1} − x_{r2})`, binomial.
    ///
    /// Hybrid of the current individual and the best-so-far. Still
    /// **prone to premature convergence** because the
    /// `F · (x_{best} − x_i)` term dominates once the population is
    /// near the best. Useful on multimodal landscapes where pure-best
    /// variants get stuck in local basins, less useful on Sphere.
    CurrentToBest1Bin,
    /// `x_{r1} + F · (x_{r2} − x_{r3}) + F · (x_{r4} − x_{r5})`,
    /// binomial. Higher variance than `Rand1Bin` thanks to two
    /// difference vectors; converges on Sphere but more slowly.
    Rand2Bin,
    /// `x_{r1} + F · (x_{r2} − x_{r3})`, exponential crossover.
    /// Identical mutation to `Rand1Bin`, different crossover mask shape.
    /// Performance comparable to `Rand1Bin` in practice.
    Rand1Exp,
}

impl DeVariant {
    /// Number of distinct random indices the variant needs (in
    /// addition to the current individual `i`).
    const fn random_indices(self) -> usize {
        match self {
            DeVariant::Rand1Bin | DeVariant::Rand1Exp => 3,
            DeVariant::Best1Bin | DeVariant::CurrentToBest1Bin => 2,
            DeVariant::Rand2Bin => 5,
        }
    }

    /// Whether this variant uses exponential crossover.
    const fn is_exponential(self) -> bool {
        matches!(self, DeVariant::Rand1Exp)
    }
}

/// Static configuration for a [`DifferentialEvolution`] run.
#[derive(Debug, Clone)]
pub struct DeConfig {
    /// Population size (≥ 5 for `Rand2Bin`, ≥ 4 otherwise).
    pub pop_size: usize,
    /// Genome dimensionality.
    pub genome_dim: usize,
    /// Search-space bounds (initialization and clamping).
    pub bounds: Bounds,
    /// Differential weight (F). Typical range [0.4, 0.9].
    pub f: f32,
    /// Crossover probability (CR). Typical range [0.1, 0.9].
    pub cr: f32,
    /// Variant.
    pub variant: DeVariant,
}

impl DeConfig {
    /// Default configuration (`Rand1Bin`, F = 0.5, CR = 0.9) for a given
    /// dimensionality.
    #[must_use]
    pub fn default_for(pop_size: usize, genome_dim: usize) -> Self {
        Self {
            pop_size,
            genome_dim,
            bounds: Bounds::new(-5.12, 5.12),
            f: 0.5,
            cr: 0.9,
            variant: DeVariant::Rand1Bin,
        }
    }
}

impl Validate for DeConfig {
    fn validate(&self) -> Result<(), ConfigError> {
        const C: &str = "DeConfig";
        let min_pop = if self.variant == DeVariant::Rand2Bin {
            5
        } else {
            4
        };
        config::at_least(C, "pop_size", self.pop_size, min_pop)?;
        config::nonzero(C, "genome_dim", self.genome_dim)?;
        config::in_range(C, "f", 0.0, 2.0, f64::from(self.f))?;
        config::in_range(C, "cr", 0.0, 1.0, f64::from(self.cr))?;
        Ok(())
    }
}

/// Generation state for [`DifferentialEvolution`].
///
/// The two-phase ask/tell handshake uses `fitness.is_empty()` as a
/// sentinel: on the very first [`Strategy::ask`] call the initial
/// population is returned unchanged; on the very first
/// [`Strategy::tell`] call `fitness` is populated and
/// `best_genome`/`best_fitness` are initialized. Subsequent
/// ask/tell cycles produce and evaluate trial vectors.
#[derive(Debug, Clone)]
pub struct DeState<B: Backend> {
    /// Current population, shape `(pop_size, D)`.
    pub population: Tensor<B, 2>,
    /// Host-side fitness cache for the current population.
    ///
    /// Empty before the first [`Strategy::tell`] call; length `pop_size`
    /// thereafter. The `is_empty()` check is the sentinel that
    /// distinguishes the initial evaluation phase from subsequent
    /// trial-vector generations.
    pub fitness: Vec<f32>,
    /// Index of the current best individual within `population`.
    pub best_index: usize,
    /// Best-so-far genome, shape `(1, D)`.
    ///
    /// `None` before the first [`Strategy::tell`] call.
    pub best_genome: Option<Tensor<B, 2>>,
    /// Best-so-far fitness across all completed generations.
    ///
    /// `f32::NEG_INFINITY` before the first [`Strategy::tell`] call (the
    /// worst value under the maximise convention).
    pub best_fitness: f32,
    /// Number of completed `tell` calls (zero-based generation index + 1).
    pub generation: usize,
}

/// Classical DE/rand/1/bin (and friends).
///
/// # Example
///
/// ```no_run
/// use burn::backend::Flex;
/// use rlevo_evolution::algorithms::de::{DeConfig, DeVariant, DifferentialEvolution};
///
/// let strategy = DifferentialEvolution::<Flex>::new();
/// let mut params = DeConfig::default_for(30, 10);
/// params.variant = DeVariant::Rand1Bin;
/// let _ = (strategy, params);
/// ```
#[derive(Debug, Clone, Copy, Default)]
pub struct DifferentialEvolution<B: Backend> {
    _backend: PhantomData<fn() -> B>,
}

impl<B: Backend> DifferentialEvolution<B> {
    /// Builds a new (stateless) strategy object.
    #[must_use]
    pub fn new() -> Self {
        Self {
            _backend: PhantomData,
        }
    }

    fn sample_initial_population(
        params: &DeConfig,
        rng: &mut dyn Rng,
        device: &<B as burn::tensor::backend::BackendTypes>::Device,
    ) -> Tensor<B, 2> {
        let (lo, hi): (f32, f32) = params.bounds.into();
        // Host-sample the initial population from a deterministic
        // `seed_stream` rather than the process-wide Flex RNG (`B::seed` +
        // `Tensor::random`), whose draws interleave with sibling tests under
        // the parallel runner and are not reproducible across schedules.
        let pop = params.pop_size;
        let genome_dim = params.genome_dim;
        let mut stream = seed_stream(rng.next_u64(), 0, SeedPurpose::Init);
        let mut rows = Vec::with_capacity(pop * genome_dim);
        for _ in 0..pop * genome_dim {
            rows.push(lo + (hi - lo) * stream.random::<f32>());
        }
        Tensor::<B, 2>::from_data(TensorData::new(rows, [pop, genome_dim]), device)
    }

    /// Samples `k` indices from `0..pop_size`, all distinct and all
    /// different from `self_idx`.
    ///
    /// # Panics
    ///
    /// Panics if `pop_size <= k`, since the rejection loop cannot make
    /// progress without enough candidates outside `self_idx`.
    fn sample_distinct_excluding(
        self_idx: usize,
        pop_size: usize,
        k: usize,
        rng: &mut dyn Rng,
    ) -> Vec<usize> {
        assert!(
            pop_size > k,
            "DE: pop_size must exceed the number of distinct indices required"
        );
        let mut chosen = Vec::with_capacity(k);
        while chosen.len() < k {
            let candidate = rng.random_range(0..pop_size);
            if candidate != self_idx && !chosen.contains(&candidate) {
                chosen.push(candidate);
            }
        }
        chosen
    }
}

impl<B: Backend> Strategy<B> for DifferentialEvolution<B>
where
    B::Device: Clone,
{
    type Params = DeConfig;
    type State = DeState<B>;
    type Genome = Tensor<B, 2>;

    /// Samples the initial population uniformly within `params.bounds`
    /// and returns a [`DeState`] with an empty fitness cache, signalling
    /// that the first ask/tell cycle should evaluate the initial
    /// population rather than generate trial vectors.
    ///
    /// Initial sampling goes through [`seed_stream`] rather than
    /// `B::seed + Tensor::random` to keep results reproducible across
    /// parallel test threads.
    fn init(
        &self,
        params: &DeConfig,
        rng: &mut dyn Rng,
        device: &<B as burn::tensor::backend::BackendTypes>::Device,
    ) -> DeState<B> {
        debug_assert!(
            params.validate().is_ok(),
            "invalid DeConfig reached init: {params:?}"
        );
        let population = Self::sample_initial_population(params, rng, device);
        DeState {
            population,
            fitness: Vec::new(),
            best_index: 0,
            best_genome: None,
            best_fitness: f32::NEG_INFINITY,
            generation: 0,
        }
    }

    /// Proposes the next population of candidate solutions.
    ///
    /// **First call (fitness cache empty):** returns the initial
    /// population from [`DeState::population`] unchanged so the caller
    /// can evaluate it before any mutation/crossover step.
    ///
    /// **Subsequent calls:** for each individual `i` in `0..pop_size`:
    ///
    /// 1. Sample the required number of distinct random indices
    ///    (excluding `i`) via [`seed_stream`] with [`SeedPurpose::Trial`].
    /// 2. Compute the mutant vector `v_i` according to
    ///    [`DeConfig::variant`].
    /// 3. Apply binomial or exponential crossover (also seeded through
    ///    [`seed_stream`] with [`SeedPurpose::Crossover`]) to blend `v_i`
    ///    with the current individual, ensuring at least one gene comes
    ///    from `v_i` (`j_rand` guarantee).
    /// 4. Clamp the trial genome to `params.bounds`.
    ///
    /// The returned state is a clone of the input state; no fitness
    /// update occurs here — that happens in [`Strategy::tell`].
    #[allow(clippy::too_many_lines, clippy::many_single_char_names)]
    fn ask(
        &self,
        params: &DeConfig,
        state: &DeState<B>,
        rng: &mut dyn Rng,
        device: &<B as burn::tensor::backend::BackendTypes>::Device,
    ) -> (Tensor<B, 2>, DeState<B>) {
        // First call: evaluate the initial population.
        if state.fitness.is_empty() {
            return (state.population.clone(), state.clone());
        }

        let DeConfig {
            pop_size,
            genome_dim,
            f,
            cr,
            variant,
            ..
        } = *params;

        let mut trial_rng =
            seed_stream(rng.next_u64(), state.generation as u64, SeedPurpose::Trial);

        // ------------------------------------------------------------------
        // 1. Build the mutant vector v_i for every i, host-side gathers.
        //    We assemble three index tensors (a, b, c [and d, e for rand2])
        //    and do the arithmetic on-device in one sweep.
        // ------------------------------------------------------------------
        let k = variant.random_indices();
        let mut rand_indices: Vec<Vec<usize>> =
            (0..k).map(|_| Vec::with_capacity(pop_size)).collect();
        for i in 0..pop_size {
            let chosen = Self::sample_distinct_excluding(i, pop_size, k, &mut trial_rng);
            for (j, idx) in chosen.into_iter().enumerate() {
                rand_indices[j].push(idx);
            }
        }

        let gather = |idxs: &[usize]| -> Tensor<B, 2> {
            #[allow(clippy::cast_possible_wrap)]
            let v: Vec<i64> = idxs.iter().map(|&i| i as i64).collect();
            let t = Tensor::<B, 1, Int>::from_data(TensorData::new(v, [pop_size]), device);
            state.population.clone().select(0, t)
        };

        let v = match variant {
            DeVariant::Rand1Bin | DeVariant::Rand1Exp => {
                let a = gather(&rand_indices[0]);
                let b = gather(&rand_indices[1]);
                let c = gather(&rand_indices[2]);
                a + (b - c).mul_scalar(f)
            }
            DeVariant::Best1Bin => {
                #[allow(clippy::single_range_in_vec_init)]
                let best = state
                    .population
                    .clone()
                    .slice([state.best_index..state.best_index + 1])
                    .expand([pop_size, genome_dim]);
                let b = gather(&rand_indices[0]);
                let c = gather(&rand_indices[1]);
                best + (b - c).mul_scalar(f)
            }
            DeVariant::CurrentToBest1Bin => {
                #[allow(clippy::single_range_in_vec_init)]
                let best = state
                    .population
                    .clone()
                    .slice([state.best_index..state.best_index + 1])
                    .expand([pop_size, genome_dim]);
                let current = state.population.clone();
                let a = gather(&rand_indices[0]);
                let b = gather(&rand_indices[1]);
                current.clone() + (best - current).mul_scalar(f) + (a - b).mul_scalar(f)
            }
            DeVariant::Rand2Bin => {
                let a = gather(&rand_indices[0]);
                let b = gather(&rand_indices[1]);
                let c = gather(&rand_indices[2]);
                let d = gather(&rand_indices[3]);
                let e = gather(&rand_indices[4]);
                a + (b - c).mul_scalar(f) + (d - e).mul_scalar(f)
            }
        };

        // ------------------------------------------------------------------
        // 2. Crossover: binomial or exponential. Always preserve at
        //    least one mutant gene per row (j_rand).
        // ------------------------------------------------------------------
        let mut cross_rng = seed_stream(
            rng.next_u64(),
            state.generation as u64,
            SeedPurpose::Crossover,
        );
        let mut cross_mask = vec![false; pop_size * genome_dim];
        if variant.is_exponential() {
            for row in 0..pop_size {
                let start = cross_rng.random_range(0..genome_dim);
                let mut len = 1;
                while len < genome_dim && cross_rng.random::<f32>() < cr {
                    len += 1;
                }
                for k in 0..len {
                    let j = (start + k) % genome_dim;
                    cross_mask[row * genome_dim + j] = true;
                }
            }
        } else {
            for row in 0..pop_size {
                let j_rand = cross_rng.random_range(0..genome_dim);
                for j in 0..genome_dim {
                    if j == j_rand || cross_rng.random::<f32>() < cr {
                        cross_mask[row * genome_dim + j] = true;
                    }
                }
            }
        }
        #[allow(clippy::cast_possible_wrap)]
        let mask_int: Vec<i64> = cross_mask.iter().map(|&b| i64::from(b)).collect();
        let mask_tensor = Tensor::<B, 2, Int>::from_data(
            TensorData::new(mask_int, [pop_size, genome_dim]),
            device,
        );
        let mask_bool = mask_tensor.equal_elem(1);

        // Where cross_mask == 1, take from v; otherwise from state.population.
        let trial = state.population.clone().mask_where(mask_bool, v);
        let (lo, hi): (f32, f32) = params.bounds.into();
        let trial = trial.clamp(lo, hi);

        (trial, state.clone())
    }

    /// Consumes the evaluated trial population and advances the state.
    ///
    /// **First call (fitness cache empty):** stores the initial
    /// population's fitness, initializes `best_genome`/`best_fitness`,
    /// and increments the generation counter. No replacement occurs
    /// because there are no previous individuals to compare against.
    ///
    /// **Subsequent calls:** applies greedy per-slot replacement — each
    /// trial individual replaces its corresponding current individual if
    /// and only if `trial_fitness[i] >= state.fitness[i]`. The best-ever
    /// genome and fitness are updated if the new generation improves on
    /// `state.best_fitness`.
    ///
    /// Returns the updated [`DeState`] and a [`StrategyMetrics`] snapshot
    /// covering the current generation's fitness distribution.
    fn tell(
        &self,
        _params: &DeConfig,
        trial: Tensor<B, 2>,
        fitness: Tensor<B, 1>,
        mut state: DeState<B>,
        _rng: &mut dyn Rng,
    ) -> (DeState<B>, StrategyMetrics) {
        let fitness_host = fitness
            .into_data()
            .into_vec::<f32>()
            .expect("fitness tensor must be readable as f32");

        // First `tell`: stash fitness for the initial population.
        if state.fitness.is_empty() {
            state.fitness.clone_from(&fitness_host);
            state.best_index = argmax_host(&fitness_host);
            state.generation += 1;
            update_best(&mut state, &trial, &fitness_host);
            let m = StrategyMetrics::from_host_fitness(
                state.generation,
                &fitness_host,
                state.best_fitness,
            );
            state.best_fitness = m.best_fitness_ever();
            state.population = trial;
            return (state, m);
        }

        // Greedy per-slot replacement: trial replaces current iff
        // trial is at least as good (canonical: fitness no lower).
        let device = trial.device();
        let pop_size = state.fitness.len();
        let mut replace_mask = vec![0i64; pop_size];
        let mut new_fit = state.fitness.clone();
        for i in 0..pop_size {
            if fitness_host[i] >= state.fitness[i] {
                replace_mask[i] = 1;
                new_fit[i] = fitness_host[i];
            }
        }

        let mask_int =
            Tensor::<B, 1, Int>::from_data(TensorData::new(replace_mask, [pop_size]), &device);
        let mask_bool_row = mask_int.equal_elem(1);
        let genome_dim = state.population.dims()[1];
        let mask_bool = mask_bool_row
            .unsqueeze_dim::<2>(1)
            .expand([pop_size, genome_dim]);
        let next_pop = state
            .population
            .clone()
            .mask_where(mask_bool, trial.clone());

        state.population = next_pop;
        state.fitness.clone_from(&new_fit);
        state.best_index = argmax_host(&new_fit);
        state.generation += 1;
        update_best(&mut state, &trial, &fitness_host);
        let m = StrategyMetrics::from_host_fitness(state.generation, &new_fit, state.best_fitness);
        state.best_fitness = m.best_fitness_ever();
        (state, m)
    }

    /// Returns the best-so-far genome and its canonical (maximise) fitness.
    ///
    /// Returns `None` before the first [`Strategy::tell`] call, when
    /// `DeState::best_genome` is still `None`.
    fn best(&self, state: &DeState<B>) -> Option<(Tensor<B, 2>, f32)> {
        state
            .best_genome
            .as_ref()
            .map(|g| (g.clone(), state.best_fitness))
    }
}

fn update_best<B: Backend>(state: &mut DeState<B>, pop: &Tensor<B, 2>, fitness: &[f32]) {
    if fitness.is_empty() {
        return;
    }
    let best_idx = argmax_host(fitness);
    let best_f = fitness[best_idx];
    if best_f > state.best_fitness {
        let device = pop.device();
        #[allow(clippy::cast_possible_wrap)]
        let idx =
            Tensor::<B, 1, Int>::from_data(TensorData::new(vec![best_idx as i64], [1]), &device);
        state.best_genome = Some(pop.clone().select(0, idx));
        state.best_fitness = best_f;
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::fitness::FromFitnessEvaluable;
    use crate::strategy::EvolutionaryHarness;
    use burn::backend::Flex;
    use rlevo_core::fitness::FitnessEvaluable;
    type TestBackend = Flex;

    #[test]
    fn default_config_validates() {
        assert!(DeConfig::default_for(30, 10).validate().is_ok());
    }

    #[test]
    fn rejects_pop_size_below_min() {
        let mut cfg = DeConfig::default_for(3, 10);
        cfg.pop_size = 3;
        assert_eq!(cfg.validate().unwrap_err().field, "pop_size");
    }

    /// [`DifferentialEvolution::sample_distinct_excluding`] must return
    /// exactly `k` indices that are pairwise distinct, all in `0..pop_size`,
    /// and all different from `self_idx`. Swept over every valid
    /// `(pop_size, k, self_idx)` triple for a handful of draws each so the
    /// rejection loop is exercised broadly (`de` §7, operator property).
    #[test]
    fn sample_distinct_excluding_yields_valid_indices() {
        use rand::SeedableRng;
        use rand::rngs::StdRng;

        let mut rng = StdRng::seed_from_u64(20_240_607);
        for pop_size in [4usize, 5, 8, 20] {
            // `k` never exceeds the largest per-variant index count (5, Rand2Bin)
            // and must stay strictly below `pop_size`.
            for k in 1..pop_size.min(6) {
                for self_idx in 0..pop_size {
                    for _ in 0..25 {
                        let chosen: Vec<usize> =
                            DifferentialEvolution::<TestBackend>::sample_distinct_excluding(
                                self_idx, pop_size, k, &mut rng,
                            );
                        assert_eq!(chosen.len(), k, "must return exactly k indices");
                        for (a, &x) in chosen.iter().enumerate() {
                            assert!(x < pop_size, "index {x} out of range for pop {pop_size}");
                            assert_ne!(x, self_idx, "index must differ from self_idx");
                            for &y in &chosen[a + 1..] {
                                assert_ne!(x, y, "indices must be pairwise distinct");
                            }
                        }
                    }
                }
            }
        }
    }

    /// The rejection loop cannot make progress when `pop_size <= k` (there are
    /// not enough candidates outside `self_idx`); the documented `assert`
    /// guards that with a panic rather than spinning forever (`de` §7).
    #[test]
    #[should_panic(expected = "pop_size must exceed")]
    fn sample_distinct_excluding_panics_when_pop_too_small() {
        use rand::SeedableRng;
        use rand::rngs::StdRng;

        let mut rng = StdRng::seed_from_u64(1);
        // k == pop_size: impossible to draw k distinct indices excluding self.
        let _ = DifferentialEvolution::<TestBackend>::sample_distinct_excluding(0, 3, 3, &mut rng);
    }

    /// Every gene of a generated trial vector stays inside `params.bounds`
    /// after the mutation + crossover + clamp pipeline (`de` §7, bounds
    /// handling). Drives the strategy one full ask/tell/ask cycle so the
    /// second `ask` returns genuine trial vectors rather than the initial
    /// population.
    #[test]
    fn trial_genes_stay_within_bounds() {
        use rand::SeedableRng;
        use rand::rngs::StdRng;

        let device = Default::default();
        let strategy = DifferentialEvolution::<TestBackend>::new();
        let mut params = DeConfig::default_for(12, 4);
        params.variant = DeVariant::Rand1Bin;
        let (lo, hi): (f32, f32) = params.bounds.into();

        let mut rng = StdRng::seed_from_u64(4242);
        let state = strategy.init(&params, &mut rng, &device);
        // First ask returns the initial population unchanged; a tell populates
        // the fitness cache so the next ask produces trial vectors.
        let (pop0, s) = strategy.ask(&params, &state, &mut rng, &device);
        let n = pop0.dims()[0];
        let fitness =
            Tensor::<TestBackend, 1>::from_data(TensorData::new(vec![0.0_f32; n], [n]), &device);
        let (s, _) = strategy.tell(&params, pop0, fitness, s, &mut rng);
        let (trial, _) = strategy.ask(&params, &s, &mut rng, &device);
        let genes: Vec<f32> = trial
            .into_data()
            .into_vec::<f32>()
            .expect("trial host-read of a tensor this test just built");
        for g in genes {
            assert!(
                g.is_finite() && g >= lo && g <= hi,
                "trial gene {g} left [{lo}, {hi}]"
            );
        }
    }

    /// Sphere landscape that returns `NaN` for half the domain. Exercises the
    /// fitness-hygiene chokepoint (ADR 0034): a `NaN` fitness must never become
    /// the reported best nor poison a population slot ("zombie slot").
    struct NanSphere;
    struct NanSphereFit;
    impl FitnessEvaluable for NanSphereFit {
        type Individual = Vec<f64>;
        type Landscape = NanSphere;
        fn evaluate(&self, x: &Self::Individual, _: &Self::Landscape) -> f64 {
            let s: f64 = x.iter().map(|v| v * v).sum();
            if x[0] > 0.0 { f64::NAN } else { s }
        }
    }

    /// A fitness function that yields `NaN` for many genomes must not crash the
    /// run and must never report a `NaN` (or otherwise non-finite) best. The
    /// harness sanitizes `NaN → −∞` at the driver chokepoint, so the poisoned
    /// slots can never out-rank a finite individual or block replacement
    /// (`de` §7, NaN regression).
    #[test]
    fn nan_fitness_never_becomes_best() {
        let device = Default::default();
        let params = DeConfig::default_for(30, 4);
        let fitness_fn = FromFitnessEvaluable::new(NanSphereFit, NanSphere);
        let mut harness = EvolutionaryHarness::<TestBackend, _, _>::new(
            DifferentialEvolution::<TestBackend>::new(),
            params,
            fitness_fn,
            99,
            device,
            40,
        )
        .expect("valid params");
        harness.reset();
        loop {
            if harness.step(()).done {
                break;
            }
        }
        let best = harness.latest_metrics().unwrap().best_fitness_ever();
        assert!(
            best.is_finite(),
            "NaN fitness poisoned best_fitness_ever: {best}"
        );
    }

    struct Sphere;
    struct SphereFit;
    impl FitnessEvaluable for SphereFit {
        type Individual = Vec<f64>;
        type Landscape = Sphere;
        fn evaluate(&self, x: &Self::Individual, _: &Self::Landscape) -> f64 {
            x.iter().map(|v| v * v).sum()
        }
    }

    fn run_de(variant: DeVariant, dim: usize, gens: usize) -> f32 {
        let device = Default::default();
        let mut params = DeConfig::default_for(30, dim);
        params.variant = variant;
        let fitness_fn = FromFitnessEvaluable::new(SphereFit, Sphere);
        let mut harness = EvolutionaryHarness::<TestBackend, _, _>::new(
            DifferentialEvolution::<TestBackend>::new(),
            params,
            fitness_fn,
            11,
            device,
            gens,
        )
        .expect("valid params");
        harness.reset();
        loop {
            if harness.step(()).done {
                break;
            }
        }
        harness.latest_metrics().unwrap().best_fitness_ever()
    }

    /// All five DE variants converge on Sphere-D10 within budget.
    ///
    /// The Burn Flex backend seeds its RNG through a process-wide
    /// mutex, so separate `#[test]` functions that call `Tensor::random`
    /// race on seeding and produce non-deterministic trajectories. This
    /// single test runs the variants sequentially inside one function
    /// so their seed state is not contended.
    ///
    /// Per-variant tolerance reflects classical characterizations:
    /// `rand1`/`rand2` converge to optimum, `best1` / current-to-best
    /// suffer from premature convergence on unimodal landscapes.
    #[test]
    fn all_variants_converge_on_sphere_d10() {
        let rand1bin = run_de(DeVariant::Rand1Bin, 10, 500);
        assert!(rand1bin < 1e-6, "DE/rand/1/bin best={rand1bin}");

        let rand2bin = run_de(DeVariant::Rand2Bin, 10, 800);
        assert!(rand2bin < 1e-6, "DE/rand/2/bin best={rand2bin}");

        let rand1exp = run_de(DeVariant::Rand1Exp, 10, 500);
        assert!(rand1exp < 1e-6, "DE/rand/1/exp best={rand1exp}");

        let best1bin = run_de(DeVariant::Best1Bin, 10, 500);
        assert!(best1bin < 1.0, "DE/best/1/bin best={best1bin}");

        let c2b = run_de(DeVariant::CurrentToBest1Bin, 10, 500);
        assert!(c2b < 2.0, "DE/current-to-best/1/bin best={c2b}");
    }
}