//! Milestone-1 gate for the real selective-scan Mamba (Phase-3 spec).
//!
//! These tests prove the `MambaBlock` forward is now a genuine S6
//! selective-state-space model — not the prior passthrough stub:
//!   - **Liveness**: every weight that was *dead* in the stub
//!     (`conv1d_weight`, `A_log`, `dt_proj`, `x_proj`, `D`) now changes
//!     the output when perturbed.
//!   - **Causality**: output at step `l` depends only on inputs `≤ l`
//!     (no future leakage through the causal conv + scan).
//!   - **Determinism**: a seeded block is bit-stable across builds.
//!   - **Scan dynamics**: a non-trivial input produces non-constant,
//!     finite output with real temporal mixing.

use std::collections::HashMap;

use rtx_tensor::{Device, Tensor};
use rtx_transformers::layers::mamba::{MambaBlock, MambaConfig};

const D_MODEL: usize = 8;
const D_STATE: usize = 16;
const D_CONV: usize = 4;
const L: usize = 6;

fn cfg() -> MambaConfig {
    MambaConfig::new(D_MODEL, D_STATE, D_CONV)
}

fn fwd(block: &MambaBlock, x: &Tensor) -> Vec<f32> {
    block
        .forward(x)
        .expect("forward")
        .output
        .to_vec()
        .expect("to_vec")
}

fn max_abs_diff(a: &[f32], b: &[f32]) -> f32 {
    a.iter()
        .zip(b)
        .map(|(x, y)| (x - y).abs())
        .fold(0.0, f32::max)
}

/// Clone all persistence tensors into a map, replacing `target` with a
/// copy whose every element is shifted by `delta`.
fn perturbed_map(
    block: &MambaBlock,
    target: &str,
    delta: f32,
    dev: &Device,
) -> HashMap<String, Tensor> {
    let mut map = HashMap::new();
    for (name, t) in block.persistence_tensors() {
        if name == target {
            let mut v = t.to_vec().expect("to_vec");
            for x in v.iter_mut() {
                *x += delta;
            }
            let pt = Tensor::from_vec(v, t.shape().dims(), dev).expect("from_vec");
            map.insert(name.to_string(), pt);
        } else {
            map.insert(name.to_string(), t.clone());
        }
    }
    map
}

/// A seeded block with `dt_bias` overridden to 0 ⇒ `Δ = softplus(0) ≈
/// 0.69`. The default init uses a deliberately small `Δ≈0.01` (the scan
/// is near-identity — correct for training stability), which makes the
/// scan-only weights' influence vanish below the f32 floor. This widens
/// `Δ` so the scan genuinely contributes to the output, letting us
/// observe each weight's effect.
fn active_block(cfg: &MambaConfig, dev: &Device, seed: u64) -> MambaBlock {
    let base = MambaBlock::new_seeded(cfg.clone(), dev, seed).expect("base");
    let d = cfg.get_d_inner();
    let mut map = HashMap::new();
    for (name, t) in base.persistence_tensors() {
        if name == "dt_bias" {
            map.insert(
                name.to_string(),
                Tensor::from_vec(vec![0.0f32; d], &[d], dev).expect("dtb"),
            );
        } else {
            map.insert(name.to_string(), t.clone());
        }
    }
    MambaBlock::from_persistence_tensors(cfg.clone(), dev, map).expect("rebuild")
}

#[test]
fn every_formerly_dead_weight_is_live() {
    let dev = Device::cpu();
    let block = active_block(&cfg(), &dev, 42);
    let x = Tensor::randn_seeded(&[1, L, D_MODEL], &dev, 7).expect("x");
    let y0 = fwd(&block, &x);

    // In the stub these were all discarded; each must now move the output.
    for name in ["conv1d_weight", "A_log", "dt_proj", "x_proj", "D"] {
        let map = perturbed_map(&block, name, 0.5, &dev);
        let b2 = MambaBlock::from_persistence_tensors(cfg(), &dev, map).expect("rebuild");
        let y2 = fwd(&b2, &x);
        let diff = max_abs_diff(&y0, &y2);
        assert!(
            diff > 1e-4,
            "weight `{name}` is DEAD — perturbing it left the output unchanged (max_abs_diff={diff:.2e})"
        );
    }
}

#[test]
fn forward_is_causal() {
    let dev = Device::cpu();
    let block = MambaBlock::new_seeded(cfg(), &dev, 11).expect("block");
    let base = Tensor::randn_seeded(&[1, L, D_MODEL], &dev, 3).expect("x");
    let y0 = fwd(&block, &base);

    // Perturb ONLY the last timestep.
    let mut xv = base.to_vec().expect("to_vec");
    for m in 0..D_MODEL {
        xv[(L - 1) * D_MODEL + m] += 1.0;
    }
    let x2 = Tensor::from_vec(xv, &[1, L, D_MODEL], &dev).expect("x2");
    let y2 = fwd(&block, &x2);

    // Outputs for steps 0..L-1 must be identical (no future leakage);
    // the last step must change.
    let prefix0 = &y0[..(L - 1) * D_MODEL];
    let prefix2 = &y2[..(L - 1) * D_MODEL];
    assert!(
        max_abs_diff(prefix0, prefix2) < 1e-6,
        "non-causal: changing input[L-1] altered an earlier output"
    );
    let last0 = &y0[(L - 1) * D_MODEL..];
    let last2 = &y2[(L - 1) * D_MODEL..];
    assert!(
        max_abs_diff(last0, last2) > 1e-4,
        "last output did not respond to the last input"
    );
}

#[test]
fn seeded_forward_is_deterministic() {
    let dev = Device::cpu();
    let a = MambaBlock::new_seeded(cfg(), &dev, 99).expect("a");
    let b = MambaBlock::new_seeded(cfg(), &dev, 99).expect("b");
    let x = Tensor::randn_seeded(&[1, L, D_MODEL], &dev, 5).expect("x");
    assert_eq!(
        fwd(&a, &x),
        fwd(&b, &x),
        "same (config, seed) must be bit-exact"
    );
}

/// Build a seeded block whose every weight is scaled by `scale`, so the
/// SSM operates in a well-conditioned regime (small `A_log` ⇒ `A≈-1` ⇒
/// the scan neither saturates nor underflows), giving a meaningful
/// finite-difference signal.
fn scaled_block(cfg: &MambaConfig, dev: &Device, seed: u64, scale: f32) -> MambaBlock {
    let base = MambaBlock::new_seeded(cfg.clone(), dev, seed).expect("base");
    let mut map = HashMap::new();
    for (name, t) in base.persistence_tensors() {
        let mut v = t.to_vec().expect("to_vec");
        for x in v.iter_mut() {
            *x *= scale;
        }
        map.insert(
            name.to_string(),
            Tensor::from_vec(v, t.shape().dims(), dev).expect("from_vec"),
        );
    }
    MambaBlock::from_persistence_tensors(cfg.clone(), dev, map).expect("rebuild")
}

fn loss(block: &MambaBlock, x: &Tensor, dov: &[f32]) -> f32 {
    let out = block
        .forward(x)
        .expect("fwd")
        .output
        .to_vec()
        .expect("to_vec");
    out.iter().zip(dov).map(|(o, g)| o * g).sum::<f32>()
}

fn perturbed_one(
    block: &MambaBlock,
    name: &str,
    idx: usize,
    eps: f32,
    cfg: &MambaConfig,
    dev: &Device,
) -> MambaBlock {
    let mut map = HashMap::new();
    for (nm, t) in block.persistence_tensors() {
        let mut v = t.to_vec().expect("to_vec");
        if nm == name {
            v[idx] += eps;
        }
        map.insert(
            nm.to_string(),
            Tensor::from_vec(v, t.shape().dims(), dev).expect("from_vec"),
        );
    }
    MambaBlock::from_persistence_tensors(cfg.clone(), dev, map).expect("rebuild")
}

#[test]
fn analytic_gradients_match_finite_differences() {
    let dev = Device::cpu();
    // Small, well-conditioned instance for a clean f32 FD signal.
    let cfg = MambaConfig::new(4, 4, 3);
    let block = scaled_block(&cfg, &dev, 21, 0.2);
    let ll = 4usize;
    let dmodel = 4usize;

    // Input and a fixed upstream gradient d_out (so L = Σ d_out·out).
    let xvec: Vec<f32> = (0..ll * dmodel)
        .map(|i| 0.5 * ((i as f32 * 0.37).sin()))
        .collect();
    let x = Tensor::from_vec(xvec, &[1, ll, dmodel], &dev).expect("x");
    let dov: Vec<f32> = (0..ll * dmodel)
        .map(|i| ((i as f32 * 0.91).cos()))
        .collect();

    let grads = block
        .backward(
            &x,
            &Tensor::from_vec(dov.clone(), &[1, ll, dmodel], &dev).expect("dov"),
        )
        .expect("backward");

    let eps = 1e-2f32;
    let mut checked = 0;
    for name in [
        "in_proj",
        "conv1d_weight",
        "conv1d_bias",
        "A_log",
        "x_proj",
        "dt_proj",
        "dt_bias",
        "D",
        "out_proj",
    ] {
        let g = grads
            .get(name)
            .unwrap_or_else(|| panic!("missing grad {name}"))
            .to_vec()
            .expect("g");
        let len = g.len();
        // Sample up to 4 spread-out indices per parameter.
        let idxs: Vec<usize> = if len <= 4 {
            (0..len).collect()
        } else {
            vec![0, len / 4, len / 2, (3 * len) / 4]
        };
        for &idx in &idxs {
            let bp = perturbed_one(&block, name, idx, eps, &cfg, &dev);
            let bm = perturbed_one(&block, name, idx, -eps, &cfg, &dev);
            let fd = (loss(&bp, &x, &dov) - loss(&bm, &x, &dov)) / (2.0 * eps);
            let an = g[idx];
            let tol = 5e-3 + 5e-2 * fd.abs();
            assert!(
                (an - fd).abs() <= tol,
                "grad mismatch for `{name}`[{idx}]: analytic={an:.6} finite-diff={fd:.6} (tol {tol:.4})"
            );
            checked += 1;
        }
    }
    assert!(
        checked >= 30,
        "expected to check ≥30 grad elements, got {checked}"
    );
}

fn named_param(block: &MambaBlock, name: &str) -> Vec<f32> {
    block
        .persistence_tensors()
        .into_iter()
        .find(|(nm, _)| *nm == name)
        .map(|(_, t)| t.to_vec().expect("to_vec"))
        .expect("param")
}

/// Milestone-3: a self-contained Adam loop drives the loss down and
/// actually moves the backbone weights — i.e. the block is *trainable*.
/// The target is a teacher block's output on the same input (a learnable
/// regression at matched capacity), so a real gradient signal must shrink
/// the loss substantially.
#[test]
fn training_loop_reduces_loss_and_moves_weights() {
    let dev = Device::cpu();
    let cfg = MambaConfig::new(4, 4, 3);
    let ll = 4usize;
    let dmodel = 4usize;

    let teacher = scaled_block(&cfg, &dev, 99, 0.2);
    let xvec: Vec<f32> = (0..ll * dmodel)
        .map(|i| 0.5 * ((i as f32 * 0.41).sin()))
        .collect();
    let x = Tensor::from_vec(xvec, &[1, ll, dmodel], &dev).expect("x");
    let target = teacher
        .forward(&x)
        .expect("teacher")
        .output
        .to_vec()
        .expect("t");

    let mut block = scaled_block(&cfg, &dev, 5, 0.2);
    // conv1d_weight is a seed-varying backbone weight (A_log/dt_bias/D
    // are deterministic init, identical between teacher and student, so
    // they carry little gradient here) — it must move if the SSM block
    // (not just a head) is being trained.
    let cw_before = named_param(&block, "conv1d_weight");

    let (lr, b1, b2, eps) = (0.03f32, 0.9f32, 0.999f32, 1e-8f32);
    let mut state: HashMap<String, (Vec<f32>, Vec<f32>)> = HashMap::new();
    let mut first_loss = 0.0f32;
    let mut last_loss = 0.0f32;

    for step in 1..=200i32 {
        let out = block.forward(&x).expect("fwd").output.to_vec().expect("o");
        let ne = out.len();
        let mut dov = vec![0.0f32; ne];
        let mut loss = 0.0f32;
        for i in 0..ne {
            let e = out[i] - target[i];
            loss += e * e;
            dov[i] = 2.0 * e / (ne as f32);
        }
        loss /= ne as f32;
        if step == 1 {
            first_loss = loss;
        }
        last_loss = loss;

        let d_out = Tensor::from_vec(dov, &[1, ll, dmodel], &dev).expect("dov");
        let grads = block.backward(&x, &d_out).expect("backward");

        let mut map = HashMap::new();
        for (name, t) in block.persistence_tensors() {
            let p = t.to_vec().expect("p");
            let g = grads.get(name).expect("g").to_vec().expect("gv");
            let entry = state
                .entry(name.to_string())
                .or_insert_with(|| (vec![0.0f32; p.len()], vec![0.0f32; p.len()]));
            let (m, v) = entry;
            let mut np = vec![0.0f32; p.len()];
            for i in 0..p.len() {
                m[i] = b1 * m[i] + (1.0 - b1) * g[i];
                v[i] = b2 * v[i] + (1.0 - b2) * g[i] * g[i];
                let mhat = m[i] / (1.0 - b1.powi(step));
                let vhat = v[i] / (1.0 - b2.powi(step));
                np[i] = p[i] - lr * mhat / (vhat.sqrt() + eps);
            }
            map.insert(
                name.to_string(),
                Tensor::from_vec(np, t.shape().dims(), &dev).expect("np"),
            );
        }
        block = MambaBlock::from_persistence_tensors(cfg.clone(), &dev, map).expect("rebuild");
    }

    assert!(
        last_loss < 0.5 * first_loss,
        "training did not reduce loss enough: {first_loss:.5} → {last_loss:.5}"
    );
    let cw_after = named_param(&block, "conv1d_weight");
    assert!(
        max_abs_diff(&cw_before, &cw_after) > 1e-3,
        "backbone weight conv1d_weight did not move during training"
    );
}

#[test]
fn output_is_finite_and_non_constant() {
    let dev = Device::cpu();
    let block = MambaBlock::new_seeded(cfg(), &dev, 1).expect("block");
    let x = Tensor::randn_seeded(&[1, L, D_MODEL], &dev, 2).expect("x");
    let y = fwd(&block, &x);
    assert!(y.iter().all(|v| v.is_finite()), "non-finite output");
    let spread = max_abs_diff(&y, &vec![y[0]; y.len()]);
    assert!(
        spread > 1e-4,
        "output is suspiciously constant ({spread:.2e})"
    );
}