// GPU grad-checks for the Phase T5 structural ops added on top of the T4 set:
// embedding (gather fwd / scatter-add bwd), reshape, transpose_3d01,
// transpose_2d, and split/merge_heads. Same harness as autograd.rs:
// L = sum(W ∘ out), W fixed random ⇒ upstream dOut = W; run backward(), then
// grad-check each leaf's .grad() against central finite differences.
//
// Gated behind `not(no_cuda)`: compiles out on a GPU-less host, runs on dash5.
#![cfg(not(no_cuda))]

use xtrain_autodiff::ops;
use xtrain_autodiff::tape::Var;
use xtrain_autodiff::{GradCheckConfig, grad_check};
use xtrain_cuda::device;
use xtrain_tensor::{Device, Tensor};

fn fill(n: usize, seed: u64) -> Vec<f32> {
    let mut state = seed
        .wrapping_mul(2862933555777941757)
        .wrapping_add(3037000493);
    (0..n)
        .map(|_| {
            state = state
                .wrapping_mul(6364136223846793005)
                .wrapping_add(1442695040888963407);
            ((state >> 33) as f32 / (1u64 << 31) as f32) - 0.5
        })
        .collect()
}

fn require_gpu() {
    assert!(
        device::device_count().expect("device count") > 0,
        "no CUDA device"
    );
    device::set_device(0).unwrap();
}

fn cuda(data: &[f32], shape: &[usize]) -> Tensor {
    Tensor::from_slice(data, shape).to_device(Device::Cuda(0))
}

fn weighted_sum(out: &Tensor, w: &[f32]) -> f32 {
    out.to_device(Device::Cpu)
        .as_slice::<f32>()
        .iter()
        .zip(w)
        .map(|(o, w)| o * w)
        .sum()
}

// Structural ops are exactly linear in their input → a large eps just sharpens
// f32 resolution (same as add/mul/transpose in autograd.rs).
fn cfg_linear() -> GradCheckConfig {
    GradCheckConfig {
        eps: 1e-2,
        rel_tol: 2e-2,
        atol: 1e-3,
    }
}

fn report(name: &str, res: &xtrain_autodiff::GradCheckResult) {
    println!(
        "{name}: max_rel_err = {:.3e} (worst num={:.5} ana={:.5} @ {})",
        res.max_rel_err, res.worst_numeric, res.worst_analytic, res.worst_index
    );
    assert!(res.passed, "{name} grad-check failed: {res:?}");
}

// L = sum(W ∘ out): a constant-W leaf mul + sum-to-scalar reduction.
fn scalar_loss(out: &Var, w: &[f32]) -> Var {
    let wt = Var::leaf(cuda(w, out.value().shape()));
    sum_all(&ops::mul(out, &wt))
}

fn sum_all(x: &Var) -> Var {
    let xv = x.value();
    let total: f32 = xv.to_device(Device::Cpu).as_slice::<f32>().iter().sum();
    let scalar = Tensor::from_slice(&[total], &[1]).to_device(xv.device());
    let shape: Vec<usize> = xv.shape().to_vec();
    Var::from_op(
        scalar,
        vec![x.clone()],
        Box::new(move |d, parents| {
            let dval = d.to_device(Device::Cpu).as_slice::<f32>()[0];
            let ones = vec![dval; shape.iter().product()];
            let g = Tensor::from_slice(&ones, &shape).to_device(Device::Cuda(0));
            Var::push_grad(&parents[0], g);
        }),
    )
}

// ---- embedding (gather fwd / scatter-add bwd) ----
// Includes a repeated id so the atomic scatter-add accumulation is exercised.
#[test]
fn embedding_bwd() {
    require_gpu();
    let (vocab, dim) = (5, 7);
    let ids_host: Vec<i32> = vec![0, 3, 1, 3, 2, 0]; // 0 and 3 repeat
    let seq = ids_host.len();
    let table_h = fill(vocab * dim, 201);
    let w = fill(seq * dim, 202);

    let ids = Tensor::from_slice(&ids_host, &[seq]).to_device(Device::Cuda(0));
    let table = Var::leaf(cuda(&table_h, &[vocab, dim]));
    let out = ops::embedding(&table, &ids);
    scalar_loss(&out, &w).backward();

    let dtable = table.grad().unwrap().to_device(Device::Cpu);
    let idf = ids_host.clone();
    let wf = w.clone();
    let lt = move |v: &[f32], s: &[usize]| {
        let ids = Tensor::from_slice(&idf, &[seq]).to_device(Device::Cuda(0));
        weighted_sum(&cuda(v, s).embedding(&ids), &wf)
    };
    report(
        "embedding dTable",
        &grad_check(
            &table_h,
            &[vocab, dim],
            &lt,
            dtable.as_slice::<f32>(),
            cfg_linear(),
        ),
    );
}

// ---- reshape ----
#[test]
fn reshape_bwd() {
    require_gpu();
    let (rows, cols) = (6, 8);
    let x_h = fill(rows * cols, 211);
    let w = fill(rows * cols, 212);

    let x = Var::leaf(cuda(&x_h, &[rows, cols]));
    let out = ops::reshape(&x, &[rows * 2, cols / 2]);
    scalar_loss(&out, &w).backward();

    let dx = x.grad().unwrap().to_device(Device::Cpu);
    let wf = w.clone();
    let lx =
        move |v: &[f32], s: &[usize]| weighted_sum(&cuda(v, s).reshape(&[rows * 2, cols / 2]), &wf);
    report(
        "reshape dX",
        &grad_check(&x_h, &[rows, cols], &lx, dx.as_slice::<f32>(), cfg_linear()),
    );
}

// ---- transpose_3d01 ([a,b,c] -> [b,a,c]) ----
#[test]
fn transpose_3d01_bwd() {
    require_gpu();
    let (a, b, c) = (3, 4, 5);
    let x_h = fill(a * b * c, 221);
    let w = fill(a * b * c, 222);

    let x = Var::leaf(cuda(&x_h, &[a, b, c]));
    let out = ops::transpose_3d01(&x);
    scalar_loss(&out, &w).backward();

    let dx = x.grad().unwrap().to_device(Device::Cpu);
    let wf = w.clone();
    let lx = move |v: &[f32], s: &[usize]| weighted_sum(&cuda(v, s).transpose_3d01(), &wf);
    report(
        "transpose_3d01 dX",
        &grad_check(&x_h, &[a, b, c], &lx, dx.as_slice::<f32>(), cfg_linear()),
    );
}

// ---- transpose_2d ----
#[test]
fn transpose_2d_bwd() {
    require_gpu();
    let (r, c) = (5, 7);
    let x_h = fill(r * c, 231);
    let w = fill(r * c, 232);

    let x = Var::leaf(cuda(&x_h, &[r, c]));
    let out = ops::transpose_2d(&x);
    scalar_loss(&out, &w).backward();

    let dx = x.grad().unwrap().to_device(Device::Cpu);
    let wf = w.clone();
    let lx = move |v: &[f32], s: &[usize]| weighted_sum(&cuda(v, s).transpose_2d(), &wf);
    report(
        "transpose_2d dX",
        &grad_check(&x_h, &[r, c], &lx, dx.as_slice::<f32>(), cfg_linear()),
    );
}

// ---- split_heads + merge_heads round-trip (identity reshuffle of [nh,seq,hd]) ----
// out = merge_heads(split_heads(x)) must equal x, and its grad must be dOut=W
// reshuffled identically — i.e. dx grad-checks against the identity composition.
#[test]
fn split_merge_heads_bwd() {
    require_gpu();
    let (nh, seq, hd) = (3, 4, 5);
    let x_h = fill(nh * seq * hd, 241);
    let w = fill(nh * seq * hd, 242);

    let x = Var::leaf(cuda(&x_h, &[nh, seq, hd]));
    let heads = ops::split_heads(&x);
    let out = ops::merge_heads(&heads); // back to [nh,seq,hd]
    scalar_loss(&out, &w).backward();

    let dx = x.grad().unwrap().to_device(Device::Cpu);
    // forward is identity, so grad-check the identity map.
    let wf = w.clone();
    let lx = move |v: &[f32], s: &[usize]| weighted_sum(&cuda(v, s), &wf);
    report(
        "split/merge_heads dX",
        &grad_check(
            &x_h,
            &[nh, seq, hd],
            &lx,
            dx.as_slice::<f32>(),
            cfg_linear(),
        ),
    );
}