// CPU-only diagnostic shared by the standalone DS4 oracle and Rust tests.
// The input is the existing 32-row, little-endian F32 logit recording.
pub fn run(
    path: &std::path::Path,
    vocab: usize,
    mut sample: impl FnMut(&[f32]) -> i32,
) -> Result<serde_json::Value, String> {
    if !(1..=1_000_000).contains(&vocab) {
        return Err("invalid sampler vocabulary size".into());
    }
    let expected = 32 * vocab * 4;
    if std::fs::metadata(path).map_err(|e| e.to_string())?.len() != expected as u64 {
        return Err("sampler recording must contain exactly 32 F32 rows".into());
    }
    let bytes = std::fs::read(path).map_err(|e| e.to_string())?;
    if bytes.len() != expected {
        return Err("sampler recording changed while reading".into());
    }
    let values = bytes
        .chunks_exact(4)
        .map(|b| f32::from_le_bytes(b.try_into().unwrap()))
        .collect::<Vec<_>>();
    for row in values.chunks_exact(vocab) {
        std::hint::black_box(sample(row));
    }
    let mut batch_ms = Vec::with_capacity(16);
    let mut token_ids = Vec::with_capacity(512);
    for _ in 0..16 {
        let started = std::time::Instant::now();
        for row in values.chunks_exact(vocab) {
            token_ids.push(std::hint::black_box(sample(row)));
        }
        batch_ms.push(started.elapsed().as_secs_f64() * 1000.0);
    }
    Ok(serde_json::json!({
        "event":"sampler_replay_benchmark", "vocab":vocab, "rows":32,
        "warmup_draws":32, "draws":512, "batch_ms":batch_ms, "token_ids":token_ids,
    }))
}