//! The training loop: sample a batch of sequences → ONE batched forward `loss` →
//! backward → grad clip → AdamW step → zero grads; with an LR schedule, periodic
//! loss logging, and periodic checkpointing.
//!
//! Since T10 the model is batched (`loss_batched`): `batch_size` sequences are
//! flattened to `[batch*seq]` and run as a SINGLE forward/backward, so the linear
//! projections become big `[batch*seq, dim]` GEMMs that fill the GPU. The
//! cross-entropy mean is over all `batch*seq` rows — already the batch-mean loss,
//! so backward yields the batch-mean gradient directly (clip pre-scale = 1.0; no
//! more "loop B times + SUM + ×1/batch" hack).

#![cfg(not(no_cuda))]

use std::path::PathBuf;
use std::time::Instant;

use xtrain_model::{TinyTransformer, batched_ids_tensor, ids_tensor};
use xtrain_optim::GpuAdamW;
use xtrain_tensor::Device;

use crate::checkpoint;
use crate::clip::clip_grad_norm_gpu;
use crate::data::Corpus;
use crate::schedule::LrSchedule;

/// Knobs for a training run.
pub struct TrainConfig {
    pub seq_len: usize,
    pub batch_size: usize,
    pub steps: usize,
    pub schedule: LrSchedule,
    pub weight_decay: f32,
    pub max_grad_norm: f32,
    pub log_every: usize,
    /// Optional checkpoint path written every `ckpt_every` steps (and at the end).
    /// When `eval_every > 0`, the checkpoint instead tracks the BEST val loss.
    pub ckpt_path: Option<PathBuf>,
    pub ckpt_every: usize,
    /// Evaluate held-out val loss every `eval_every` steps (0 = never). Each eval
    /// averages cross-entropy over `eval_batches` fixed windows of the val corpus.
    pub eval_every: usize,
    pub eval_batches: usize,
    /// Seed for reproducible sequence sampling.
    pub seed: u64,
}

/// Outcome of a run: per-step train losses and (step, val_loss) eval points.
pub struct TrainResult {
    pub train_losses: Vec<f32>,
    pub evals: Vec<(usize, f32)>,
    pub best_val: Option<f32>,
}

/// Train `model` on `corpus` for `cfg.steps` AdamW steps. Returns the per-step
/// train-loss trace plus any (step, val_loss) eval points. Logs progress, and —
/// when `valid` is given and `cfg.eval_every > 0` — evaluates held-out val loss
/// periodically and checkpoints the BEST val model (else checkpoints on a fixed
/// cadence, as in T6). Logs progress.
pub fn train(
    model: &TinyTransformer,
    device: Device,
    corpus: &Corpus,
    valid: Option<&Corpus>,
    cfg: &TrainConfig,
) -> TrainResult {
    let params = model.params();
    let mut opt = GpuAdamW::new(cfg.weight_decay);
    let mut rng = cfg.seed;
    let mut losses = Vec::with_capacity(cfg.steps);
    let mut evals = Vec::new();
    let mut best_val: Option<f32> = None;
    let start = Instant::now();
    let mut tokens_seen: u64 = 0;
    // Best-val checkpointing only kicks in when we actually evaluate.
    let track_best = valid.is_some() && cfg.eval_every > 0;

    for step in 0..cfg.steps {
        let lr = cfg.schedule.lr(step);

        // Sample `batch_size` sequences and run them as ONE batched forward/
        // backward. The CE mean over all batch*seq rows is the batch-mean loss, so
        // backward already yields the batch-mean gradient (clip pre-scale = 1.0).
        let mut inputs = Vec::with_capacity(cfg.batch_size);
        let mut targets_v = Vec::with_capacity(cfg.batch_size);
        for _ in 0..cfg.batch_size {
            let (input, target) = corpus.sample(cfg.seq_len, &mut rng);
            inputs.push(input);
            targets_v.push(target);
        }
        let ids = batched_ids_tensor(&inputs, device);
        let targets = batched_ids_tensor(&targets_v, device);
        // Training mode → dropout active (T18; no-op when cfg.dropout == 0). Set
        // each step so it is restored after a periodic eval flips to eval mode.
        model.train();
        let loss = model.loss_batched(&ids, &targets, cfg.batch_size);
        let step_loss = read_scalar(&loss);
        loss.backward();
        tokens_seen += (cfg.batch_size * cfg.seq_len) as u64;
        losses.push(step_loss);

        // Backward already produced the batch-mean gradient — just clip it.
        let gnorm = clip_grad_norm_gpu(&params, cfg.max_grad_norm, 1.0);
        opt.step(lr, &params);
        for p in &params {
            p.zero_grad();
        }

        if step % cfg.log_every == 0 || step == cfg.steps - 1 {
            let elapsed = start.elapsed().as_secs_f32();
            let tps = tokens_seen as f32 / elapsed.max(1e-6);
            println!(
                "step {step:5}/{}: loss {step_loss:.4} lr {lr:.2e} gnorm {gnorm:.3} \
                 ({tps:.0} tok/s)",
                cfg.steps
            );
        }

        // Periodic held-out eval (deterministic windows, no grad).
        if let Some(v) = valid {
            if cfg.eval_every > 0 && ((step + 1) % cfg.eval_every == 0 || step == cfg.steps - 1) {
                let vl = eval_loss(model, device, v, cfg.seq_len, cfg.eval_batches);
                evals.push((step, vl));
                let improved = best_val.map(|b| vl < b).unwrap_or(true);
                println!(
                    "  eval @ step {step}: val loss {vl:.4}{}",
                    if improved { " (best)" } else { "" }
                );
                if improved {
                    best_val = Some(vl);
                    if let Some(path) = &cfg.ckpt_path {
                        checkpoint::save(path, &params).expect("best checkpoint save");
                    }
                }
            }
        }

        // Fixed-cadence checkpointing (only when not tracking best val).
        if !track_best {
            if let Some(path) = &cfg.ckpt_path {
                if cfg.ckpt_every > 0 && (step + 1) % cfg.ckpt_every == 0 {
                    checkpoint::save(path, &params).expect("checkpoint save");
                }
            }
        }
    }

    // Without periodic eval, still persist the final params (T6 behaviour). With
    // best-val tracking the checkpoint already holds the best model — don't clobber.
    if !track_best {
        if let Some(path) = &cfg.ckpt_path {
            checkpoint::save(path, &params).expect("final checkpoint save");
            println!("saved checkpoint → {}", path.display());
        }
    }
    TrainResult {
        train_losses: losses,
        evals,
        best_val,
    }
}

/// Mean cross-entropy over `batches` deterministic, non-overlapping windows of
/// the validation corpus (no backward — eval only). Deterministic so val loss is
/// comparable across steps and runs (and across models — the v0-vs-v1 metric).
pub fn eval_loss(
    model: &TinyTransformer,
    device: Device,
    valid: &Corpus,
    seq: usize,
    batches: usize,
) -> f32 {
    if valid.len() <= seq + 1 {
        return f32::NAN;
    }
    // Eval mode → dropout is identity (T18).
    model.eval();
    let n_win = (valid.len() - 1) / seq; // disjoint windows that fit
    let batches = batches.max(1).min(n_win.max(1));
    let stride = (n_win / batches).max(1);
    let mut sum = 0.0f32;
    let mut count = 0usize;
    for i in 0..batches {
        let s = (i * stride) * seq;
        if s + seq + 1 > valid.len() {
            break;
        }
        let input: Vec<i32> = valid.tokens[s..s + seq].to_vec();
        let target: Vec<i32> = valid.tokens[s + 1..s + seq + 1].to_vec();
        let ids = ids_tensor(&input, device);
        let targets = ids_tensor(&target, device);
        let loss = model.loss(&ids, &targets);
        sum += read_scalar(&loss);
        count += 1;
    }
    if count == 0 {
        f32::NAN
    } else {
        sum / count as f32
    }
}

fn read_scalar(v: &xtrain_autodiff::tape::Var) -> f32 {
    v.value().to_device(Device::Cpu).as_slice::<f32>()[0]
}