"""Fine-tune ChordTransformer on the personal (user) chord corpus.

Requires a pre-trained checkpoint produced by scripts/pretrain.py.

Usage:
    # Full run (fine-tuning + plot + report)
    python scripts/train.py

    # Skip training; re-plot and report from existing CSV
    python scripts/train.py --skip-training

Outputs written:
    checkpoints/finetuned.pt          best checkpoint
    checkpoints/finetuned.log.csv     per-epoch metrics
    checkpoints/finetuned_curves.png  train/val loss plot
"""

from __future__ import annotations

import argparse
import csv
import logging
import math
import sys
from pathlib import Path

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
import torch

sys.path.insert(0, str(Path(__file__).resolve().parent.parent))

from src.model import ChordTransformer
from src.train import TrainConfig, train
from src.tokenizer import TOKEN_TO_ID

# ---------------------------------------------------------------------------
# Paths
# ---------------------------------------------------------------------------

DATA_DIR   = Path("data/processed/user")
INIT_FROM  = Path("checkpoints/pretrained.pt")
CHECKPOINT = Path("checkpoints/finetuned.pt")
LOG_CSV    = Path("checkpoints/finetuned.log.csv")
CURVES_PNG = Path("checkpoints/finetuned_curves.png")
REPORT_TXT = Path("checkpoints/finetuned.report.txt")

# ---------------------------------------------------------------------------
# Training config
# ---------------------------------------------------------------------------

TRAIN_CFG = TrainConfig(
    data_dir=DATA_DIR,
    output=CHECKPOINT,
    init_from=INIT_FROM,
    # Small corpus (~45 train files) → ~6 batches/epoch.
    # 50 epochs × 6 = ~300 gradient steps; patience=10 gives a 60-step window.
    epochs=50,
    batch_size=8,
    lr=1e-5,
    warmup_steps=10,
    patience=10,
    seed=42,
    device="auto",
    # Must match pretrained checkpoint (max_seq_len=320).
    max_seq_len=320,
)


# ---------------------------------------------------------------------------
# Plotting
# ---------------------------------------------------------------------------

def plot_curves(log_csv: Path, out_png: Path) -> None:
    epochs, train_losses, val_losses, val_ppls = [], [], [], []
    with open(log_csv, newline="") as fh:
        for row in csv.DictReader(fh):
            epochs.append(int(row["epoch"]))
            train_losses.append(float(row["train_loss"]))
            val_losses.append(float(row["val_loss"]))
            val_ppls.append(float(row["val_ppl"]))

    fig, (ax1, ax2) = plt.subplots(1, 2, figsize=(11, 4))

    ax1.plot(epochs, train_losses, label="train loss", linewidth=1.5)
    ax1.plot(epochs, val_losses,   label="val loss",   linewidth=1.5)
    best_epoch = epochs[val_losses.index(min(val_losses))]
    ax1.axvline(best_epoch, color="grey", linestyle="--", linewidth=0.8,
                label=f"best epoch {best_epoch}")
    ax1.set_xlabel("epoch")
    ax1.set_ylabel("cross-entropy loss")
    ax1.set_title("Fine-tuning loss")
    ax1.legend()
    ax1.grid(True, alpha=0.3)

    ax2.plot(epochs, val_ppls, color="tab:orange", linewidth=1.5)
    ax2.axvline(best_epoch, color="grey", linestyle="--", linewidth=0.8)
    ax2.set_xlabel("epoch")
    ax2.set_ylabel("perplexity")
    ax2.set_title("Val perplexity")
    ax2.grid(True, alpha=0.3)

    fig.tight_layout()
    out_png.parent.mkdir(parents=True, exist_ok=True)
    fig.savefig(out_png, dpi=150)
    plt.close(fig)
    print(f"[plot] saved -> {out_png}")


# ---------------------------------------------------------------------------
# Report
# ---------------------------------------------------------------------------

def write_report(log_csv: Path, checkpoint: Path, report_path: Path) -> None:
    rows = []
    with open(log_csv, newline="") as fh:
        rows = list(csv.DictReader(fh))

    if not rows:
        print("[report] log CSV is empty — nothing to report")
        return

    val_losses = [float(r["val_loss"]) for r in rows]
    best_idx   = val_losses.index(min(val_losses))
    best_row   = rows[best_idx]

    best_loss = float(best_row["val_loss"])
    conv_epoch = next(
        (int(r["epoch"]) for r in rows if float(r["val_loss"]) <= best_loss * 1.01),
        int(best_row["epoch"]),
    )

    n_params = None
    if checkpoint.exists():
        ckpt = torch.load(checkpoint, weights_only=True)
        model = ChordTransformer(**ckpt["model_config"])
        tied = model.token_emb.weight.numel()
        n_params = sum(p.numel() for p in model.parameters()) - tied

    lines = []
    lines += [
        "",
        "=" * 52,
        "  FINE-TUNING REPORT",
        "=" * 52,
        f"  Total epochs run      : {len(rows)}",
        f"  Best epoch (val loss) : {best_row['epoch']}",
        f"  Convergence epoch     : {conv_epoch}  (val ≤ best+1 %)",
        f"  Best val loss         : {best_loss:.4f}",
        f"  Best val perplexity   : {float(best_row['val_ppl']):.2f}",
        f"  Final train loss      : {float(rows[-1]['train_loss']):.4f}",
    ]
    if n_params is not None:
        lines.append(f"  Unique parameters     : {n_params:,}")
    lines += [
        f"  Checkpoint            : {checkpoint}",
        f"  Log CSV               : {log_csv}",
        "=" * 52,
        "",
        f"  {'epoch':>5}  {'train':>8}  {'val':>8}  {'ppl':>7}  {'lr':>10}",
        f"  {'-'*5}  {'-'*8}  {'-'*8}  {'-'*7}  {'-'*10}",
    ]
    for r in rows:
        marker = " ←" if int(r["epoch"]) == int(best_row["epoch"]) else ""
        lines.append(
            f"  {int(r['epoch']):>5}  {float(r['train_loss']):>8.4f}"
            f"  {float(r['val_loss']):>8.4f}  {float(r['val_ppl']):>7.2f}"
            f"  {float(r['lr']):>10.2e}{marker}"
        )
    lines.append("")

    report_path.parent.mkdir(parents=True, exist_ok=True)
    report_path.write_text("\n".join(lines), encoding="utf-8")
    print(f"[report] saved -> {report_path}")


# ---------------------------------------------------------------------------
# Main
# ---------------------------------------------------------------------------

def main() -> None:
    ap = argparse.ArgumentParser(description=__doc__,
                                 formatter_class=argparse.RawDescriptionHelpFormatter)
    ap.add_argument("--skip-training", action="store_true",
                    help="Skip training; re-plot and report from existing CSV.")
    args = ap.parse_args()

    logging.basicConfig(
        level=logging.INFO,
        format="%(asctime)s %(levelname)s %(message)s",
        datefmt="%H:%M:%S",
    )

    if not args.skip_training:
        if not INIT_FROM.exists():
            print(f"ERROR: pre-trained checkpoint not found: {INIT_FROM}", file=sys.stderr)
            print("Run python scripts/pretrain.py first.", file=sys.stderr)
            sys.exit(1)
        if not DATA_DIR.exists():
            print(f"ERROR: data directory not found: {DATA_DIR}", file=sys.stderr)
            print("Run: python scripts/prepare_data.py --input-dir data/raw_user --output-dir data/processed/user", file=sys.stderr)
            sys.exit(1)
        n_train = len(list((DATA_DIR / "train").glob("*.pt")))
        n_batches = (n_train + TRAIN_CFG.batch_size - 1) // TRAIN_CFG.batch_size
        est_epoch_s = n_batches * 1.5
        device_label = "GPU" if torch.cuda.is_available() else "CPU"
        print(
            f"[train] {n_train} train files, {n_batches} batches/epoch\n"
            f"[train] estimated time on {device_label}: "
            f"~{est_epoch_s/60:.0f} min/epoch, "
            f"~{TRAIN_CFG.epochs * est_epoch_s / 3600:.1f} h total\n"
            f"[train] (early stopping with patience={TRAIN_CFG.patience} may reduce this)\n"
        )
        train(TRAIN_CFG)
    else:
        if not LOG_CSV.exists():
            print(f"ERROR: log CSV not found: {LOG_CSV}", file=sys.stderr)
            sys.exit(1)
        print(f"[skip-training] using existing log: {LOG_CSV}")

    plot_curves(LOG_CSV, CURVES_PNG)
    write_report(LOG_CSV, CHECKPOINT, REPORT_TXT)


if __name__ == "__main__":
    main()