"""
Component 5: Training pipeline for the 420M code model.

Features:
- FP16 mixed precision
- Gradient checkpointing
- Gradient accumulation
- 8-bit optimizer attempt with safe fallback
- Checkpoint save every N steps
- Resume from checkpoint
- Early stopping
- Live progress with loss, LR, ETA, VRAM
"""

from __future__ import annotations

import argparse
import json
import math
import os
import sys
import time
from pathlib import Path
from typing import Any, Dict, Optional, Tuple

import torch
import yaml
from torch.optim import AdamW
from torch.utils.data import DataLoader
from tqdm import tqdm

# Ensure src imports work from project root.
PROJECT_ROOT = Path(__file__).resolve().parents[1]
if str(PROJECT_ROOT) not in sys.path:
    sys.path.insert(0, str(PROJECT_ROOT))

from src.model_architecture.code_transformer import CodeTransformerLM, ModelConfig, get_model_presets  # noqa: E402
from src.training_pipeline.tokenized_dataset import CausalCollator, TokenizedJsonlDataset  # noqa: E402


def parse_args() -> argparse.Namespace:
    parser = argparse.ArgumentParser(description="Run Component 5 training.")
    parser.add_argument("--config", default="configs/component5_training_config.yaml")
    return parser.parse_args()


def load_yaml(path: Path) -> Dict[str, Any]:
    if not path.exists():
        raise FileNotFoundError(f"Config not found: {path}")
    with path.open("r", encoding="utf-8") as f:
        data = yaml.safe_load(f)
    if not isinstance(data, dict):
        raise ValueError("Invalid YAML format.")
    return data


def load_model_config(path: Path) -> ModelConfig:
    cfg = load_yaml(path)
    preset = cfg.get("preset")
    model_cfg = cfg.get("model", {})
    if preset:
        presets = get_model_presets()
        if preset not in presets:
            raise ValueError(f"Unknown model preset: {preset}")
        base = presets[preset].__dict__.copy()
        base.update(model_cfg)
        return ModelConfig(**base)
    return ModelConfig(**model_cfg)


def make_optimizer(model: torch.nn.Module, train_cfg: Dict[str, Any]) -> Tuple[torch.optim.Optimizer, str]:
    lr = float(train_cfg["learning_rate"])
    wd = float(train_cfg["weight_decay"])
    betas = tuple(float(x) for x in train_cfg.get("betas", [0.9, 0.95]))
    prefer_8bit = bool(train_cfg.get("prefer_8bit_adam", True))

    if prefer_8bit:
        try:
            import bitsandbytes as bnb  # type: ignore

            optimizer = bnb.optim.Adam8bit(model.parameters(), lr=lr, betas=betas, weight_decay=wd)
            return optimizer, "Adam8bit"
        except Exception:
            pass

    optimizer = AdamW(model.parameters(), lr=lr, betas=betas, weight_decay=wd)
    return optimizer, "AdamW"


def cosine_lr(base_lr: float, step: int, warmup_steps: int, max_steps: int, min_lr_ratio: float) -> float:
    if step < warmup_steps:
        return base_lr * (step / max(1, warmup_steps))
    progress = (step - warmup_steps) / max(1, max_steps - warmup_steps)
    progress = min(1.0, max(0.0, progress))
    cosine = 0.5 * (1.0 + math.cos(math.pi * progress))
    min_lr = base_lr * min_lr_ratio
    return min_lr + (base_lr - min_lr) * cosine


def set_optimizer_lr(optimizer: torch.optim.Optimizer, lr: float) -> None:
    for pg in optimizer.param_groups:
        pg["lr"] = lr


def get_vram_gb() -> float:
    if not torch.cuda.is_available():
        return 0.0
    return torch.cuda.memory_allocated() / (1024**3)


def save_checkpoint(
    ckpt_dir: Path,
    step: int,
    model: CodeTransformerLM,
    optimizer: torch.optim.Optimizer,
    scaler: Optional[torch.cuda.amp.GradScaler],
    best_val: float,
    no_improve_evals: int,
    config: Dict[str, Any],
) -> Path:
    ckpt_dir.mkdir(parents=True, exist_ok=True)
    ckpt_path = ckpt_dir / f"step_{step}.pt"
    payload = {
        "step": step,
        "model_state": model.state_dict(),
        "optimizer_state": optimizer.state_dict(),
        "scaler_state": scaler.state_dict() if scaler is not None else None,
        "best_val": best_val,
        "no_improve_evals": no_improve_evals,
        "config": config,
    }
    torch.save(payload, ckpt_path)
    latest = ckpt_dir / "latest.pt"
    torch.save(payload, latest)
    return ckpt_path


def load_checkpoint(
    ckpt_path: Path,
    model: CodeTransformerLM,
    optimizer: torch.optim.Optimizer,
    scaler: Optional[torch.cuda.amp.GradScaler],
    device: torch.device,
) -> Tuple[int, float, int]:
    payload = torch.load(ckpt_path, map_location=device)
    model.load_state_dict(payload["model_state"])
    optimizer.load_state_dict(payload["optimizer_state"])
    if scaler is not None and payload.get("scaler_state") is not None:
        scaler.load_state_dict(payload["scaler_state"])
    step = int(payload.get("step", 0))
    best_val = float(payload.get("best_val", 1e9))
    no_improve = int(payload.get("no_improve_evals", 0))
    return step, best_val, no_improve


@torch.no_grad()
def evaluate_loss(
    model: CodeTransformerLM,
    val_loader: DataLoader,
    device: torch.device,
    use_fp16: bool,
    max_batches: int = 50,
) -> float:
    model.eval()
    losses = []
    amp_enabled = use_fp16 and device.type == "cuda"
    for i, (input_ids, labels) in enumerate(val_loader):
        if i >= max_batches:
            break
        input_ids = input_ids.to(device, non_blocking=True)
        labels = labels.to(device, non_blocking=True)
        with torch.amp.autocast("cuda", enabled=amp_enabled, dtype=torch.float16):
            out = model(input_ids=input_ids, labels=labels)
        losses.append(float(out["loss"].item()))
    model.train()
    if not losses:
        return 1e9
    return sum(losses) / len(losses)


def train() -> None:
    args = parse_args()
    cfg = load_yaml(Path(args.config))
    train_cfg = cfg["training"]
    data_cfg = cfg["data"]
    resume_cfg = cfg.get("resume", {})

    device = torch.device("cuda" if torch.cuda.is_available() else "cpu")
    if device.type != "cuda":
        raise RuntimeError("CUDA GPU is required for this training setup.")

    model_cfg = load_model_config(Path(cfg["model"]["model_config_path"]))
    model_cfg.max_seq_len = int(train_cfg["max_seq_len"])
    model_cfg.gradient_checkpointing = bool(train_cfg.get("use_gradient_checkpointing", True))

    model = CodeTransformerLM(model_cfg)
    model.enable_gradient_checkpointing(model_cfg.gradient_checkpointing)
    model = model.to(device)

    use_fp16 = bool(train_cfg.get("use_fp16", True))
    scaler = torch.amp.GradScaler("cuda", enabled=use_fp16)

    optimizer, optimizer_name = make_optimizer(model, train_cfg)

    tokenized_path = str(data_cfg["tokenized_jsonl_path"])
    train_ds = TokenizedJsonlDataset(
        path=tokenized_path,
        split="train",
        val_ratio=float(data_cfg.get("val_ratio", 0.02)),
        split_seed=int(data_cfg.get("split_seed", 17)),
    )
    val_ds = TokenizedJsonlDataset(
        path=tokenized_path,
        split="val",
        val_ratio=float(data_cfg.get("val_ratio", 0.02)),
        split_seed=int(data_cfg.get("split_seed", 17)),
    )

    collator = CausalCollator(pad_token_id=0, max_seq_len=int(train_cfg["max_seq_len"]))
    train_loader = DataLoader(
        train_ds,
        batch_size=int(train_cfg["micro_batch_size"]),
        shuffle=True,
        num_workers=int(data_cfg.get("num_workers", 0)),
        pin_memory=True,
        collate_fn=collator,
    )
    val_loader = DataLoader(
        val_ds,
        batch_size=int(train_cfg["micro_batch_size"]),
        shuffle=False,
        num_workers=0,
        pin_memory=True,
        collate_fn=collator,
    )

    out_dir = Path(train_cfg["output_dir"])
    out_dir.mkdir(parents=True, exist_ok=True)

    global_step = 0
    best_val = 1e9
    no_improve = 0

    resume_from = str(resume_cfg.get("resume_from", "none")).strip().lower()
    if resume_from != "none":
        if resume_from == "latest":
            ckpt_path = out_dir / "latest.pt"
        else:
            ckpt_path = Path(resume_cfg["resume_from"])
        if ckpt_path.exists():
            global_step, best_val, no_improve = load_checkpoint(
                ckpt_path=ckpt_path,
                model=model,
                optimizer=optimizer,
                scaler=scaler,
                device=device,
            )
            print(f"[resume] loaded checkpoint {ckpt_path} at step {global_step}")
        else:
            print(f"[resume] checkpoint not found, starting fresh: {ckpt_path}")

    max_steps = int(train_cfg["max_steps"])
    grad_accum = int(train_cfg["grad_accum_steps"])
    log_every = int(train_cfg["log_every"])
    eval_every = int(train_cfg["eval_every"])
    save_every = int(train_cfg["save_every"])
    warmup_steps = int(train_cfg["warmup_steps"])
    min_lr_ratio = float(train_cfg["min_lr_ratio"])
    grad_clip = float(train_cfg["grad_clip_norm"])
    max_vram_gb = float(train_cfg.get("max_vram_gb", 7.0))
    patience = int(train_cfg.get("early_stopping_patience_evals", 20))
    min_delta = float(train_cfg.get("early_stopping_min_delta", 5e-4))
    base_lr = float(train_cfg["learning_rate"])

    model.train()
    start_time = time.time()
    running_loss = 0.0
    running_count = 0

    pbar = tqdm(total=max_steps, initial=global_step, desc="train", dynamic_ncols=True)

    while global_step < max_steps:
        for input_ids, labels in train_loader:
            if global_step >= max_steps:
                break

            current_lr = cosine_lr(base_lr, global_step, warmup_steps, max_steps, min_lr_ratio)
            set_optimizer_lr(optimizer, current_lr)

            input_ids = input_ids.to(device, non_blocking=True)
            labels = labels.to(device, non_blocking=True)

            amp_enabled = use_fp16 and device.type == "cuda"
            with torch.amp.autocast("cuda", enabled=amp_enabled, dtype=torch.float16):
                out = model(input_ids=input_ids, labels=labels)
                loss = out["loss"] / grad_accum

            scaler.scale(loss).backward()

            running_loss += float(loss.item()) * grad_accum
            running_count += 1

            if running_count % grad_accum == 0:
                scaler.unscale_(optimizer)
                torch.nn.utils.clip_grad_norm_(model.parameters(), grad_clip)
                scaler.step(optimizer)
                scaler.update()
                optimizer.zero_grad(set_to_none=True)

                global_step += 1
                pbar.update(1)

                elapsed = time.time() - start_time
                steps_done = max(1, global_step)
                steps_left = max(0, max_steps - global_step)
                eta_sec = (elapsed / steps_done) * steps_left
                avg_loss = running_loss / max(1, running_count)
                vram = get_vram_gb()

                if vram > max_vram_gb:
                    raise RuntimeError(
                        f"VRAM safety threshold exceeded: {vram:.2f} GB > {max_vram_gb:.2f} GB. "
                        "Reduce max_seq_len or grad_accum/micro_batch settings."
                    )

                if global_step % log_every == 0:
                    pbar.set_postfix(
                        {
                            "loss": f"{avg_loss:.4f}",
                            "lr": f"{current_lr:.2e}",
                            "vram_gb": f"{vram:.2f}",
                            "eta_min": f"{eta_sec/60.0:.1f}",
                        }
                    )

                if global_step % save_every == 0:
                    ckpt_path = save_checkpoint(
                        ckpt_dir=out_dir,
                        step=global_step,
                        model=model,
                        optimizer=optimizer,
                        scaler=scaler,
                        best_val=best_val,
                        no_improve_evals=no_improve,
                        config=cfg,
                    )
                    print(f"\n[checkpoint] saved {ckpt_path}")

                if global_step % eval_every == 0:
                    val_loss = evaluate_loss(model, val_loader, device, use_fp16=use_fp16)
                    print(f"\n[eval] step={global_step} val_loss={val_loss:.4f} best={best_val:.4f}")
                    if val_loss < (best_val - min_delta):
                        best_val = val_loss
                        no_improve = 0
                    else:
                        no_improve += 1
                    if no_improve >= patience:
                        print(
                            f"\n[early_stop] no improvement for {no_improve} evals "
                            f"(patience={patience}). Stopping training."
                        )
                        global_step = max_steps
                        break

    pbar.close()
    final_ckpt = save_checkpoint(
        ckpt_dir=out_dir,
        step=global_step,
        model=model,
        optimizer=optimizer,
        scaler=scaler,
        best_val=best_val,
        no_improve_evals=no_improve,
        config=cfg,
    )
    print("Training completed.")
    print(f"Optimizer used: {optimizer_name}")
    print(f"Final checkpoint: {final_ckpt}")


def main() -> None:
    try:
        train()
    except Exception as exc:
        print("Component 5 training failed.")
        print(f"What went wrong: {exc}")
        print(
            "Fix suggestion: lower max_seq_len, keep micro_batch_size=1, "
            "increase grad_accum_steps, and verify checkpoint/output paths."
        )
        raise SystemExit(1)


if __name__ == "__main__":
    main()