"""Training execution helper (Quick Start)."""

from pathlib import Path
from typing import Optional

import torch
import torch.nn as nn
from torch.utils.data import DataLoader

from llm_lab.config import TrainConfig
from .trainer import Trainer
from .checkpoint import CheckpointManager
from llm_lab.utils import auto_configure, set_seed


def _setup_and_train(
    model: nn.Module,
    train_dataloader: DataLoader,
    val_dataloader: Optional[DataLoader],
    config: TrainConfig,
    seq_len: int,
    auto_config: bool,
    fallback_checkpoint_dir: str,
) -> Trainer:
    """Shared setup logic: auto-config, Drive check, seed, train."""
    if auto_config:
        config = auto_configure(config)

    # Check Google Drive mount (Colab)
    if "/content/drive" in config.checkpoint_dir:
        drive_path = Path("/content/drive/MyDrive")
        if not drive_path.exists():
            print("\n⚠️ Google Drive is not mounted!")
            print("  Run in Colab: from google.colab import drive; drive.mount('/content/drive')")
            print("  Switching to local path.")
            config.checkpoint_dir = fallback_checkpoint_dir

    set_seed(config.seed)

    trainer = Trainer(model, train_dataloader, val_dataloader, config, seq_len)
    trainer.train()

    return trainer


def start_training(
    model: nn.Module,
    train_dataloader: DataLoader,
    val_dataloader: Optional[DataLoader] = None,
    config: Optional[TrainConfig] = None,
    seq_len: int = 2048,
    auto_config: bool = True,
) -> Trainer:
    """Starts training (one-line execution).

    Usage (Colab):
    ```python
    from model import LLMModel, ModelConfig
    from data_pipeline import setup_data_pipeline, DataConfig
    from trainer import start_training, TrainConfig

    # 1. Create model
    model_config = ModelConfig.base_1b()
    model = LLMModel(model_config)

    # 2. Data pipeline
    tok, train_dl, val_dl = setup_data_pipeline("pretrained")

    # 3. Start training (automatic checkpoint restoration)
    trainer = start_training(model, train_dl, val_dl)
    ```
    """
    return _setup_and_train(
        model=model,
        train_dataloader=train_dataloader,
        val_dataloader=val_dataloader,
        config=config or TrainConfig(),
        seq_len=seq_len,
        auto_config=auto_config,
        fallback_checkpoint_dir="./checkpoints",
    )


def start_cpt(
    model: nn.Module,
    train_dataloader: DataLoader,
    val_dataloader: Optional[DataLoader] = None,
    config: Optional[TrainConfig] = None,
    pretrained_checkpoint_dir: str = "/content/drive/MyDrive/llm-1b-lab/checkpoints",
    seq_len: int = 2048,
    auto_config: bool = True,
) -> Trainer:
    """Starts Continual Pre-Training from a pretrained checkpoint.

    Loads pretrained model weights, creates a fresh optimizer,
    and trains with the new config (typically lower LR + new data).

    Args:
        model: The LLM model (same architecture as the pretrained checkpoint).
        train_dataloader: Mixed data dataloader (code + general).
        val_dataloader: Validation dataloader.
        config: CPT training config. Defaults to TrainConfig.code_cpt_1b().
        pretrained_checkpoint_dir: Path to the base pretrained checkpoint directory.
        seq_len: Sequence length (should match model config).
        auto_config: Whether to auto-detect GPU and adjust settings.

    Returns:
        Trainer instance after training completes.

    Usage (Colab):
    ```python
    from llm_lab.config import ModelConfig, DataConfig, TrainConfig
    from llm_lab.model import LLMModel
    from llm_lab.data import setup_cpt_data_pipeline
    from llm_lab.training import start_cpt

    model = LLMModel(ModelConfig.base_1b())
    tok, train_dl, val_dl = setup_cpt_data_pipeline()
    trainer = start_cpt(model, train_dl, val_dl)
    ```
    """
    # Load pretrained weights (model only, no optimizer)
    CheckpointManager.load_model_only(
        model=model,
        checkpoint_dir=pretrained_checkpoint_dir,
        device=torch.device("cuda" if torch.cuda.is_available() else "cpu"),
    )

    # Note: Trainer._try_resume() will look in the CPT checkpoint dir.
    # If no CPT checkpoint exists yet, it starts from step 0 with
    # the pretrained weights we just loaded.
    return _setup_and_train(
        model=model,
        train_dataloader=train_dataloader,
        val_dataloader=val_dataloader,
        config=config or TrainConfig.code_cpt_1b(),
        seq_len=seq_len,
        auto_config=auto_config,
        fallback_checkpoint_dir="./checkpoints-cpt-code",
    )