"""Data loader for the encoder subproject.

Reuses the parent's `FinetuneDataset` verbatim — the parent's tokenized arrays
at `data/synthetic/` are already shaped `(N, 64, 15)`, which is exactly what
the per-transaction encoder needs. This module is a thin orchestrator that
resolves data paths (via the `encoder/data/synthetic -> ../../data/synthetic`
symlink), builds train/val/test loaders, and exposes a fingerprint-verification
helper so accidental data regeneration breaks fast.

Why we don't define a new Dataset class: the encoder's input contract is
identical to the parent's (`(B, 64, 15) int64` plus fraud + amount_range
labels). The only thing that changes is what the model does with those
tokens. Keeping the Dataset shared guarantees apples-to-apples comparison.
"""

from __future__ import annotations

from pathlib import Path

import numpy as np
import torch
from torch.utils.data import DataLoader

from src.training.finetune import FinetuneDataset


def load_data_arrays(
    data_dir: Path | str,
) -> tuple[np.ndarray, np.ndarray, np.ndarray | None, dict[str, np.ndarray]]:
    """Load raw token arrays + split indices from `data_dir`.

    Returns:
        token_ids: (N, 64, 15) int16
        sequence_labels: (N,) int8 (fraud)
        ar_targets: (N,) int8 last-transaction amount_range, or None if file absent
        splits: dict with keys 'train' / 'val' / 'test', each int64 indices
    """
    data_dir = Path(data_dir)
    token_ids = np.load(data_dir / "token_ids.npy")
    sequence_labels = np.load(data_dir / "sequence_labels.npy")
    splits = dict(np.load(data_dir / "split_indices.npz"))

    ar_path = data_dir / "amount_range_labels.npy"
    ar_targets: np.ndarray | None = None
    if ar_path.exists():
        # Parent stores per-transaction amount_range as (N, 64). The head
        # targets the LAST transaction's amount bucket, so we slice [:, -1].
        ar_all = np.load(ar_path)
        ar_targets = ar_all[:, -1]

    return token_ids, sequence_labels, ar_targets, splits


def verify_fingerprint(data_dir: Path | str, expected: str) -> None:
    """Raise if data fingerprint differs from `expected`.

    Catches the silent failure where data has been regenerated under us —
    in which case head-to-head comparison numbers against the parent's
    already-published eval.md.json would not be apples-to-apples.
    """
    fp_path = Path(data_dir) / "fingerprint.txt"
    if not fp_path.exists():
        raise FileNotFoundError(
            f"No fingerprint.txt at {fp_path}. Encoder relies on the parent's "
            f"data/synthetic/ for head-to-head; regenerate via parent's "
            f"`python -m scripts.generate` if missing.",
        )
    actual = fp_path.read_text().strip()
    if actual != expected:
        raise ValueError(
            f"Data fingerprint mismatch:\n"
            f"  expected: {expected}\n"
            f"  actual:   {actual}\n"
            f"Data has been regenerated since this config was pinned. Head-to-head "
            f"comparison against the parent's eval.md.json would not be valid.",
        )


def build_loaders(
    data_dir: Path | str,
    batch_size: int = 32,
    label_fraction: float = 1.0,
    seed: int = 42,
    num_workers: int = 4,
) -> tuple[DataLoader, DataLoader, DataLoader]:
    """Build train/val/test DataLoaders.

    Args:
        data_dir: path to the tokenized synthetic arrays (symlink to parent OK).
        batch_size: applied to all three loaders.
        label_fraction: subsample fraction of `train` indices for the
            label-scarcity sweep (1.0 = full, 0.10 = 10%, 0.01 = 1%). Val and
            test are never subsampled.
        seed: RNG seed for the train-subset selection. Same seed as the parent's
            scarcity protocol so the head-to-head selects the same training
            subsets across both architectures.
        num_workers: DataLoader worker count for train. Val/test use half.

    Returns:
        (train_loader, val_loader, test_loader)
    """
    token_ids, sequence_labels, ar_targets, splits = load_data_arrays(data_dir)

    train_indices = splits["train"]
    if label_fraction < 1.0:
        # np.random.RandomState (not Generator) to match the parent's
        # subsampling RNG exactly. Same seed -> identical train subset.
        rng = np.random.RandomState(seed)
        n_keep = max(1, int(len(train_indices) * label_fraction))
        train_indices = rng.choice(train_indices, n_keep, replace=False)

    train_ds = FinetuneDataset(token_ids, sequence_labels, train_indices, ar_targets)
    val_ds = FinetuneDataset(token_ids, sequence_labels, splits["val"], ar_targets)
    test_ds = FinetuneDataset(token_ids, sequence_labels, splits["test"], ar_targets)

    train_loader = DataLoader(
        train_ds,
        batch_size=batch_size,
        shuffle=True,
        num_workers=num_workers,
        pin_memory=torch.cuda.is_available(),
        drop_last=True,
    )
    eval_workers = max(0, num_workers // 2)
    val_loader = DataLoader(
        val_ds,
        batch_size=batch_size,
        shuffle=False,
        num_workers=eval_workers,
        pin_memory=torch.cuda.is_available(),
    )
    test_loader = DataLoader(
        test_ds,
        batch_size=batch_size,
        shuffle=False,
        num_workers=eval_workers,
        pin_memory=torch.cuda.is_available(),
    )
    return train_loader, val_loader, test_loader