"""CUAD dataset loading and contract-level (leakage-safe) splitting."""
from __future__ import annotations

import pandas as pd
from sklearn.model_selection import GroupShuffleSplit

from src.config import CUAD_HF_ID, CUAD_PARQUET, SEED, TEST_SIZE


def load_cuad(force_download: bool = False) -> pd.DataFrame:
    """Load CUAD as a DataFrame. Cached locally as parquet after first run."""
    if CUAD_PARQUET.exists() and not force_download:
        return pd.read_parquet(CUAD_PARQUET)

    try:
        from datasets import load_dataset
    except ImportError as e:
        raise RuntimeError(
            "datasets library required for initial download; install requirements.txt"
        ) from e

    ds = load_dataset(CUAD_HF_ID)
    df = ds["train"].to_pandas()
    CUAD_PARQUET.parent.mkdir(parents=True, exist_ok=True)
    df.to_parquet(CUAD_PARQUET, index=False)
    return df


def contract_group_split(
    df: pd.DataFrame, test_size: float = TEST_SIZE, seed: int = SEED
) -> tuple[pd.DataFrame, pd.DataFrame]:
    """Split by file_name so no contract appears in both train and test.

    Returns (train_df, test_df). Index reset.
    """
    splitter = GroupShuffleSplit(n_splits=1, test_size=test_size, random_state=seed)
    train_idx, test_idx = next(splitter.split(df, groups=df["file_name"]))
    train_df = df.iloc[train_idx].reset_index(drop=True)
    test_df = df.iloc[test_idx].reset_index(drop=True)
    return train_df, test_df


def naive_random_split(
    df: pd.DataFrame, test_size: float = TEST_SIZE, seed: int = SEED
) -> tuple[pd.DataFrame, pd.DataFrame]:
    """Naive row-level shuffle split.

    Provided for the leakage-comparison demo only; do not use for real eval.
    """
    shuffled = df.sample(frac=1.0, random_state=seed).reset_index(drop=True)
    cut = int(len(shuffled) * (1 - test_size))
    return shuffled.iloc[:cut].reset_index(drop=True), shuffled.iloc[cut:].reset_index(drop=True)


def summarize(df: pd.DataFrame) -> dict:
    """Quick descriptive stats used in EDA and tests."""
    return {
        "n_rows": int(len(df)),
        "n_contracts": int(df["file_name"].nunique()),
        "n_labels": int(df["label"].nunique()),
        "mean_clause_chars": float(df["clause"].str.len().mean()),
        "label_top5": df["label"].value_counts().head(5).to_dict(),
    }