"""Data loading, cleaning, and splitting. Pure functions over file paths.
Nothing here trains a model or logs to W&B — that's the train.py boundary."""

from __future__ import annotations

import pandas as pd

from . import config


def load_raw(filename: str) -> pd.DataFrame:
    """Load a raw dataset by filename from data/raw/."""
    path = config.RAW_DIR / filename
    return pd.read_csv(path) if path.suffix == ".csv" else pd.read_parquet(path)


def chronological_split(
    df: pd.DataFrame,
    time_col: str,
    train_frac: float = 0.8,
    val_frac: float = 0.1,
) -> tuple[pd.DataFrame, pd.DataFrame, pd.DataFrame]:
    """Time-ordered train/val/test split. Never shuffle a time series."""
    df_sorted = df.sort_values(time_col).reset_index(drop=True)
    n = len(df_sorted)
    train_end = int(n * train_frac)
    val_end = int(n * (train_frac + val_frac))
    return df_sorted.iloc[:train_end], df_sorted.iloc[train_end:val_end], df_sorted.iloc[val_end:]