"""Split validation helpers."""

from __future__ import annotations

import pandas as pd


def split_cat_sets(split_df: pd.DataFrame) -> tuple[set[str], set[str]]:
    """Return train/test cat sets for the cat-heldout split."""

    train_cats = set(split_df.loc[split_df["cat_heldout_split"] == "train", "cat_id"])
    test_cats = set(split_df.loc[split_df["cat_heldout_split"] == "test", "cat_id"])
    return train_cats, test_cats


def cat_overlap(split_df: pd.DataFrame) -> set[str]:
    """Return cats appearing in both cat-heldout train and test."""

    train_cats, test_cats = split_cat_sets(split_df)
    return train_cats & test_cats


def assert_zero_cat_overlap(split_df: pd.DataFrame) -> None:
    """Raise when the cat-heldout split is not subject-disjoint."""

    overlap = cat_overlap(split_df)
    if overlap:
        raise AssertionError(f"expected zero cat overlap, found {sorted(overlap)}")