"""SHA256 helpers for canonical-indices cache invalidation + result-record
provenance. Computing the digest of every upstream parquet a loader reads,
embedded into ``meta.attrs["data_sha256"]``, makes silent dataset drift
detectable: if a panel parquet mutates, the canonical-indices cache key
changes and any downstream ``RunRecord`` carries a different hash.
"""

from __future__ import annotations

import hashlib
from pathlib import Path


_BUF_SIZE = 1 << 20  # 1 MB


def sha256_file(path: str | Path) -> str:
    """Return the hex SHA-256 of ``path`` (full file)."""
    p = Path(path)
    if not p.exists():
        raise FileNotFoundError(f"sha256_file: {p} does not exist")
    h = hashlib.sha256()
    with p.open("rb") as fh:
        while chunk := fh.read(_BUF_SIZE):
            h.update(chunk)
    return h.hexdigest()


def sha256_dataset(paths: list[str | Path]) -> dict[str, str]:
    """Return ``{path_str: sha256}`` for every existing path in ``paths``."""
    return {str(Path(p)): sha256_file(p) for p in paths}


def sha256_combined(paths: list[str | Path]) -> str:
    """Return a single hex digest combining every file in ``paths``.

    Used as a cache-key suffix for canonical-indices: if ANY upstream
    parquet mutates, the suffix changes and the cache regenerates.
    """
    h = hashlib.sha256()
    for p in sorted(str(Path(x)) for x in paths):
        sub = sha256_file(p).encode()
        h.update(p.encode() + b":" + sub + b"\n")
    return h.hexdigest()[:16]