"""Content-hash helpers for integrity / tamper verification.

A tiny, dependency-free module so the *same* hashing code backs two controls:

- **ADR-0011** (manual-upload safety gate) records ``sha256`` in an upload's
  provenance at validation time.
- **ADR-0010** (dataset integrity on load) will reuse :func:`verify_sha256` to
  refuse a tampered file at ``resolve_to_local_path`` time.

Streamed so a multi-GB h5ad never has to be read into memory at once.
"""
from __future__ import annotations

import hashlib
from pathlib import Path

# 4 MiB — matches the download chunk size used in src/core/data_io.py.
_DEFAULT_CHUNK = 4 * 1024 * 1024


def compute_sha256(path: str | Path, chunk_size: int = _DEFAULT_CHUNK) -> str:
    """Return the hex SHA-256 of the file at ``path``, read in chunks.

    Raises ``FileNotFoundError`` if the path does not exist (callers that stage
    a file should have materialized it first).
    """
    h = hashlib.sha256()
    with open(path, "rb") as fh:
        for chunk in iter(lambda: fh.read(chunk_size), b""):
            h.update(chunk)
    return h.hexdigest()


def verify_sha256(path: str | Path, expected: str, chunk_size: int = _DEFAULT_CHUNK) -> bool:
    """Return True iff the file at ``path`` hashes to ``expected`` (case-insensitive).

    A missing/empty ``expected`` returns False — an absent baseline is not a
    pass. (ADR-0010 handles "no baseline recorded" as a distinct, explicit case
    at its call site; this helper only answers "does it match".)
    """
    if not expected:
        return False
    return compute_sha256(path, chunk_size).lower() == str(expected).strip().lower()