"""Library scanning and the stable per-book key shared across progress.py, blocks.py and
remote.py."""

from __future__ import annotations

import hashlib
import re
from pathlib import Path

# Books downloaded from the server are named "<server id, 4+ digits> - <title>.epub".
_SERVER_FILE_RE = re.compile(r"^(\d{4,7}) - ")


def server_id_of(path: Path) -> int | None:
    m = _SERVER_FILE_RE.match(path.name)
    return int(m.group(1)) if m else None


def server_key(server_id: int) -> str:
    return f"s{server_id}"


def _book_id(path: Path) -> str:
    """A book's key for reading state: the server's id for a downloaded book (the same whether
    or not, and under whatever name, the file is there), else a hash of its path."""
    sid = server_id_of(path)
    if sid is not None:
        return server_key(sid)
    return hashlib.sha256(str(path.resolve()).encode()).hexdigest()[:16]


def scan_library(library_dir: Path) -> list[Path]:
    if not library_dir.exists():
        return []
    return sorted(library_dir.rglob("*.epub"))