"""Library boundary and exclusion policy — the single source used everywhere.

One function decides what is inside the library, what is a supported photo, and
what is excluded (`_IGNORE/` and generated thumbnail/cache directories). Symlinks
that resolve outside the library root are rejected rather than silently followed,
so discovery can never escape the configured boundary.

Extracted from photo_analyzer.discover_photos / SUPPORTED_EXTENSIONS
(see donor_ledger.yaml: pa-discovery).
"""

from __future__ import annotations

import os
from pathlib import Path
from typing import Iterable, Iterator

SUPPORTED_EXTENSIONS = {
    ".jpg", ".jpeg", ".png", ".webp", ".heic", ".heif", ".tiff", ".tif"
}
EXCLUDED_DIR_NAMES = {"_IGNORE", ".@__thumb"}


class PathPolicyError(ValueError):
    """A path violates the library boundary (e.g. a symlink escaping the root)."""


def is_supported(path: os.PathLike | str) -> bool:
    return Path(path).suffix.lower() in SUPPORTED_EXTENSIONS


def is_excluded(path: os.PathLike | str) -> bool:
    return any(part in EXCLUDED_DIR_NAMES for part in Path(path).parts)


def normalize_root(root: os.PathLike | str) -> Path:
    return Path(root).expanduser().resolve()


def resolve_within(root: Path, path: os.PathLike | str) -> Path:
    """Resolve ``path`` (following symlinks) and confirm it stays within ``root``.

    Raises ``PathPolicyError`` if the resolved path escapes the root.
    """
    root = Path(root).resolve()
    resolved = Path(path).resolve()
    if resolved != root and root not in resolved.parents:
        raise PathPolicyError(f"path escapes library root {root}: {path} -> {resolved}")
    return resolved


def iter_supported_files(root: os.PathLike | str) -> Iterator[Path]:
    """Yield supported, non-excluded files under ``root`` in deterministic order.

    Excluded directories are never descended into. A symlinked file whose target
    escapes the root raises ``PathPolicyError`` (unsafe paths fail rather than
    leaking outside the library).
    """
    root = normalize_root(root)
    found: list[Path] = []
    for dirpath, dirnames, filenames in os.walk(root, followlinks=False):
        dirnames[:] = sorted(d for d in dirnames if d not in EXCLUDED_DIR_NAMES)
        for name in filenames:
            candidate = Path(dirpath) / name
            if not is_supported(candidate) or is_excluded(candidate):
                continue
            if candidate.is_symlink():
                resolve_within(root, candidate)  # raises on escape
            found.append(candidate)
    return iter(sorted(found))


def discover(roots: Iterable[os.PathLike | str]) -> list[Path]:
    """Deterministic, de-duplicated discovery across one or more roots."""
    seen: set[Path] = set()
    for root in roots:
        seen.update(iter_supported_files(root))
    return sorted(seen)