import re
from dataclasses import dataclass
from pathlib import PurePosixPath


SEMESTER_RE = re.compile(r"^\d+\.Semester$")


@dataclass(frozen=True)
class PathMetadata:
    semester: str
    fach: str
    typ: str | None


def parse_path(file_path: str, ingest_root: str) -> PathMetadata | None:
    """Parse a Nextcloud file path into structured metadata.

    Returns None when the path is outside the ingest root or does not match
    the expected `<root>/<N>.Semester/<Fach>/[<typ>/...]/<file>` pattern.

    Caller must pass a file path. Directory paths (with or without trailing
    slash) produce undefined results — `PurePosixPath` strips trailing slashes,
    so a directory ending in `/foo/` is indistinguishable from a file `/foo`.
    """
    norm_path = file_path.lstrip("/")
    norm_root = ingest_root.strip("/")

    if not norm_path.startswith(norm_root + "/"):
        return None

    relative = norm_path[len(norm_root) + 1:]
    parts = PurePosixPath(relative).parts

    # Layout: [semester, fach, *inner, filename] — minimum 3 parts
    if len(parts) < 3:
        return None

    semester, fach = parts[0], parts[1]

    if not SEMESTER_RE.fullmatch(semester):
        return None

    inner = parts[2:-1]  # everything between fach and filename
    typ = inner[0] if inner else None

    return PathMetadata(semester=semester, fach=fach, typ=typ)