"""The media-kind discriminator for a library asset — the single home of one invariant.

`MediaAsset.media_kind` is `"video" | "audio" | "file"`. It decides which library actions apply:
media (video/audio) gets play/edit/share; a generic uploaded file gets download + share only, and
is never rendered inline (see the force-download serving in the file/watch routes). Every finalize
path computes it HERE — the download worker + the upload ingest via `derive`, the editor by
carrying the source's kind over — so the rule lives in code, not in a comment that can drift (an
earlier ad-hoc `vcodec is None and acodec is not None` audio check lived only in links.py).
"""

VIDEO = "video"
AUDIO = "audio"
FILE = "file"


def derive(vcodec: str | None, acodec: str | None) -> str:
    """Media kind from the probed codecs: a video stream -> video, audio-only -> audio, neither
    (a non-media upload: pdf/zip/docx/…) -> file."""
    if vcodec:
        return VIDEO
    if acodec:
        return AUDIO
    return FILE