"""generate_thumbnail task: PIL/ffmpeg thumbnail generation on the thumbnail queue.

Lives separately from import_file because thumbnails can be regenerated en
masse (FC-2c adds the 'regenerate all' admin action) and they're CPU-bound
so they deserve their own queue lane.
"""

from pathlib import Path

from sqlalchemy.exc import DBAPIError, OperationalError

from ..celery_app import celery
from ..models import ImageRecord
from ..services.importer import is_video
from ..services.thumbnailer import Thumbnailer
from ._sync_engine import sync_session_factory as _sync_session_factory

IMAGES_ROOT = Path("/images")

THUMB_MAGIC_JPEG = b"\xff\xd8\xff"
THUMB_MAGIC_PNG = b"\x89PNG\r\n\x1a\n"


def _thumb_is_valid(path: Path) -> bool:
    """Return True iff `path` exists and starts with a JPEG or PNG magic header.

    The on-disk thumbnail format is set by services/thumbnailer.py — JPEG for
    opaque sources, PNG for alpha sources. Anything else (missing file, OSError,
    truncated, wrong magic) is invalid.
    """
    try:
        with path.open("rb") as f:
            head = f.read(12)
    except OSError:
        return False
    if len(head) < 8:
        return False
    if head[:3] == THUMB_MAGIC_JPEG:
        return True
    if head[:8] == THUMB_MAGIC_PNG:
        return True
    return False


@celery.task(
    name="backend.app.tasks.thumbnail.generate_thumbnail",
    bind=True,
    autoretry_for=(OperationalError, DBAPIError, OSError),
    retry_backoff=5,
    retry_backoff_max=60,
    retry_jitter=True,
    max_retries=3,
    soft_time_limit=120,
    time_limit=180,
)
def generate_thumbnail(self, image_id: int) -> dict:
    SessionLocal = _sync_session_factory()
    with SessionLocal() as session:
        record = session.get(ImageRecord, image_id)
        if record is None:
            return {"status": "missing", "image_id": image_id}

        thumbnailer = Thumbnailer(images_root=IMAGES_ROOT)
        source = Path(record.path)
        try:
            if is_video(source):
                result = thumbnailer.generate_video_thumbnail(source, record.sha256)
            else:
                result = thumbnailer.generate_image_thumbnail(source, record.sha256)
        except Exception as exc:  # pragma: no cover — thumbnail failure is non-fatal
            return {"status": "failed", "image_id": image_id, "error": f"{type(exc).__name__}: {exc}"}

        record.thumbnail_path = str(result.path)
        session.add(record)
        session.commit()
        return {"status": "ok", "image_id": image_id, "path": str(result.path)}