"""Shared upload validation: size + format constraints applied before any
heavy work (detection, embedding, storage)."""
import io

from PIL import Image, UnidentifiedImageError

ALLOWED_FORMATS = {"JPEG", "PNG", "WEBP"}
MAX_FILE_BYTES = 10 * 1024 * 1024  # 10 MB


class InvalidUploadError(Exception):
    """Raised when an uploaded file fails size/format validation."""


def decode_image(raw: bytes) -> Image.Image:
    """Decode + validate raw upload bytes. Raises InvalidUploadError on any failure."""
    if len(raw) > MAX_FILE_BYTES:
        mb = len(raw) / (1024 * 1024)
        raise InvalidUploadError(
            f"File is {mb:.1f} MB; max is {MAX_FILE_BYTES // (1024 * 1024)} MB."
        )
    try:
        pil = Image.open(io.BytesIO(raw))
        pil.load()
    except (UnidentifiedImageError, OSError) as exc:
        raise InvalidUploadError("File is not a valid image.") from exc
    if pil.format not in ALLOWED_FORMATS:
        raise InvalidUploadError(
            f"Format {pil.format!r} not allowed. Use JPEG, PNG, or WebP."
        )
    return pil