"""Icon normalization — anything Pillow can decode becomes a 256x256 PNG.

Intune wants exactly 256x256 PNG, but nothing a human picks (a photo from the
web, a vendor logo, a screenshot) arrives that way. Rather than reject it, fit
it into the box: aspect ratio preserved, transparent letterbox around it.
"""
from __future__ import annotations

import io

from PIL import Image, ImageOps, UnidentifiedImageError

ICON_SIZE = (256, 256)


class ImageError(ValueError):
    """Raised when the bytes are not a decodable raster image."""


def normalize_icon(data: bytes) -> bytes:
    """Return `data` as exactly-256x256 PNG bytes.

    A file that *already* is a 256x256 PNG is returned byte-identical — the
    icon store is content-addressed by SHA256, so re-encoding an unchanged
    upload would move it to a new path on every save for no reason.

    Raises `ImageError` if the bytes cannot be decoded as an image.
    """
    try:
        img = Image.open(io.BytesIO(data))
        img.load()
    except (UnidentifiedImageError, OSError, ValueError) as exc:
        raise ImageError("Not a readable image file.") from exc

    if img.format == "PNG" and img.size == ICON_SIZE:
        return data

    # `contain` scales down to fit inside the box without cropping; a smaller
    # source is left at its own size rather than blown up into mush.
    fitted = ImageOps.contain(img.convert("RGBA"), ICON_SIZE, Image.LANCZOS)

    canvas = Image.new("RGBA", ICON_SIZE, (0, 0, 0, 0))
    canvas.paste(
        fitted,
        ((ICON_SIZE[0] - fitted.width) // 2, (ICON_SIZE[1] - fitted.height) // 2),
    )

    out = io.BytesIO()
    canvas.save(out, format="PNG", optimize=True)
    return out.getvalue()