"""Real, decodable images built with the standard library, for tests."""

from __future__ import annotations

import struct
import zlib


def png(width: int = 2, height: int = 2) -> bytes:
    """A valid grey PNG of ``width`` x ``height``."""
    def chunk(kind: bytes, body: bytes) -> bytes:
        return (struct.pack(">I", len(body)) + kind + body
                + struct.pack(">I", zlib.crc32(kind + body) & 0xFFFFFFFF))

    rows = b"".join(b"\x00" + b"\x80" * width for _ in range(height))
    return (b"\x89PNG\r\n\x1a\n"
            + chunk(b"IHDR", struct.pack(">IIBBBBB", width, height, 8, 0, 0, 0, 0))
            + chunk(b"IDAT", zlib.compress(rows)) + chunk(b"IEND", b""))