#!/usr/bin/env python3
# build: assemble the installable GRUB theme into dist/udt/.
# Copyright (C) 2026 Danilo M. <danix@danix.xyz>
# Licensed under the GNU General Public License v2 only.
"""Assemble dist/udt/ from the theme.txt udt-palette rendered.

Usage: build [--selftest]

Writes the PNG slices, the .pf2 fonts and a GRUB-readable copy of the
current wallpaper next to theme.txt. Everything is staged in a temporary
directory and swapped in at the end, so a failed run leaves the previous
dist/udt/ untouched. Never touches /boot: root copies dist/udt/ there.
"""
import struct
import sys
import zlib

PNG_SIG = b"\x89PNG\r\n\x1a\n"


def chunk(tag, data):
    return (struct.pack(">I", len(data)) + tag + data
            + struct.pack(">I", zlib.crc32(tag + data)))


def png(w, h, rgba):
    """A w x h PNG of one colour: 8-bit RGBA, not interlaced, which GRUB reads."""
    row = b"\0" + bytes(rgba) * w  # filter byte 0, then the pixels
    return (PNG_SIG
            + chunk(b"IHDR", struct.pack(">IIBBBBB", w, h, 8, 6, 0, 0, 0))
            + chunk(b"IDAT", zlib.compress(row * h))
            + chunk(b"IEND", b""))


def unpng(data):
    """Decode what png() writes: (width, height, set of RGBA pixels)."""
    assert data[:8] == PNG_SIG
    pos, idat = 8, b""
    while pos < len(data):
        n, tag = struct.unpack(">I4s", data[pos:pos + 8])
        body = data[pos + 8:pos + 8 + n]
        assert struct.unpack(">I", data[pos + 8 + n:pos + 12 + n])[0] == zlib.crc32(tag + body)
        if tag == b"IHDR":
            w, h = struct.unpack(">II", body[:8])
        elif tag == b"IDAT":
            idat += body
        pos += 12 + n
    raw = zlib.decompress(idat)
    stride = 1 + 4 * w
    px = set()
    for y in range(h):
        row = raw[y * stride:(y + 1) * stride]
        assert row[0] == 0
        px |= {tuple(row[1 + 4 * x:5 + 4 * x]) for x in range(w)}
    return w, h, px


def selftest():
    w, h, px = unpng(png(3, 2, (0x12, 0x34, 0x56, 0xd9)))
    assert (w, h, px) == (3, 2, {(0x12, 0x34, 0x56, 0xd9)}), (w, h, px)
    print("selftest ok")


def main():
    if sys.argv[1:] == ["--selftest"]:
        return selftest()


if __name__ == "__main__":
    main()