#!/usr/bin/env python3
"""Convert an XWD dump (from xwd) into a PNG, without external image libraries.

Usage: xwd -root -silent | python3 xwd2png.py out.png
"""
import struct
import sys
import zlib


def read_header(data):
    fields = struct.unpack(">25I", data[:100])
    names = [
        "header_size", "file_version", "pixmap_format", "pixmap_depth",
        "pixmap_width", "pixmap_height", "xoffset", "byte_order",
        "bitmap_unit", "bitmap_bit_order", "bitmap_pad", "bits_per_pixel",
        "bytes_per_line", "visual_class", "red_mask", "green_mask",
        "blue_mask", "bits_per_rgb", "colormap_entries", "ncolors",
        "window_width", "window_height", "window_x", "window_y",
        "window_bdrwidth",
    ]
    return dict(zip(names, fields))


def write_png(path, width, height, rgb_rows):
    raw = b"".join(b"\x00" + row for row in rgb_rows)

    def chunk(tag, payload):
        return (struct.pack(">I", len(payload)) + tag + payload +
                struct.pack(">I", zlib.crc32(tag + payload) & 0xFFFFFFFF))

    header = struct.pack(">IIBBBBB", width, height, 8, 2, 0, 0, 0)
    png = (b"\x89PNG\r\n\x1a\n" + chunk(b"IHDR", header) +
           chunk(b"IDAT", zlib.compress(raw, 6)) + chunk(b"IEND", b""))
    with open(path, "wb") as handle:
        handle.write(png)


def main():
    out_path = sys.argv[1] if len(sys.argv) > 1 else "out.png"
    data = sys.stdin.buffer.read()
    head = read_header(data)
    width = head["pixmap_width"]
    height = head["pixmap_height"]
    bpp = head["bits_per_pixel"]
    stride = head["bytes_per_line"]
    offset = head["header_size"] + head["ncolors"] * 12
    pixels = data[offset:]

    if bpp == 32:
        rows = []
        for y in range(height):
            line = pixels[y * stride:y * stride + width * 4]
            out = bytearray(width * 3)
            for x in range(width):
                src = x * 4
                dst = x * 3
                out[dst] = line[src + 2]
                out[dst + 1] = line[src + 1]
                out[dst + 2] = line[src]
            rows.append(bytes(out))
    elif bpp == 24:
        rows = []
        for y in range(height):
            line = pixels[y * stride:y * stride + width * 3]
            out = bytearray(width * 3)
            for x in range(width):
                src = x * 3
                dst = x * 3
                out[dst] = line[src + 2]
                out[dst + 1] = line[src + 1]
                out[dst + 2] = line[src]
            rows.append(bytes(out))
    else:
        sys.stderr.write("unsupported bits_per_pixel=%s\n" % bpp)
        return 2

    write_png(out_path, width, height, rows)
    print("%s %dx%d" % (out_path, width, height))
    return 0


if __name__ == "__main__":
    sys.exit(main())