Unswizzle 24-bit color w/ Alpha

import png, struct, os, math, pdb
import sys

xy = True  #Set to false for yx swizzle order

def chunks(l, n):
    """Yield successive n-sized chunks from l."""
    for i in range(0, len(l), n):
        yield l[i:i + n]

def unswizzle(writebuf, readbuf, writeoffset, segwidth, segheight, datawidth):
    global readoffset, buffer
##    print(segwidth, segheight)
    if segwidth == 2 and segheight == 2:
        if xy:
            writebuf[writeoffset:writeoffset+8] = readbuf[readoffset:readoffset+8]
            writebuf[writeoffset + datawidth:writeoffset + datawidth + 8] = readbuf[readoffset + 8:readoffset + 16]
        else:
            writebuf[writeoffset:writeoffset+4] = readbuf[readoffset:readoffset+4]
            writebuf[writeoffset + datawidth:writeoffset + datawidth+4] = readbuf[readoffset+4:readoffset+8]
            writebuf[writeoffset+4:writeoffset+8] = readbuf[readoffset+8:readoffset+12]
            writebuf[writeoffset + datawidth + 4:writeoffset + datawidth + 8] = readbuf[readoffset+12:readoffset+16]
        readoffset += 16
    else:
        if xy:
            unswizzle(writebuf, readbuf, writeoffset, segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + segwidth * 4 // 2, segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + datawidth * (segheight // 2), segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + datawidth * (segheight // 2) + segwidth * 4 // 2, segwidth // 2, segheight // 2, datawidth)
        else:
            unswizzle(writebuf, readbuf, writeoffset, segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + datawidth * (segheight // 2), segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + segwidth * 4 // 2, segwidth // 2, segheight // 2, datawidth)
            unswizzle(writebuf, readbuf, writeoffset + datawidth * (segheight // 2) + segwidth * 4 // 2, segwidth // 2, segheight // 2, datawidth)

with open('ch01.png.phyre', 'rb') as f:
    f.seek(0xF8F)
    bitmap = f.read()
width = 1024
height = 1024
buffer = bytearray(len(bitmap))         #Output buffer
readoffset = 0                          #Input position counter
if width == height:
    unswizzle(buffer, bitmap, 0, width, height, width * 4)
elif width > height:
    for w in range(0, width, height):
        unswizzle(buffer, bitmap, w, height, height, width * 4)
elif height > width:
    for h in range(0, height * width, width ** 2):
        unswizzle(buffer, bitmap, h, width, width, width * 4)
else:
    print('error')
p = list(chunks(buffer, width * 4))
with open('output1.png', 'wb') as f:
    w = png.Writer(width, height, alpha=True)
    w.write(f, p)