unpack_bsdiff.py

import sys, struct, subprocess

fd = open(sys.argv[1], "rb")

fd.seek(0x3c, 0)

hdr = fd.read(12)

magic, blocksize = struct.unpack(">4sQ", hdr)
assert magic == b"pbzx"

outfd = open(sys.argv[2], "wb")

while True:
    hdr = fd.read(16)
    if not hdr:
        break

    uncompressed_size, compressed_size = struct.unpack(">QQ", hdr)
    
    print(f"{compressed_size:#x} -> {uncompressed_size:#x}")

    blk = fd.read(compressed_size)
    print(f"@{fd.tell():#x}")
    
    if uncompressed_size == compressed_size:
        outfd.write(blk)
    else:
        ret = subprocess.run(["xzcat"], input=blk, capture_output=True, check=True)
        outfd.write(ret.stdout)