If you should ever find yourself in the position of having to figure out how to compress XFDL files with the asc-gzip encoding, and I don’t wish it on you, here’s Python code to do it. Obviously, you’ll need some imports and error handling and optimization.

This thread gave me pointers to figure this out; Bryan was just a little off because he was using a gzip library rather than a zlib one.

# compress according to wacky XFDL compression scheme
def compress(fc):
    CHUNK_SIZE = 60000
    out = ''
    for i in range(0, len(fc), CHUNK_SIZE):
        chunk = fc[i:i + CHUNK_SIZE]
        chunklen = len(chunk)
        compressedchunk = zlib.compress(chunk)
        compressedchunklen = len(compressedchunk)
        out += chr(compressedchunklen / 256)
        out += chr(compressedchunklen % 256)
        out += chr(chunklen / 256)
        out += chr(chunklen % 256)
        out += compressedchunk
        
    f = StringIO.StringIO()
    f.write('application/x-xfdl;content-encoding="asc-gzip"n')
    b64 = base64.standard_b64encode(out)
    
    for i in range(0, len(b64), 76):
        f.write(b64[i:i+76])
        f.write('rn')
    ret = f.getvalue()
    f.close()
    return ret