decompress.py

import sys

def main():
    input_file = sys.argv[1]
    output_file = sys.argv[2]

    with open(input_file, 'rb') as f_in, open(output_file, 'wb') as f_out:
        decompressed_length = int.from_bytes(f_in.read(4), 'little')
        bytes_written = 0

        output_buffer = [0] * 4096
        output_buffer_pos = 0xFEE

        def get_byte(offset):
            nonlocal output_buffer

            return output_buffer[offset % len(output_buffer)]

        def write_byte(b):
            nonlocal output_buffer, output_buffer_pos, bytes_written, f_out

            output_buffer[output_buffer_pos] = b

            output_buffer_pos += 1
            output_buffer_pos %= len(output_buffer)

            bytes_written += 1

            f_out.write(bytes([b]))

        while True:
            # >= to catch an over-run, if we're unlucky enough for one to
            # happen
            if bytes_written >= decompressed_length:
                return

            control = ord(f_in.read(1))
            control_bits = [(control >> i) & 1 for i in range(8)]

            for control_bit in control_bits:
                # same deal as above
                if bytes_written >= decompressed_length:
                    return

                if control_bit == 1:
                    # literal
                    literal = ord(f_in.read(1))

                    write_byte(literal)
                else:
                    # reference
                    reference = int.from_bytes(f_in.read(2), 'big')
                    offset = (((reference >> 4) & 0xF) << 8) | (reference >> 8)
                    length = (reference & 0xF) + 3

                    for i in range(length):
                        write_byte(get_byte(offset + i))

if __name__ == '__main__':
    main()