adaptive-arithmetic-compress.py

# 
# Compression application using adaptive arithmetic coding
# 
# Usage: python adaptive-arithmetic-compress.py InputFile OutputFile
# Then use the corresponding adaptive-arithmetic-decompress.py application to recreate the original input file.
# Note that the application starts with a flat frequency table of 257 symbols (all set to a frequency of 1),
# and updates it after each byte encoded. The corresponding decompressor program also starts with a flat
# frequency table and updates it after each byte decoded. It is by design that the compressor and
# decompressor have synchronized states, so that the data can be decompressed properly.
# 
# Copyright (c) Project Nayuki
# MIT License. See readme file.
# https://www.nayuki.io/page/reference-arithmetic-coding
# 

import contextlib, sys
import arithmeticcoding


# Command line main application function.
def main(args):
	# Handle command line arguments
	if len(args) != 2:
		sys.exit("Usage: python adaptive-arithmetic-compress.py InputFile OutputFile")
	inputfile, outputfile = args
	
	# Perform file compression
	with open(inputfile, "rb") as inp, \
			contextlib.closing(arithmeticcoding.BitOutputStream(open(outputfile, "wb"))) as bitout:
		compress(inp, bitout)


def compress(inp, bitout):
	initfreqs = arithmeticcoding.FlatFrequencyTable(257)
	freqs = arithmeticcoding.SimpleFrequencyTable(initfreqs)
	enc = arithmeticcoding.ArithmeticEncoder(32, bitout)
	while True:
		# Read and encode one byte
		symbol = inp.read(1)
		if len(symbol) == 0:
			break
		enc.write(freqs, symbol[0])
		freqs.increment(symbol[0])
	enc.write(freqs, 256)  # EOF
	enc.finish()  # Flush remaining code bits


# Main launcher
if __name__ == "__main__":
	main(sys.argv[1 : ])