compression.py

import zlib
import struct
import sys

# 4 byte header
# crc32 uLong 
#
# 0   1
# +---+---+
# |CMF|FLG|
# +---+---+
# bits 0 to 3  CM     Compression method
# bits 4 to 7  CINFO  Compression info

def comp_data(data, cvalue=9):
	'''
	Takes in a string and computes
	the comp obj.
	data = string wanting compression
	cvalue = 0-9 comp value (default 6)
	'''
	cdata = zlib.compress(data,cvalue)
	return cdata

def crc32_data(data):
	'''
	Takes in a string and computes crc32 value.
	data = string before compression

	returns:
	HEX bytes of data
	'''
	crc = zlib.crc32(data) & 0xFFFFFFFF
	return crc

def build_header(data, crc):
	'''
	Takes comp data, org crc32 value,
	and adds self header.
	data =  comp data
	crc = crc32 value
	'''
	header = struct.pack("!I",crc)
	built_data = header + data
	return built_data

def dec_data(data):
	'''
	Takes:
	Custom / standard header data
	data = comp data with zlib header

	returns:
	dict with crc32 cheack and dec data string
	ex. {"crc32" : true, "dec_data" : "-SNIP-"}
	'''
	comp_crc32 = struct.unpack("!I", data[:4])[0]
	dec_data = zlib.decompress(data[4:])
	dec_crc32 = zlib.crc32(dec_data) & 0xFFFFFFFF
	if comp_crc32 == dec_crc32:
		crc32 = True
	else:
		crc32 = False
	return { "header_crc32" : comp_crc32, "dec_crc32" : dec_crc32, "crc32_check" : crc32, "data" : dec_data }

data = "Killswitch-GUI is a sick handle"
start_crc32 = crc32_data(data)
print "-Starting str: ", data
print "-un-compressed data size: ", str(sys.getsizeof(data))
print "-Start crc32 value: " + str(start_crc32)
comp_data = comp_data(data)
print "-Compression data size: " + str(sys.getsizeof(comp_data))
final_comp_data = build_header(comp_data, start_crc32)
print "-Compression data with CRC header built size: " + str(sys.getsizeof(final_comp_data))
print "-Starting decompress of data!"
dec_data = dec_data(final_comp_data)
print "-Final return data dict: ", dec_data