This recipe introduces a novel way of compressing text and is meant primarily as an exercise. The procedures work best on standard 7-BIT ASCII and worst on binary encoded data. Please note that function "encode" returns a string and a key that must be passed to function "decode" in order to recover the original data.
Python, 48 lines
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Copy to clipboard1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 | '''Module for string compression.
This module provides two functions for
compressing and decompressing strings.'''
__version__ = '1.0'
import sys as _sys
################################################################################
def encode(string, divide=1024):
'Compress a string.'
def encode(s, k, b):
i = 0
for c in s:
i *= b
i += k.index(c) + 1
s = ''
while i:
s = chr(i % 255 + 1) + s
i /= 255
return s
key = ''.join(byte for byte in map(chr, xrange(256)) if byte in string)
divide = divide * 256 / len(key)
base = len(key) + 1
return '\0'.join(encode(string[index:index+divide], key, base) for index in xrange(0, len(string), divide)), key
def decode(string, key):
'Decompress a string.'
def decode(s, k, b):
i = 0
for c in s:
i *= 255
i += ord(c) - 1
s = ''
while i:
s = k[i % b - 1] + s
i /= b
return s
base = len(key) + 1
return ''.join(decode(string, key, base) for string in string.split('\0'))
################################################################################
if __name__ == '__main__':
_sys.stdout.write('Content-Type: text/plain\n\n')
_sys.stdout.write(file(_sys.argv[0]).read())
|
The second argument to function "encode" is "divide" and is used for tuning the encoding algorithm. Larger numbers may allow better compression at the expense of speed. Internally, the function auto- matically adapts this factor according to type of data received in the first argument.
Tags: algorithms
