A simple noisy channel that flips a single bit in a block of N with probability P.

channel.py

# A command-line noisy channel
# 
# USAGE: python channel.py BLOCK_SIZE FLIP_PROBABILITY
#
# EXAMPLE:
#    $ echo "0000000" | python channel.py 7 0.8
#    0000100
#
# AUTHOR: Mark Reid <[email protected]>
# CREATED: 2013-10-21
import sys
import random

def flip(c):
    """Return 0 if given character c is 1 and return 1 if c is 0"""
    if c == "0":
        return "1"
    elif c == "1":
        return "0"
    else:
        print("ERROR: Not a bit!")

def corrupt(s,k,p):
    """For each block of k bits, flip a single bit in k with probability p"""
    while len(s) > 0:
        chunk = list(s[0:k])
        if p > random.random():
            i = random.randint(0,len(chunk)-1)
            chunk[i] = flip(chunk[i])

        sys.stdout.write(''.join(chunk))
        s = s[k:]

if __name__ == "__main__":
    if len(sys.argv) < 2:
        print("USAGE: python channel.py BLOCK_SIZE FLIP_PROBABILITY")
    else:
        p = float(sys.argv.pop())
        k = int(sys.argv.pop())
        input_string = sys.stdin.read().strip()
        corrupt(input_string,k,p)