main.py

#!/usr/bin/env python
#
# Decode binary message in Bush's 'Sixteen Stone' album cover.
#

PO = False # 'print out'

#
# test.data has ASCII chars 7 bits long. To decode, set DATA_FILE to
# "test.data", bit_size to 7 and do not reverse bits.
#

DATA_FILE = "cover.data"

def get_file_lines():
    fh = open(DATA_FILE, "r")
    lines = []
    for line in fh:
        lines.append(line)
    fh.close()
    return lines

def get_file_string(reverse=False):
    lines = []
    with open(DATA_FILE, "r") as fh:
        for line in fh:
            lines.append(line.strip())
    lines = reverse and reversed(lines) or lines
    return "".join(lines)

def get_char(binary, one_or_zero):
    binary = binary.replace("?", one_or_zero)
    intval = int(binary, 2)
    char = chr(intval)
    return char, intval

def get_letter(binary):
    char0, intval0 = get_char(binary, "0")
    char1, intval1 = get_char(binary, "1")
    char = None
    intval = None
    if char0 == char1:
        char = char0
        intval = intval0
    else:
        char = "({}|{})".format(char0, char1)
        intval = [intval0, intval1]
    if PO: print "Binary:", binary, "Int:", intval, "Char:", char
    return char

def rev(val):
    return val[::-1]

def alg_num1(string, bit_size):
    if PO: print "String:", string, "len:", len(string)
    start = bit_size * -1
    offset = len(string)
    letters = []
    while offset > 0:
        bits = string[start:offset]
        if not bits: break
        #print "bits:", bits, "start:", start, "offset:", offset, "size:", bit_size
        letters.append(get_letter(bits))
        start = start - bit_size
        offset = offset - bit_size
    print "Message:", "".join(letters)

def alg_num2(string, bit_size):
    if PO: print "String:", string, "len:", len(string)
    start = bit_size * -1
    offset = len(string)
    letters = []
    while offset > 0:
        bits = string[start:offset]
        if not bits: break
        #print "bits:", bits, "start:", start, "offset:", offset, "size:", bit_size
        letters.append(get_letter(bits))
        start = start - bit_size
        offset = offset - bit_size
    print "Message:", "".join(letters)

#
# Algorithms
#
# Last line in data:
#   0110111000110
#    
#
# 1. Each chunk is N bits long reading left to right, decoding (of course) from
#    right to left:
#    - If bit is 7, then bits are read as 0110111 000110? <- '?' would contain a bit from the next line, if there was one
#    
# 2. Each chunk is N bits long reading right to left, decoding from right to
#    left:
#    - If bit is 7, then bits are read as 0110001 110110? <- Again, '?' would contain next line's bit, if it existed
#
# An option was added to read the file from either left to right (top to bottom)
# (reverse=False) or read right to left (bottom to top) (reverse=True)
#
# For bits that are unknown, both characters are decoded when the bit is '0'
# or '1'. It is denoted as (L|R) where L denotes if the bit is '0', R denotes
# when the bit is '1'. As an example, you may see something like '(A|B)'. 'A' is
# what the char would be if the bit was '0', 'B' if '1'.
#

print("Alg #1 - File is read forward")
alg_num1(get_file_string(reverse=False), 7)
print("Alg #1 - File is read reversed")
alg_num1(get_file_string(reverse=True), 7)