ROT

#!/usr/bin/env python
import sys
import getopt

class ROT(object):

    def __init__(self):
        self._characterList = self._generate()

    def _generate(self):
        '''
        Generate a list of ASCII characters resembling a-z.
        '''
        start = chr(ord("a"))
        end = chr(ord("z"))
        chacterList = []

        current = start
        while current is not end:
            chacterList.append(current)
            current = chr(ord(current) + 1) # Shift ASCII character one space
        chacterList.append(current)

        return chacterList

    def shift(self, cypher, increment):
        '''
        Using the characterSet as reference, shift every ASCII character in the
        cypher by the given increment.
        '''
        register = {}
        for ascii in self._characterList:
            register[ord(ascii)] = ascii

        plainText = []
        for ascii in cypher:
            ascii = ascii.lower() # Turn all to lowercase.

            cyph = register.get(ord(ascii) + increment, None)
            if not cyph and ord(ascii) != 32: # ASCII 32 is space.
                cyph = register.get((ord(ascii) - 25) + (increment - 1)) # When end of list is reached, jump back to start.
            elif ord(ascii) == 32:
                cyph = " "

            if not cyph:
                pass
            else:
                plainText.append(cyph)
        return plainText

    def length(self):
        '''
        Return the length of the characterset.
        '''
        return len(self._characterList)


usage_doc = '''ROT encryption implementation.
Usage:
    ./rot.py -c <cypher>
Note:
    Current implementation does not allow for any special characters apart from spaces.
    '''
def usage():
    print usage_doc

def main():
    try:
        long_options = ['help', 'cypher=']
        opts, _ = getopt.getopt(sys.argv[1:], "hc:", long_options)
    except getopt.GetoptError:
        usage()
        return -1

    cypher = ""
    for o, a in opts:
        if o in ('-h', '--help'):
            usage()
            return 0
        if o in ('-c', '--cypher='):
            cypher = a

    # Example: hello world (cycle 8)
    if not len(cypher):
        cypher = "zwddg ogjdv!"
    rot = ROT()

    # Do an increment for every character in the lower-case alphabet.
    increment = 0
    while increment is not rot.length():
        plainText = rot.shift(cypher, increment)
        plainString = str(increment) + ": " + ''.join(plainText)
        print plainString
        increment = increment + 1

    plainText = rot.shift(cypher, increment)
    plainString = str(increment) + ": " + ''.join(plainText)
    print plainString

if __name__ == "__main__":
    err_code = main()
    sys.exit(err_code)