#!/usr/bin/env python

"""
    code.py - Jim Mahoney, Sep 25, demo for python class

 Encode text using the classic 'ROT13' algorithm,
 which shifts each letter by 13 characters.
 (See for example http://en.wikipedia.org/wiki/ROT13 .)

 This version is fairly primitive, since this is early in the term,
 
 First, it assumes that all input consists *only* of the lowercase
 letters 'a' through 'z'.  (A better version would also do uppercase,
 and skip any other characters.)

 Second, it works on files whose names are hard-coded into this program.
 To see it both encode and decode (which are the same, since 13+13=26),
 it converts
                plain.txt  => coded.txt
 and
                coded.txt  => decoded.txt

 Note the use of subroutines to break the task into several pieces.

 To run it :

   # create plain.txt
   $ ./code.py
   # look at coded.txt and decoded.txt

 To test the rot13 subroutine :

   $ python
   >>> import code
   rot13 :   converting 'plain.txt' to 'encoded.txt'  - done.
   rot13 :   converting 'encoded.txt' to 'decoded.txt'  - done.
   >>> code.rot13('impossible')
   'vzcbffvoyr'
"""    

def rot13(plain_text):
    """convert a string to one shifted by 13 characters"""
    coded_text = ""
    for char in plain_text:
        zero_to_25 = ord(char) - ord('a')
        coded_char = (zero_to_25 + 13) % 26;
        coded_text = coded_text + chr(coded_char + ord('a'))
    return coded_text

def convert(in_filename, out_filename):
    print " rot13 : ", 
    print " converting '" + in_filename + "' to '" + out_filename + "'", 
    input_file = open(in_filename, 'r')
    output_file = open(out_filename, 'w')
    lines = input_file.readlines()
    for line in lines:
        line = line[0:-1]                # remove last newline character
        # print "plain is '" + line + "'"
        coded_line = rot13(line)
        # print "code is '" + coded_line + "'"        
        coded_line = coded_line + "\n"   # add newline back in
        output_file.write(coded_line)
    print " - done."

convert('plain.txt',   'encoded.txt')
convert('encoded.txt', 'decoded.txt')