#! /usr/bin/env python

"""Encodes and decode text via Caesar (rot13) rotation.

This program filters text through rot13 translation.  Typically it
reads text from standard input and writes to standard output.  With
the -f option, it will read input from the file, but still writes to
standard output.

Usage: %(program)s [-f file] [-h]

Where:

    --help
    -h        - print this message

    file      - optional file name to read, otherwise standard input is
                read.
"""

import sys
import string
import getopt

program = sys.argv[0]

def usage(status, msg=''):
    stdout = sys.stdout
    try:
	sys.stdout = sys.stderr
	if msg:
	    print msg
	print __doc__ % globals()
    finally:
	sys.stdout = stdout
	sys.exit(status)


def main():
    table = string.maketrans(
	'nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM',
	'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ')

    try:
	opts, args = getopt.getopt(sys.argv[1:], 'hf:', ['file=', 'help'])
    except getopt.error, msg:
	usage(1, msg)

    # default values
    help = 0
    file = None
    
    for opt, arg in opts:
	if opt in ('-h', '--help'):
	    help = 1
	elif opt in ('-f', '--file'):
	    file = arg

    # sanity checks
    if args:
	usage(1)

    if help:
	usage(0)

    if file:
	try:
	    fp = open(file, 'r')
	    text = fp.read()
	    fp.close()
	except IOError, (errno, msg):
	    print '%s: %s' % (msg, file)
	    sys.exit(1)
    else:
	text = sys.stdin.read()

    print string.translate(text, table)


if __name__ == '__main__':
    main()