#!/usr/bin/env ruby
# 
# License: BSD
# 
# Copyright (c) 2006-2007, Gregory Fleischer (gfleischer@gmail.com)
# 
# All rights reserved.
# 
# Redistribution and use in source and binary forms, with or without
# modification, are permitted provided that the following conditions
# are met:
#   
#   1. Redistributions of source code must retain the above copyright
#      notice, this list of conditions and the following disclaimer.
#   2. Redistributions in binary form must reproduce the above copyright
#      notice, this list of conditions and the following disclaimer in
#      the documentation and/or other materials provided with the
#      distribution.
#   3. The names of the authors may not be used to endorse or promote
#      products derived from this software without specific prior
#      written permission.
#   
# THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
# IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.
# 

require 'getoptlong'

opts = GetoptLong.new(
                      ['--help', '-h', GetoptLong::NO_ARGUMENT],
                      ['--key', '-k', GetoptLong::REQUIRED_ARGUMENT],
                      ['--encrypt', '-e', GetoptLong::OPTIONAL_ARGUMENT],
                      ['--decrypt', '-d', GetoptLong::OPTIONAL_ARGUMENT]
                      )

def usage
  puts "usage: #{$0} --key <key> --encrypt|--decrypt"
end

encrypt=false
decrypt=false
key=nil
text=nil
opts.each do |opt,arg|
  case opt
  when '--help'
    usage()
    exit 0
  when '--key'
    key = arg.dup()
  when '--encrypt'
    text = arg.dup()
    encrypt = true
  when '--decrypt'
    text = arg.dup()
    decrypt = true
  end
end

if key.nil?
  puts "missing key"
  usage()
  exit 1
end

if not encrypt and not decrypt
  puts "must specify either encrypt or decrypt"
  usage()
  exit 1
end

unless key.match(/^[A-Za-z]+$/)
  puts "invalid key"
  usage()
  exit 1
end

if text.nil? || 0 == text.length
  text = STDIN.read()
end

text.gsub!(/[^a-zA-Z]/) {|c|}

if key.match(/^[a-z]+$/)
  text.downcase! unless text.nil?
  alpha = ('a'..'z').to_a
  offset = 'a'[0]
elsif key.match(/^[A-Z]+$/)
  text.upcase! unless text.nil?
  alpha = ('A'..'Z').to_a
  offset = 'A'[0]
else
  puts "invalid key #{key}"
  exit 1
end

keylen = key.length
i=0
text.each_byte do |c|
  k = key[i % keylen] - offset
  c -= offset
  if encrypt
    print alpha[(c + k) % 26]
  else
    print alpha[(c - k) % 26]
  end
  i += 1
end

puts ""

# eof