atbash.rb

=begin
Atbash cipher
The Atbash cipher is a type of ancient encryption created to encode messages in Hebrew.

At its heart is a very simple substitution cipher that transposes all the letters in the alphabet backwards. The first letter in the alphabet is replaced by the the last letter so A -> Z, B -> Y etc.

Plain:  abcdefghijklmnopqrstuvwxyz
Cipher: zyxwvutsrqponmlkjihgfedcba
It is obviously a very weak encryption mechanism!

It is also an example of a symmetric encryption - the same key (our algorithm in this case) is used to both encrypt and decrypt the message.

Examples
Encoding test gives gvhg
Encoding gvhg gives test
Your algorithm should

convert all input to lower case
preserve spaces (don't convert them)
output the encoded message
=end

input_text = ARGV[0].downcase.chars # Take input and convert to array
alphabet = ('a'..'z').to_a
cipher = alphabet.reverse # Create each part of the cipher
cipher_hash = Hash[*alphabet.zip(cipher).flatten] # Marry up the matching values and convert into a hash
output = ''

input_text.each do | letter |
	# If we find the letter in the hash we use if, if not its a special character and we add that in as is
	cipher_hash[letter] != nil ? output += cipher_hash[letter] : output += letter
end

puts output