%% Ceasar cypher in Erlang for the rosetta code wiki.
%% Implemented by J.W. Luiten

-module(ceasar).
-export([main/2]).

%% rot: rotate Char by Key places
rot(Char,Key) when (Char >= $A) and (Char =< $Z) or
                   (Char >= $a) and (Char =< $z) ->
  Offset = $A + Char band 32,
  N = Char - Offset,
  Offset + (N + Key) rem 26;
rot(Char, _Key) ->
  Char.

%% key: normalize key.
key(Key) when Key < 0 ->
  26 + Key rem 26;
key(Key) when Key > 25 ->
  Key rem 26;
key(Key) ->
  Key.

main(PlainText, Key) ->
  Encode = key(Key),
  Decode = key(-Key),

  io:format("Plaintext ----> ~s~n", [PlainText]),

  CypherText = lists:map(fun(Char) -> rot(Char, Encode) end, PlainText),
  io:format("Cyphertext ---> ~s~n", [CypherText]),

  PlainText = lists:map(fun(Char) -> rot(Char, Decode) end, CypherText).