'use strict';

var ALPHABET = 'abcdefghijklmnopqrstuvwxyz';

function randomKey() {

var i, result = '';

for ( i = 0; i < 100; i++ ) {

result += ALPHABET[randomUpTo(ALPHABET.length)];

}

return result;

}

function randomUpTo(n) {

return Math.floor(Math.random() * n);

}

module.exports = function (userDefinedKey) {

var key;

function addEncodedCharacter(character, index, array) {

/*jshint validthis:true */

var i = ALPHABET.indexOf(character) + ALPHABET.indexOf(key[index]);

if (i >= ALPHABET.length) { i -= ALPHABET.length; }

this.push(ALPHABET[i]);

}

function addDecodedCharacter(character, index, array) {

/*jshint validthis:true */

var i = ALPHABET.indexOf(character) - ALPHABET.indexOf(key[index]);

if (i < 0) { i += ALPHABET.length; }

this.push(ALPHABET[i]);

}

this.encode = function (plaintext) {

var characters = [];

plaintext.split('').forEach( addEncodedCharacter, characters );

return characters.join('');

};

this.decode = function (ciphertext) {

var characters = [];

ciphertext.split('').forEach( addDecodedCharacter, characters );

return characters.join('');

};

this.key = userDefinedKey || randomKey();

key = this.key;

if (userDefinedKey === '' || key.match(/[\dA-Z]/)) {

throw new Error('Bad key');

}

};