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Filename : alphabet.js
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'use strict';

var randomFromSeed = require('./random/random-from-seed');

var ORIGINAL = '0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ_-';
var alphabet;
var previousSeed;

var shuffled;

function reset() {
    shuffled = false;
}

function setCharacters(_alphabet_) {
    if (!_alphabet_) {
        if (alphabet !== ORIGINAL) {
            alphabet = ORIGINAL;
            reset();
        }
        return;
    }

    if (_alphabet_ === alphabet) {
        return;
    }

    if (_alphabet_.length !== ORIGINAL.length) {
        throw new Error('Custom alphabet for shortid must be ' + ORIGINAL.length + ' unique characters. You submitted ' + _alphabet_.length + ' characters: ' + _alphabet_);
    }

    var unique = _alphabet_.split('').filter(function(item, ind, arr){
       return ind !== arr.lastIndexOf(item);
    });

    if (unique.length) {
        throw new Error('Custom alphabet for shortid must be ' + ORIGINAL.length + ' unique characters. These characters were not unique: ' + unique.join(', '));
    }

    alphabet = _alphabet_;
    reset();
}

function characters(_alphabet_) {
    setCharacters(_alphabet_);
    return alphabet;
}

function setSeed(seed) {
    randomFromSeed.seed(seed);
    if (previousSeed !== seed) {
        reset();
        previousSeed = seed;
    }
}

function shuffle() {
    if (!alphabet) {
        setCharacters(ORIGINAL);
    }

    var sourceArray = alphabet.split('');
    var targetArray = [];
    var r = randomFromSeed.nextValue();
    var characterIndex;

    while (sourceArray.length > 0) {
        r = randomFromSeed.nextValue();
        characterIndex = Math.floor(r * sourceArray.length);
        targetArray.push(sourceArray.splice(characterIndex, 1)[0]);
    }
    return targetArray.join('');
}

function getShuffled() {
    if (shuffled) {
        return shuffled;
    }
    shuffled = shuffle();
    return shuffled;
}

/**
 * lookup shuffled letter
 * @param index
 * @returns {string}
 */
function lookup(index) {
    var alphabetShuffled = getShuffled();
    return alphabetShuffled[index];
}

function get () {
  return alphabet || ORIGINAL;
}

module.exports = {
    get: get,
    characters: characters,
    seed: setSeed,
    lookup: lookup,
    shuffled: getShuffled
};