/**
 * @module lib/merge-sort
 * @license MIT Copyright 2014 Daniel Imms (http://www.growingwiththeweb.com)
 */
'use strict';

var defaultCompare = require('./common/default-compare');

/**
 * Sorts an array using top down merge sort.
 * @param {Array} array The array to sort.
 * @param {function} compare The compare function.
 * @returns The sorted array.
 */
function sort(array, compare) {
  if (array.length <= 1) {
    return array;
  }

  var i;
  var middle = Math.floor(array.length / 2);
  var left = new Array(middle);
  var right = new Array(array.length - middle);

  for (i = 0; i < left.length; i++) {
    left[i] = array[i];
  }
  for (i = 0; i < right.length; i++) {
    right[i] = array[i + left.length];
  }

  return merge(sort(left, compare), sort(right, compare), compare);
}

/**
 * Merges two arrays in to a new array.
 * @param {Array} left The first array.
 * @param {Array} left The second array.
 * @param {function} compare The compare function.
 * @return The merged array.
 */
function merge(left, right, compare) {
  var result = [];
  var leftIndex = 0;
  var rightIndex = 0;

  while (leftIndex < left.length || rightIndex < right.length) {
    if (leftIndex < left.length && rightIndex < right.length) {
      if (compare(left[leftIndex], right[rightIndex]) <= 0) {
        result.push(left[leftIndex++]);
      } else {
        result.push(right[rightIndex++]);
      }
    } else if (leftIndex < left.length) {
      result.push(left[leftIndex++]);
    } else if (rightIndex < right.length) {
      result.push(right[rightIndex++]);
    }
  }
  return result;
}

/**
 * Sorts an array using top down merge sort.
 * @param {Array} array The array to sort.
 * @param {function} customCompare A custom compare function.
 * @returns The sorted array.
 */
module.exports = function (array, customCompare) {
  var compare = customCompare || defaultCompare;
  return sort(array, compare);
};