/**
 * Receives an array and retrieves it sorted by merge
 * [1, 5, 2, 4, 3, 6, 7] => [1, 2, 3, 4, 5, 6, 7]
 * @param {Array} items
 * @returns {Array}
 */
function mergeSort (items) {
  if (items.length < 2) {
    return items
  }
  let middle = Math.floor(items.length / 2)
  let left = items.slice(0, middle)
  let right = items.slice(middle)
  let params = _merge(mergeSort(left), mergeSort(right))

  params.unshift(0, items.length)
  items.splice(...params)
  return items

  function _merge (left, right) {
    const result = []
    let il = 0
    let ir = 0

    while (il < left.length && ir < right.length) {
      result.push(left[il] < right[ir] ? left[il++] : right[ir++])
    }
    return result.concat(left.slice(il)).concat(right.slice(ir))
  }
}