Merge Sort

// Merge sort using get/set helpers
function mergeSort(lo, hi) {
  if (hi - lo <= 1) return;
  const mid = (lo + hi) >> 1;
  mergeSort(lo, mid);
  mergeSort(mid, hi);
  // Copy both halves into temp arrays first
  const left = [], right = [];
  for (let i = lo; i < mid; i++) left.push(get(i));
  for (let i = mid; i < hi; i++) right.push(get(i));
  let i = 0, j = 0, k = lo;
  while (i < left.length && j < right.length) {
    if (left[i] <= right[j]) {
      set(k++, left[i++]);
    } else {
      set(k++, right[j++]);
    }
  }
  while (i < left.length) set(k++, left[i++]);
  while (j < right.length) set(k++, right[j++]);
}
mergeSort(0, length);