For implementing reverse() on large, sparse arrays.
function SparseReverse(array, len) {
  var keys = GetSortedArrayKeys(array, %GetArrayKeys(array, len));
  var high_counter = keys.length - 1;
  var low_counter = 0;
  while (low_counter <= high_counter) {
    var i = keys[low_counter];
    var j = keys[high_counter];

    var j_complement = len - j - 1;
    var low, high;

    if (j_complement <= i) {
      high = j;
      while (keys[--high_counter] == j) { }
      low = j_complement;
    }
    if (j_complement >= i) {
      low = i;
      while (keys[++low_counter] == i) { }
      high = len - i - 1;
    }

    var current_i = array[low];
    if (!IS_UNDEFINED(current_i) || low in array) {
      var current_j = array[high];
      if (!IS_UNDEFINED(current_j) || high in array) {
        array[low] = current_j;
        array[high] = current_i;
      } else {
        array[high] = current_i;
        delete array[low];
      }
    } else {
      var current_j = array[high];
      if (!IS_UNDEFINED(current_j) || high in array) {
        array[low] = current_j;
        delete array[high];
      }
    }
  }
}