void mergeSortBy()

in lib/src/algorithms.dart [245:271]

• 20 lines of code
• 3 McCabe index (conditional complexity)

void mergeSortBy<E, K>(List<E> elements, K Function(E element) keyOf,
    int Function(K a, K b) compare,
    [int start = 0, int? end]) {
  end = RangeError.checkValidRange(start, end, elements.length);
  var length = end - start;
  if (length < 2) return;
  if (length < _mergeSortLimit) {
    _movingInsertionSort(elements, keyOf, compare, start, end, elements, start);
    return;
  }
  // Special case the first split instead of directly calling
  // _mergeSort, because the _mergeSort requires its target to
  // be different from its source, and it requires extra space
  // of the same size as the list to sort.
  // This split allows us to have only half as much extra space,
  // and it ends up in the original place.
  var middle = start + (length >> 1);
  var firstLength = middle - start;
  var secondLength = end - middle;
  // secondLength is always the same as firstLength, or one greater.
  var scratchSpace = List<E>.filled(secondLength, elements[start]);
  _mergeSort(elements, keyOf, compare, middle, end, scratchSpace, 0);
  var firstTarget = end - firstLength;
  _mergeSort(elements, keyOf, compare, start, middle, elements, firstTarget);
  _merge(keyOf, compare, elements, firstTarget, end, scratchSpace, 0,
      secondLength, elements, start);
}