private static int firstChild(int index) {
    return (index << 2) - 2;
  }
@param elements the number of elements that can be stored in this heap. Currently the heap cannot be resized or
                shrunk/trimmed after initial creation. elements-1 is the maximum id that can be stored in this heap
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ArrayLongMinHeap.java