code: function siftup(compare, a, i, j, k) {
  
  	var current = k - i;
  
  	// while we are not the root
  
  	while (current !== 0) {
  
  		// address of the parent in a zero-based
  		// d-ary heap
  
  		var parent = i + (current - 1 >>> 1);
  
  		// if current value is greater than its parent
  		// then we are done
  
  		if (compare(a[i + current], a[parent]) >= 0) return i + current;
  
  		// otherwise
  		// swap with parent
  
  		var tmp = a[i + current];
  		a[i + current] = a[parent];
  		a[parent] = tmp;
  
  		current = parent - i;
  	}
  
  	return i + current;
  }
docs: Sifts up a node.
  
  @param {function} compare the comparison function
  @param {array} a the array where the heap is stored
  @param {int} i is the root element
  @param {int} j - 1 is the last leaf
  @param {int} k is the target node