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
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;
}