prio-queue: add prio_queue_replace()

Add a function to replace the top element of the queue that basically
does the same as prio_queue_get() followed by prio_queue_put(), but
without the work by prio_queue_get() to rebalance the heap.  It can be
used to optimize loops that get one element and then immediately add
another one.  That's common e.g., with commit history traversal, where
we get out a commit and then put in its parents.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

prio-queue.c

@@ -58,22 +58,10 @@ void prio_queue_put(struct prio_queue *queue, void *thing)
 	}
 }
 
-void *prio_queue_get(struct prio_queue *queue)
+static void sift_down_root(struct prio_queue *queue)
 {
-	void *result;
 	size_t ix, child;
 
-	if (!queue->nr)
-		return NULL;
-	if (!queue->compare)
-		return queue->array[--queue->nr].data; /* LIFO */
-
-	result = queue->array[0].data;
-	if (!--queue->nr)
-		return result;
-
-	queue->array[0] = queue->array[queue->nr];
-
 	/* Push down the one at the root */
 	for (ix = 0; ix * 2 + 1 < queue->nr; ix = child) {
 		child = ix * 2 + 1; /* left */
@@ -86,6 +74,23 @@ void *prio_queue_get(struct prio_queue *queue)
 
 		swap(queue, child, ix);
 	}
+}
+
+void *prio_queue_get(struct prio_queue *queue)
+{
+	void *result;
+
+	if (!queue->nr)
+		return NULL;
+	if (!queue->compare)
+		return queue->array[--queue->nr].data; /* LIFO */
+
+	result = queue->array[0].data;
+	if (!--queue->nr)
+		return result;
+
+	queue->array[0] = queue->array[queue->nr];
+	sift_down_root(queue);
 	return result;
 }
 
@@ -97,3 +102,17 @@ void *prio_queue_peek(struct prio_queue *queue)
 		return queue->array[queue->nr - 1].data;
 	return queue->array[0].data;
 }
+
+void prio_queue_replace(struct prio_queue *queue, void *thing)
+{
+	if (!queue->nr) {
+		prio_queue_put(queue, thing);
+	} else if (!queue->compare) {
+		queue->array[queue->nr - 1].ctr = queue->insertion_ctr++;
+		queue->array[queue->nr - 1].data = thing;
+	} else {
+		queue->array[0].ctr = queue->insertion_ctr++;
+		queue->array[0].data = thing;
+		sift_down_root(queue);
+	}
+}