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);
+	}
+}

prio-queue.h

@@ -52,6 +52,14 @@ void *prio_queue_get(struct prio_queue *);
  */
 void *prio_queue_peek(struct prio_queue *);
 
+/*
+ * Replace the "thing" that compares the smallest with a new "thing",
+ * like prio_queue_get()+prio_queue_put() would do, but in a more
+ * efficient way.  Does the same as prio_queue_put() if the queue is
+ * empty.
+ */
+void prio_queue_replace(struct prio_queue *queue, void *thing);
+
 void clear_prio_queue(struct prio_queue *);
 
 /* Reverse the LIFO elements */

t/unit-tests/u-prio-queue.c

@@ -13,6 +13,7 @@ static int intcmp(const void *va, const void *vb, void *data UNUSED)
 #define STACK	 -3
 #define GET	 -4
 #define REVERSE  -5
+#define REPLACE  -6
 
 static int show(int *v)
 {
@@ -51,6 +52,15 @@ static void test_prio_queue(int *input, size_t input_size,
 		case REVERSE:
 			prio_queue_reverse(&pq);
 			break;
+		case REPLACE:
+			peek = prio_queue_peek(&pq);
+			cl_assert(i + 1 < input_size);
+			cl_assert(input[i + 1] >= 0);
+			cl_assert(j < result_size);
+			cl_assert_equal_i(result[j], show(peek));
+			j++;
+			prio_queue_replace(&pq, &input[++i]);
+			break;
 		default:
 			prio_queue_put(&pq, &input[i]);
 			break;
@@ -81,6 +91,13 @@ void test_prio_queue__empty(void)
 		   ((int []){ 1, 2, MISSING, 1, 2, MISSING }));
 }
 
+void test_prio_queue__replace(void)
+{
+	TEST_INPUT(((int []){ REPLACE, 6, 2, 4, REPLACE, 5, 7, GET,
+			      REPLACE, 1, DUMP }),
+		   ((int []){ MISSING, 2, 4, 5, 1, 6, 7 }));
+}
+
 void test_prio_queue__stack(void)
 {
 	TEST_INPUT(((int []){ STACK, 8, 1, 5, 4, 6, 2, 3, DUMP }),
@@ -92,3 +109,9 @@ void test_prio_queue__reverse_stack(void)
 	TEST_INPUT(((int []){ STACK, 1, 2, 3, 4, 5, 6, REVERSE, DUMP }),
 		   ((int []){ 1, 2, 3, 4, 5, 6 }));
 }
+
+void test_prio_queue__replace_stack(void)
+{
+	TEST_INPUT(((int []){ STACK, 8, 1, 5, REPLACE, 4, 6, 2, 3, DUMP }),
+		   ((int []){ 5, 3, 2, 6, 4, 1, 8 }));
+}