prio-queue: add 'peek' operation

When consuming a priority queue, it can be convenient to inspect
the next object that will be dequeued without actually dequeueing
it. Our existing library did not have such a 'peek' operation, so
add it as prio_queue_peek().

Add a reference-level comparison in t/helper/test-prio-queue.c
so this method is exercised by t0009-prio-queue.sh.

Signed-off-by: Derrick Stolee <[email protected]>

Author: derrickstolee

prio-queue.c

@@ -85,3 +85,12 @@ void *prio_queue_get(struct prio_queue *queue)
 	}
 	return result;
 }
+
+void *prio_queue_peek(struct prio_queue *queue)
+{
+	if (!queue->nr)
+		return NULL;
+	if (!queue->compare)
+		return queue->array[queue->nr - 1].data;
+	return queue->array[0].data;
+}

prio-queue.h

@@ -46,6 +46,12 @@ extern void prio_queue_put(struct prio_queue *, void *thing);
  */
 extern void *prio_queue_get(struct prio_queue *);
 
+/*
+ * Gain access to the "thing" that would be returned by
+ * prio_queue_get, but do not remove it from the queue.
+ */
+extern void *prio_queue_peek(struct prio_queue *);
+
 extern void clear_prio_queue(struct prio_queue *);
 
 /* Reverse the LIFO elements */

t/helper/test-prio-queue.c

@@ -22,9 +22,13 @@ int cmd__prio_queue(int argc, const char **argv)
 	struct prio_queue pq = { intcmp };
 
 	while (*++argv) {
-		if (!strcmp(*argv, "get"))
-			show(prio_queue_get(&pq));
-		else if (!strcmp(*argv, "dump")) {
+		if (!strcmp(*argv, "get")) {
+			void *peek = prio_queue_peek(&pq);
+			void *get = prio_queue_get(&pq);
+			if (peek != get)
+				BUG("peek and get results do not match");
+			show(get);
+		} else if (!strcmp(*argv, "dump")) {
 			int *v;
 			while ((v = prio_queue_get(&pq)))
 			       show(v);