[PATCH v2 2/4] commit-queue: LIFO or priority queue of commits

From: Junio C Hamano <gitster@pobox.com>
Date: Jun 9, 2013, 23:24 UTC
Message-ID: <1370820277-30158-3-git-send-email-gitster@pobox.com>
In-Reply-To: <1370820277-30158-1-git-send-email-gitster@pobox.com>

Traditionally we used a singly linked list of commits to hold a set of in-flight commits while traversing history. The most typical use of the list is to add commits that are newly discovered to it, keep the list sorted by commit timestamp, pick up the newest one from the list, and keep digging. The cost of keeping the singly linked list sorted is nontrivial, and this typical use pattern better matches a priority queue.

Introduce a commit-queue structure, that can be used either as a LIFO stack, or a priority queue. This will be used in the next patch to hold in-flight commits during sort-in-topological-order.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 Makefile       |  2 ++
 commit-queue.c | 71 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 commit-queue.h | 31 +++++++++++++++++++++++++
 3 files changed, 104 insertions(+)
 create mode 100644 commit-queue.c
 create mode 100644 commit-queue.h

diff --git a/Makefile b/Makefile
index 598d631..3cf55e9 100644
--- a/Makefile
+++ b/Makefile
@@ -634,6 +634,7 @@ LIB_H += cache.h
 LIB_H += color.h
 LIB_H += column.h
 LIB_H += commit.h
+LIB_H += commit-queue.h
 LIB_H += compat/bswap.h
 LIB_H += compat/cygwin.h
 LIB_H += compat/mingw.h
@@ -757,6 +758,7 @@ LIB_OBJS += color.o
 LIB_OBJS += column.o
 LIB_OBJS += combine-diff.o
 LIB_OBJS += commit.o
+LIB_OBJS += commit-queue.o
 LIB_OBJS += compat/obstack.o
 LIB_OBJS += compat/terminal.o
 LIB_OBJS += config.o
diff --git a/commit-queue.c b/commit-queue.c
new file mode 100644
index 0000000..77d4b02
--- /dev/null
+++ b/commit-queue.c
@@ -0,0 +1,71 @@
+#include "cache.h"
+#include "commit.h"
+#include "commit-queue.h"
+
+void clear_commit_queue(struct commit_queue *queue)
+{
+	free(queue->array);
+	queue->nr = 0;
+	queue->alloc = 0;
+	queue->array = NULL;
+}
+
+void commit_queue_put(struct commit_queue *queue, struct commit *commit)
+{
+	commit_compare_fn compare = queue->compare;
+	int ix, parent;
+
+	/* Append at the end */
+	ALLOC_GROW(queue->array, queue->nr + 1, queue->alloc);
+	queue->array[queue->nr++] = commit;
+	if (!compare)
+		return; /* LIFO */
+
+	/* Bubble up the new one */
+	for (ix = queue->nr - 1; ix; ix = parent) {
+		parent = (ix - 1) / 2;
+		if (compare(queue->array[parent], queue->array[ix],
+			    queue->cb_data) < 0)
+			break;
+
+		commit = queue->array[parent];
+		queue->array[parent] = queue->array[ix];
+		queue->array[ix] = commit;
+	}
+}
+
+struct commit *commit_queue_get(struct commit_queue *queue)
+{
+	struct commit *result, *swap;
+	int ix, child;
+	commit_compare_fn compare = queue->compare;
+
+	if (!queue->nr)
+		return NULL;
+	if (!compare)
+		return queue->array[--queue->nr]; /* LIFO */
+
+	result = queue->array[0];
+	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 */
+		if ((child + 1 < queue->nr) &&
+		    (compare(queue->array[child], queue->array[child + 1],
+			     queue->cb_data) >= 0))
+			child++; /* use right child */
+
+		if (compare(queue->array[ix], queue->array[child],
+			    queue->cb_data) < 0)
+			break;
+
+		swap = queue->array[child];
+		queue->array[child] = queue->array[ix];
+		queue->array[ix] = swap;
+	}
+	return result;
+}
diff --git a/commit-queue.h b/commit-queue.h
new file mode 100644
index 0000000..7c5dc4c
--- /dev/null
+++ b/commit-queue.h
@@ -0,0 +1,31 @@
+#ifndef COMMIT_QUEUE_H
+#define COMMIT_QUEUE_H
+
+/*
+ * Compare two commits; the third parameter is cb_data in the
+ * commit_queue structure.
+ */
+typedef int (*commit_compare_fn)(struct commit *, struct commit *, void *);
+
+struct commit_queue {
+	commit_compare_fn compare;
+	void *cb_data;
+	int alloc, nr;
+	struct commit **array;
+};
+
+/*
+ * Add the commit to the queue
+ */
+extern void commit_queue_put(struct commit_queue *, struct commit *);
+
+/*
+ * Extract the commit that compares the smallest out of the queue,
+ * or NULL.  If compare function is NULL, the queue acts as a LIFO
+ * stack.
+ */
+extern struct commit *commit_queue_get(struct commit_queue *);
+
+extern void clear_commit_queue(struct commit_queue *);
+
+#endif /* COMMIT_QUEUE_H */

-- 
1.8.3-451-gb703ddf