Merge branch 'rs/commit-list-sort-in-batch'

Setting up a revision traversal with many starting points was inefficient
as these were placed in a date-order priority queue one-by-one.

By René Scharfe (3) and Junio C Hamano (1)
* rs/commit-list-sort-in-batch:
  mergesort: rename it to llist_mergesort()
  revision: insert unsorted, then sort in prepare_revision_walk()
  commit: use mergesort() in commit_list_sort_by_date()
  add mergesort() for linked lists

8 files changed, 188 insertions, 7 deletions

diff --git a/.gitignore b/.gitignore
index 5a0782fe81..83a5c9df1b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -181,6 +181,7 @@
 /test-index-version
 /test-line-buffer
 /test-match-trees
+/test-mergesort
 /test-mktemp
 /test-parse-options
 /test-path-utils
diff --git a/Makefile b/Makefile
index 172e924a29..f1caae8a82 100644
--- a/Makefile
+++ b/Makefile
@@ -481,6 +481,7 @@ TEST_PROGRAMS_NEED_X += test-genrandom
 TEST_PROGRAMS_NEED_X += test-index-version
 TEST_PROGRAMS_NEED_X += test-line-buffer
 TEST_PROGRAMS_NEED_X += test-match-trees
+TEST_PROGRAMS_NEED_X += test-mergesort
 TEST_PROGRAMS_NEED_X += test-mktemp
 TEST_PROGRAMS_NEED_X += test-parse-options
 TEST_PROGRAMS_NEED_X += test-path-utils
@@ -591,6 +592,7 @@ LIB_H += log-tree.h
 LIB_H += mailmap.h
 LIB_H += merge-file.h
 LIB_H += merge-recursive.h
+LIB_H += mergesort.h
 LIB_H += notes.h
 LIB_H += notes-cache.h
 LIB_H += notes-merge.h
@@ -695,6 +697,7 @@ LIB_OBJS += mailmap.o
 LIB_OBJS += match-trees.o
 LIB_OBJS += merge-file.o
 LIB_OBJS += merge-recursive.o
+LIB_OBJS += mergesort.o
 LIB_OBJS += name-hash.o
 LIB_OBJS += notes.o
 LIB_OBJS += notes-cache.o
diff --git a/commit.c b/commit.c
index 4b39c19123..b80a45281c 100644
--- a/commit.c
+++ b/commit.c
@@ -7,6 +7,7 @@
 #include "revision.h"
 #include "notes.h"
 #include "gpg-interface.h"
+#include "mergesort.h"
 
 int save_commit_buffer = 1;
 
@@ -360,6 +361,21 @@ struct commit_list *commit_list_insert(struct commit *item, struct commit_list *
 	return new_list;
 }
 
+void commit_list_reverse(struct commit_list **list_p)
+{
+	struct commit_list *prev = NULL, *curr = *list_p, *next;
+
+	if (!list_p)
+		return;
+	while (curr) {
+		next = curr->next;
+		curr->next = prev;
+		prev = curr;
+		curr = next;
+	}
+	*list_p = prev;
+}
+
 unsigned commit_list_count(const struct commit_list *l)
 {
 	unsigned c = 0;
@@ -390,15 +406,31 @@ struct commit_list * commit_list_insert_by_date(struct commit *item, struct comm
 	return commit_list_insert(item, pp);
 }
 
+static int commit_list_compare_by_date(const void *a, const void *b)
+{
+	unsigned long a_date = ((const struct commit_list *)a)->item->date;
+	unsigned long b_date = ((const struct commit_list *)b)->item->date;
+	if (a_date < b_date)
+		return 1;
+	if (a_date > b_date)
+		return -1;
+	return 0;
+}
+
+static void *commit_list_get_next(const void *a)
+{
+	return ((const struct commit_list *)a)->next;
+}
+
+static void commit_list_set_next(void *a, void *next)
+{
+	((struct commit_list *)a)->next = next;
+}
 
 void commit_list_sort_by_date(struct commit_list **list)
 {
-	struct commit_list *ret = NULL;
-	while (*list) {
-		commit_list_insert_by_date((*list)->item, &ret);
-		*list = (*list)->next;
-	}
-	*list = ret;
+	*list = llist_mergesort(*list, commit_list_get_next, commit_list_set_next,
+				commit_list_compare_by_date);
 }
 
 struct commit *pop_most_recent_commit(struct commit_list **list,
diff --git a/commit.h b/commit.h
index 154c0e34ff..f8d250d6f6 100644
--- a/commit.h
+++ b/commit.h
@@ -57,6 +57,7 @@ unsigned commit_list_count(const struct commit_list *l);
 struct commit_list *commit_list_insert_by_date(struct commit *item,
 				    struct commit_list **list);
 void commit_list_sort_by_date(struct commit_list **list);
+void commit_list_reverse(struct commit_list **list_p);
 
 void free_commit_list(struct commit_list *list);
 
diff --git a/mergesort.c b/mergesort.c
new file mode 100644
index 0000000000..e5fdf2ee4a
--- /dev/null
+++ b/mergesort.c
@@ -0,0 +1,73 @@
+#include "cache.h"
+#include "mergesort.h"
+
+struct mergesort_sublist {
+	void *ptr;
+	unsigned long len;
+};
+
+static void *get_nth_next(void *list, unsigned long n,
+			  void *(*get_next_fn)(const void *))
+{
+	while (n-- && list)
+		list = get_next_fn(list);
+	return list;
+}
+
+static void *pop_item(struct mergesort_sublist *l,
+		      void *(*get_next_fn)(const void *))
+{
+	void *p = l->ptr;
+	l->ptr = get_next_fn(l->ptr);
+	l->len = l->ptr ? (l->len - 1) : 0;
+	return p;
+}
+
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *))
+{
+	unsigned long l;
+
+	if (!list)
+		return NULL;
+	for (l = 1; ; l *= 2) {
+		void *curr;
+		struct mergesort_sublist p, q;
+
+		p.ptr = list;
+		q.ptr = get_nth_next(p.ptr, l, get_next_fn);
+		if (!q.ptr)
+			break;
+		p.len = q.len = l;
+
+		if (compare_fn(p.ptr, q.ptr) > 0)
+			list = curr = pop_item(&q, get_next_fn);
+		else
+			list = curr = pop_item(&p, get_next_fn);
+
+		while (p.ptr) {
+			while (p.len || q.len) {
+				void *prev = curr;
+
+				if (!p.len)
+					curr = pop_item(&q, get_next_fn);
+				else if (!q.len)
+					curr = pop_item(&p, get_next_fn);
+				else if (compare_fn(p.ptr, q.ptr) > 0)
+					curr = pop_item(&q, get_next_fn);
+				else
+					curr = pop_item(&p, get_next_fn);
+				set_next_fn(prev, curr);
+			}
+			p.ptr = q.ptr;
+			p.len = l;
+			q.ptr = get_nth_next(p.ptr, l, get_next_fn);
+			q.len = q.ptr ? l : 0;
+
+		}
+		set_next_fn(curr, NULL);
+	}
+	return list;
+}
diff --git a/mergesort.h b/mergesort.h
new file mode 100644
index 0000000000..644cff1f96
--- /dev/null
+++ b/mergesort.h
@@ -0,0 +1,17 @@
+#ifndef MERGESORT_H
+#define MERGESORT_H
+
+/*
+ * Sort linked list in place.
+ * - get_next_fn() returns the next element given an element of a linked list.
+ * - set_next_fn() takes two elements A and B, and makes B the "next" element
+ *   of A on the list.
+ * - compare_fn() takes two elements A and B, and returns negative, 0, positive
+ *   as the same sign as "subtracting" B from A.
+ */
+void *llist_mergesort(void *list,
+		      void *(*get_next_fn)(const void *),
+		      void (*set_next_fn)(void *, void *),
+		      int (*compare_fn)(const void *, const void *));
+
+#endif
diff --git a/revision.c b/revision.c
index b3554ed11b..92095f5fcb 100644
--- a/revision.c
+++ b/revision.c
@@ -2076,11 +2076,13 @@ int prepare_revision_walk(struct rev_info *revs)
 		if (commit) {
 			if (!(commit->object.flags & SEEN)) {
 				commit->object.flags |= SEEN;
-				commit_list_insert_by_date(commit, &revs->commits);
+				commit_list_insert(commit, &revs->commits);
 			}
 		}
 		e++;
 	}
+	commit_list_reverse(&revs->commits);
+	commit_list_sort_by_date(&revs->commits);
 	if (!revs->leak_pending)
 		free(list);
 
diff --git a/test-mergesort.c b/test-mergesort.c
new file mode 100644
index 0000000000..3f388b4ce0
--- /dev/null
+++ b/test-mergesort.c
@@ -0,0 +1,52 @@
+#include "cache.h"
+#include "mergesort.h"
+
+struct line {
+	char *text;
+	struct line *next;
+};
+
+static void *get_next(const void *a)
+{
+	return ((const struct line *)a)->next;
+}
+
+static void set_next(void *a, void *b)
+{
+	((struct line *)a)->next = b;
+}
+
+static int compare_strings(const void *a, const void *b)
+{
+	const struct line *x = a, *y = b;
+	return strcmp(x->text, y->text);
+}
+
+int main(int argc, const char **argv)
+{
+	struct line *line, *p = NULL, *lines = NULL;
+	struct strbuf sb = STRBUF_INIT;
+
+	for (;;) {
+		if (strbuf_getwholeline(&sb, stdin, '\n'))
+			break;
+		line = xmalloc(sizeof(struct line));
+		line->text = strbuf_detach(&sb, NULL);
+		if (p) {
+			line->next = p->next;
+			p->next = line;
+		} else {
+			line->next = NULL;
+			lines = line;
+		}
+		p = line;
+	}
+
+	lines = llist_mergesort(lines, get_next, set_next, compare_strings);
+
+	while (lines) {
+		printf("%s", lines->text);
+		lines = lines->next;
+	}
+	return 0;
+}