git-rev-list: add "--dense" flag

This is what the recent git-rev-list changes have all been gearing up for.

When we use a path filter to git-rev-list, the new "--dense" flag asks
git-rev-list to compress the history so that it _only_ contains commits
that change files in the path filter.  It also rewrites the parent
information so that tools like "gitk" will see the result as a dense
history tree.

For example, on the current kernel archive:

	[torvalds@g5 linux]$ git-rev-list HEAD | wc -l
	9904
	[torvalds@g5 linux]$ git-rev-list HEAD -- kernel | wc -l
	5442
	[torvalds@g5 linux]$ git-rev-list --dense HEAD -- kernel | wc -l
	356

which shows that while we have almost ten thousand commits, we can prune
down the work to slightly more than half by only following the merges
that are interesting. But further, we can then compress the history to
just 356 entries that actually make changes to the kernel subdirectory.

To see this in action, try something like

	gitk --dense -- gitk

to see just the history that affects gitk.  Or, to show that true
parallel development still remains parallel, do

	gitk --dense -- daemon.c

which shows some parallel commits in the current git tree.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

rev-list.c

@@ -11,6 +11,7 @@
 #define INTERESTING	(1u << 1)
 #define COUNTED		(1u << 2)
 #define SHOWN		(1u << 3)
+#define TREECHANGE	(1u << 4)
 
 static const char rev_list_usage[] =
 	"git-rev-list [OPTION] commit-id <commit-id>\n"
@@ -27,6 +28,7 @@ static const char rev_list_usage[] =
 		      "  --merge-order [ --show-breaks ]\n"
 		      "  --topo-order";
 
+static int dense = 0;
 static int unpacked = 0;
 static int bisect_list = 0;
 static int tag_objects = 0;
@@ -79,6 +81,26 @@ static void show_commit(struct commit *commit)
 	fflush(stdout);
 }
 
+static void rewrite_one(struct commit **pp)
+{
+	for (;;) {
+		struct commit *p = *pp;
+		if (p->object.flags & (TREECHANGE | UNINTERESTING))
+			return;
+		/* Only single-parent commits don't have TREECHANGE */
+		*pp = p->parents->item;
+	}
+}
+
+static void rewrite_parents(struct commit *commit)
+{
+	struct commit_list *parent = commit->parents;
+	while (parent) {
+		rewrite_one(&parent->item);
+		parent = parent->next;
+	}
+}
+
 static int filter_commit(struct commit * commit)
 {
 	if (stop_traversal && (commit->object.flags & BOUNDARY))
@@ -95,6 +117,11 @@ static int filter_commit(struct commit * commit)
 		return STOP;
 	if (no_merges && (commit->parents && commit->parents->next))
 		return CONTINUE;
+	if (paths && dense) {
+		if (!(commit->object.flags & TREECHANGE))
+			return CONTINUE;
+		rewrite_parents(commit);
+	}
 	return DO;
 }
 
@@ -404,6 +431,14 @@ static struct diff_options diff_opt = {
 	.change = file_change,
 };
 
+static int same_tree(struct tree *t1, struct tree *t2)
+{
+	is_different = 0;
+	if (diff_tree_sha1(t1->object.sha1, t2->object.sha1, "", &diff_opt) < 0)
+		return 0;
+	return !is_different;
+}
+
 static struct commit *try_to_simplify_merge(struct commit *commit, struct commit_list *parent)
 {
 	if (!commit->tree)
@@ -415,11 +450,7 @@ static struct commit *try_to_simplify_merge(struct commit *commit, struct commit
 		parse_commit(p);
 		if (!p->tree)
 			continue;
-		is_different = 0;
-		if (diff_tree_sha1(commit->tree->object.sha1,
-				   p->tree->object.sha1, "", &diff_opt) < 0)
-			continue;
-		if (!is_different)
+		if (same_tree(commit->tree, p->tree))
 			return p;
 	}
 	return NULL;
@@ -485,6 +516,27 @@ static void add_parents_to_list(struct commit *commit, struct commit_list **list
 	}
 }
 
+static void compress_list(struct commit_list *list)
+{
+	while (list) {
+		struct commit *commit = list->item;
+		struct commit_list *parent = commit->parents;
+		list = list->next;
+
+		/*
+		 * Exactly one parent? Check if it leaves the tree
+		 * unchanged
+		 */
+		if (parent && !parent->next) {
+			struct tree *t1 = commit->tree;
+			struct tree *t2 = parent->item->tree;
+			if (!t1 || !t2 || same_tree(t1, t2))
+				continue;
+		}
+		commit->object.flags |= TREECHANGE;
+	}
+}
+
 static struct commit_list *limit_list(struct commit_list *list)
 {
 	struct commit_list *newlist = NULL;
@@ -514,6 +566,8 @@ static struct commit_list *limit_list(struct commit_list *list)
 	}
 	if (tree_objects)
 		mark_edges_uninteresting(newlist);
+	if (paths && dense)
+		compress_list(newlist);
 	if (bisect_list)
 		newlist = find_bisection(newlist);
 	return newlist;
@@ -700,6 +754,10 @@ int main(int argc, const char **argv)
 		        limited = 1;
 			continue;
 		}
+		if (!strcmp(arg, "--dense")) {
+			dense = 1;
+			continue;
+		}
 		if (!strcmp(arg, "--")) {
 			paths = get_pathspec(prefix, argv + i + 1);
 			if (paths) {