Implement git_merge_base()

It's implemented in revwalk.c so it has access to the revision
walker's commit cache and related functions. The algorithm is the one
used by git, modified so it fits better with the library's functions.

include/git2/revwalk.h

@@ -232,6 +232,9 @@ GIT_EXTERN(void) git_revwalk_free(git_revwalk *walk);
  */
 GIT_EXTERN(git_repository *) git_revwalk_repository(git_revwalk *walk);
 
+GIT_EXTERN(int) git_merge_base(git_oid *out, git_repository *repo, git_oid *one, git_oid *two);
+
+
 /** @} */
 GIT_END_DECL
 #endif

src/revwalk.c

@@ -15,13 +15,19 @@
 
 #include <regex.h>
 
+#define PARENT1  (1 << 0)
+#define PARENT2  (1 << 1)
+#define RESULT   (1 << 2)
+#define STALE    (1 << 3)
+
 typedef struct commit_object {
 	git_oid oid;
 	uint32_t time;
 	unsigned int seen:1,
 			 uninteresting:1,
 			 topo_delay:1,
-			 parsed:1;
+			 parsed:1,
+			 flags : 4;
 
 	unsigned short in_degree;
 	unsigned short out_degree;
@@ -55,6 +61,14 @@ struct git_revwalk {
 	unsigned int sorting;
 };
 
+static int commit_time_cmp(void *a, void *b)
+{
+	commit_object *commit_a = (commit_object *)a;
+	commit_object *commit_b = (commit_object *)b;
+
+	return (commit_a->time < commit_b->time);
+}
+
 static commit_list *commit_list_insert(commit_object *item, commit_list **list_p)
 {
 	commit_list *new_list = git__malloc(sizeof(commit_list));
@@ -67,6 +81,20 @@ static commit_list *commit_list_insert(commit_object *item, commit_list **list_p
 	return new_list;
 }
 
+static commit_list *commit_list_insert_by_date(commit_object *item, commit_list **list_p)
+{
+	commit_list **pp = list_p;
+	commit_list *p;
+
+	while ((p = *pp) != NULL) {
+		if (commit_time_cmp(p->item, item) < 0)
+			break;
+
+		pp = &p->next;
+	}
+
+	return commit_list_insert(item, pp);
+}
 static void commit_list_free(commit_list **list_p)
 {
 	commit_list *list = *list_p;
@@ -92,14 +120,6 @@ static commit_object *commit_list_pop(commit_list **stack)
 	return item;
 }
 
-static int commit_time_cmp(void *a, void *b)
-{
-	commit_object *commit_a = (commit_object *)a;
-	commit_object *commit_b = (commit_object *)b;
-
-	return (commit_a->time < commit_b->time);
-}
-
 static uint32_t object_table_hash(const void *key, int hash_id)
 {
 	uint32_t r;
@@ -244,6 +264,144 @@ static int commit_parse(git_revwalk *walk, commit_object *commit)
 	return error == GIT_SUCCESS ? GIT_SUCCESS : git__rethrow(error, "Failed to parse commit");
 }
 
+static commit_object *interesting(commit_list *list)
+{
+	while (list) {
+		commit_object *commit = list->item;
+		list = list->next;
+		if (commit->flags & STALE)
+			continue;
+
+		return commit;
+	}
+
+	return NULL;
+}
+
+static int merge_bases_many(commit_list **out, git_revwalk *walk, commit_object *one, git_vector *twos)
+{
+	int error;
+	unsigned int i;
+	commit_object *two;
+	commit_list *list = NULL, *result = NULL;
+
+	/* if the commit is repeated, we have a our merge base already */
+	git_vector_foreach(twos, i, two) {
+		if (one == two)
+			return commit_list_insert(one, out) ? GIT_SUCCESS : GIT_ENOMEM;
+	}
+
+	if ((error = commit_parse(walk, one)) < GIT_SUCCESS)
+	    return error;
+
+	one->flags |= PARENT1;
+	if (commit_list_insert(one, &list) == NULL)
+		return GIT_ENOMEM;
+
+	git_vector_foreach(twos, i, two) {
+		commit_parse(walk, two);
+		two->flags |= PARENT2;
+		if (commit_list_insert_by_date(two, &list) == NULL)
+			return GIT_ENOMEM;
+	}
+
+	/* as long as there are non-STALE commits */
+	while (interesting(list)) {
+		commit_object *commit = list->item;
+		commit_list *next;
+		int flags;
+
+		next = list->next;
+		git__free(list);
+		list = next;
+
+		flags = commit->flags & (PARENT1 | PARENT2 | STALE);
+		if (flags == (PARENT1 | PARENT2)) {
+			if (!(commit->flags & RESULT)) {
+				commit->flags |= RESULT;
+				if (commit_list_insert_by_date(commit, &result) == NULL)
+					return GIT_ENOMEM;
+			}
+			/* we mark the parents of a merge stale */
+			flags |= STALE;
+		}
+
+		for (i = 0; i < commit->out_degree; i++) {
+			commit_object *p = commit->parents[i];
+			if ((p->flags & flags) == flags)
+				continue;
+
+			if ((error = commit_parse(walk, p)) < GIT_SUCCESS)
+				return error;
+
+			p->flags |= flags;
+			if (commit_list_insert_by_date(p, &list) == NULL)
+				return GIT_ENOMEM;
+		}
+	}
+
+	commit_list_free(&list);
+	/* filter out any stale commits in the results */
+	list = result;
+	result = NULL;
+
+	while (list) {
+		struct commit_list *next = list->next;
+		if (!(list->item->flags & STALE))
+			if (commit_list_insert_by_date(list->item, &result) == NULL)
+				return GIT_ENOMEM;
+
+		    free(list);
+		    list = next;
+	}
+
+	*out = result;
+	return GIT_SUCCESS;
+}
+
+int git_merge_base(git_oid *out, git_repository *repo, git_oid *one, git_oid *two)
+{
+	git_revwalk *walk;
+	git_vector list;
+	commit_list *result;
+	commit_object *commit;
+	void *contents[1];
+	int error;
+
+	error = git_revwalk_new(&walk, repo);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	commit = commit_lookup(walk, two);
+	if (commit == NULL)
+		goto cleanup;
+
+	/* This is just one value, so we can do it on the stack */
+	memset(&list, 0x0, sizeof(git_vector));
+	contents[0] = commit;
+	list.length = 1;
+	list.contents = contents;
+
+	commit = commit_lookup(walk, one);
+	if (commit == NULL)
+		goto cleanup;
+
+	error = merge_bases_many(&result, walk, commit, &list);
+	if (error < GIT_SUCCESS)
+		return error;
+
+	if (result == NULL)
+		return GIT_ENOTFOUND;
+
+	git_oid_cpy(out, &result->item->oid);
+	commit_list_free(&result);
+
+cleanup:
+	git_revwalk_free(walk);
+
+	return GIT_SUCCESS;
+}
+
 static void mark_uninteresting(commit_object *commit)
 {
 	unsigned short i;

tests-clar/revwalk/mergebase.c

@@ -0,0 +1,37 @@
+#include "clar_libgit2.h"
+
+static git_repository *_repo;
+
+void test_revwalk_mergebase__initialize(void)
+{
+	cl_git_pass(git_repository_open(&_repo, cl_fixture("testrepo.git")));
+}
+
+void test_revwalk_mergebase__cleanup(void)
+{
+	git_repository_free(_repo);
+}
+
+void test_revwalk_mergebase__single1(void)
+{
+	git_oid result, one, two, expected;
+
+	git_oid_fromstr(&one, "c47800c7266a2be04c571c04d5a6614691ea99bd ");
+	git_oid_fromstr(&two, "9fd738e8f7967c078dceed8190330fc8648ee56a");
+	git_oid_fromstr(&expected, "5b5b025afb0b4c913b4c338a42934a3863bf3644");
+
+	cl_git_pass(git_merge_base(&result, _repo, &one, &two));
+	cl_assert(git_oid_cmp(&result, &expected) == 0);
+}
+
+void test_revwalk_mergebase__single2(void)
+{
+	git_oid result, one, two, expected;
+
+	git_oid_fromstr(&one, "763d71aadf09a7951596c9746c024e7eece7c7af");
+	git_oid_fromstr(&two, "a65fedf39aefe402d3bb6e24df4d4f5fe4547750");
+	git_oid_fromstr(&expected, "c47800c7266a2be04c571c04d5a6614691ea99bd");
+
+	cl_git_pass(git_merge_base(&result, _repo, &one, &two));
+	cl_assert(git_oid_cmp(&result, &expected) == 0);
+}