1 files changed, 147 insertions, 0 deletions
diff --git a/merge-base.c b/merge-base.c
new file mode 100644
index 0000000000..923256c821
--- /dev/null
+++ b/merge-base.c
@@ -0,0 +1,147 @@
+#include <stdlib.h>
+#include "cache.h"
+#include "commit.h"
+
+#define PARENT1 1
+#define PARENT2 2
+#define UNINTERESTING 4
+
+static struct commit *interesting(struct commit_list *list)
+{
+	while (list) {
+		struct commit *commit = list->item;
+		list = list->next;
+		if (commit->object.flags & UNINTERESTING)
+			continue;
+		return commit;
+	}
+	return NULL;
+}
+
+/*
+ * A pathological example of how this thing works.
+ *
+ * Suppose we had this commit graph, where chronologically
+ * the timestamp on the commit are A <= B <= C <= D <= E <= F
+ * and we are trying to figure out the merge base for E and F
+ * commits.
+ *
+ *                  F
+ *                 / \
+ *            E   A   D
+ *             \ /   /  
+ *              B   /
+ *               \ /
+ *                C
+ *
+ * First we push E and F to list to be processed.  E gets bit 1
+ * and F gets bit 2.  The list becomes:
+ *
+ *     list=F(2) E(1), result=empty
+ *
+ * Then we pop F, the newest commit, from the list.  Its flag is 2.
+ * We scan its parents, mark them reachable from the side that F is
+ * reachable from, and push them to the list:
+ *
+ *     list=E(1) D(2) A(2), result=empty
+ *
+ * Next pop E and do the same.
+ *
+ *     list=D(2) B(1) A(2), result=empty
+ *
+ * Next pop D and do the same.
+ *
+ *     list=C(2) B(1) A(2), result=empty
+ *
+ * Next pop C and do the same.
+ *
+ *     list=B(1) A(2), result=empty
+ *
+ * Now it is B's turn.  We mark its parent, C, reachable from B's side,
+ * and push it to the list:
+ *
+ *     list=C(3) A(2), result=empty
+ *
+ * Now pop C and notice it has flags==3.  It is placed on the result list,
+ * and the list now contains:
+ *
+ *     list=A(2), result=C(3)
+ *
+ * We pop A and do the same.
+ * 
+ *     list=B(3), result=C(3)
+ *
+ * Next, we pop B and something very interesting happens.  It has flags==3
+ * so it is also placed on the result list, and its parents are marked
+ * uninteresting, retroactively, and placed back on the list:
+ *
+ *    list=C(7), result=C(7) B(3)
+ * 
+ * Now, list does not have any interesting commit.  So we find the newest
+ * commit from the result list that is not marked uninteresting.  Which is
+ * commit B.
+ */
+
+static struct commit *common_ancestor(struct commit *rev1, struct commit *rev2)
+{
+	struct commit_list *list = NULL;
+	struct commit_list *result = NULL;
+
+	if (rev1 == rev2)
+		return rev1;
+
+	parse_commit(rev1);
+	parse_commit(rev2);
+
+	rev1->object.flags |= 1;
+	rev2->object.flags |= 2;
+	insert_by_date(rev1, &list);
+	insert_by_date(rev2, &list);
+
+	while (interesting(list)) {
+		struct commit *commit = list->item;
+		struct commit_list *tmp = list, *parents;
+		int flags = commit->object.flags & 7;
+
+		list = list->next;
+		free(tmp);
+		if (flags == 3) {
+			insert_by_date(commit, &result);
+
+			/* Mark children of a found merge uninteresting */
+			flags |= UNINTERESTING;
+		}
+		parents = commit->parents;
+		while (parents) {
+			struct commit *p = parents->item;
+			parents = parents->next;
+			if ((p->object.flags & flags) == flags)
+				continue;
+			parse_commit(p);
+			p->object.flags |= flags;
+			insert_by_date(p, &list);
+		}
+	}
+	return interesting(result);
+}
+
+int main(int argc, char **argv)
+{
+	struct commit *rev1, *rev2, *ret;
+	unsigned char rev1key[20], rev2key[20];
+
+	if (argc != 3 ||
+	    get_sha1(argv[1], rev1key) ||
+	    get_sha1(argv[2], rev2key)) {
+		usage("git-merge-base <commit-id> <commit-id>");
+	}
+	rev1 = lookup_commit_reference(rev1key);
+	rev2 = lookup_commit_reference(rev2key);
+	if (!rev1 || !rev2)
+		return 1;
+	ret = common_ancestor(rev1, rev2);
+	if (!ret)
+		return 1;
+	printf("%s\n", sha1_to_hex(ret->object.sha1));
+	return 0;
+}