Also add some illustration requested by Linus.
Signed-off-by: Junio C Hamano [EMAIL PROTECTED]
---
merge-base.c | 74 +-
1 files changed, 68 insertions(+), 6 deletions(-)
5cbb01b3bb1828759596bff71e16cfcee798491c
diff --git a/merge-base.c b/merge-base.c
--- a/merge-base.c
+++ b/merge-base.c
@@ -6,18 +6,82 @@
#define PARENT2 2
#define UNINTERESTING 4
-static int interesting(struct commit_list *list)
+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 1;
+ return commit;
}
- return 0;
+ 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;
@@ -58,9 +122,7 @@ static struct commit *common_ancestor(st
insert_by_date(p, list);
}
}
- if (!result)
- return NULL;
- return result-item;
+ return interesting(result);
}
int main(int argc, char **argv)
-
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