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git/merge-base.c
Junio C Hamano c82a22c39c merge-base: Clarify the comments on post processing.
The comment fooled myself believing that we still had an
unsolved horizon effect.

Signed-off-by: Junio C Hamano <junkio@cox.net>
2006-05-16 17:18:22 -07:00

264 lines
6.2 KiB
C

#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.
*
*
* Another pathological example how this thing used to fail to mark an
* ancestor of a merge base as UNINTERESTING before we introduced the
* postprocessing phase (mark_reachable_commits).
*
* 2
* H
* 1 / \
* G A \
* |\ / \
* | B \
* | \ \
* \ C F
* \ \ /
* \ D /
* \ | /
* \| /
* E
*
* list A B C D E F G H
* G1 H2 - - - - - - 1 2
* H2 E1 B1 - 1 - - 1 - 1 2
* F2 E1 B1 A2 2 1 - - 1 2 1 2
* E3 B1 A2 2 1 - - 3 2 1 2
* B1 A2 2 1 - - 3 2 1 2
* C1 A2 2 1 1 - 3 2 1 2
* D1 A2 2 1 1 1 3 2 1 2
* A2 2 1 1 1 3 2 1 2
* B3 2 3 1 1 3 2 1 2
* C7 2 3 7 1 3 2 1 2
*
* At this point, unfortunately, everybody in the list is
* uninteresting, so we fail to complete the following two
* steps to fully marking uninteresting commits.
*
* D7 2 3 7 7 3 2 1 2
* E7 2 3 7 7 7 2 1 2
*
* and we ended up showing E as an interesting merge base.
* The postprocessing phase re-injects C and continues traversal
* to contaminate D and E.
*/
static int show_all = 0;
static void mark_reachable_commits(struct commit_list *result,
struct commit_list *list)
{
struct commit_list *tmp;
/*
* Postprocess to fully contaminate the well.
*/
for (tmp = result; tmp; tmp = tmp->next) {
struct commit *c = tmp->item;
/* Reinject uninteresting ones to list,
* so we can scan their parents.
*/
if (c->object.flags & UNINTERESTING)
commit_list_insert(c, &list);
}
while (list) {
struct commit *c = list->item;
struct commit_list *parents;
tmp = list;
list = list->next;
free(tmp);
/* Anything taken out of the list is uninteresting, so
* mark all its parents uninteresting. We do not
* parse new ones (we already parsed all the relevant
* ones).
*/
parents = c->parents;
while (parents) {
struct commit *p = parents->item;
parents = parents->next;
if (!(p->object.flags & UNINTERESTING)) {
p->object.flags |= UNINTERESTING;
commit_list_insert(p, &list);
}
}
}
}
static int merge_base(struct commit *rev1, struct commit *rev2)
{
struct commit_list *list = NULL;
struct commit_list *result = NULL;
struct commit_list *tmp = NULL;
if (rev1 == rev2) {
printf("%s\n", sha1_to_hex(rev1->object.sha1));
return 0;
}
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 *parents;
int flags = commit->object.flags & 7;
tmp = list;
list = list->next;
free(tmp);
if (flags == 3) {
insert_by_date(commit, &result);
/* Mark parents 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);
}
}
if (!result)
return 1;
if (result->next && list)
mark_reachable_commits(result, list);
while (result) {
struct commit *commit = result->item;
result = result->next;
if (commit->object.flags & UNINTERESTING)
continue;
printf("%s\n", sha1_to_hex(commit->object.sha1));
if (!show_all)
return 0;
commit->object.flags |= UNINTERESTING;
}
return 0;
}
static const char merge_base_usage[] =
"git-merge-base [--all] <commit-id> <commit-id>";
int main(int argc, char **argv)
{
struct commit *rev1, *rev2;
unsigned char rev1key[20], rev2key[20];
setup_git_directory();
git_config(git_default_config);
while (1 < argc && argv[1][0] == '-') {
char *arg = argv[1];
if (!strcmp(arg, "-a") || !strcmp(arg, "--all"))
show_all = 1;
else
usage(merge_base_usage);
argc--; argv++;
}
if (argc != 3)
usage(merge_base_usage);
if (get_sha1(argv[1], rev1key))
die("Not a valid object name %s", argv[1]);
if (get_sha1(argv[2], rev2key))
die("Not a valid object name %s", argv[2]);
rev1 = lookup_commit_reference(rev1key);
rev2 = lookup_commit_reference(rev2key);
if (!rev1 || !rev2)
return 1;
return merge_base(rev1, rev2);
}