b4ad66b778
Also add some illustration requested by Linus. Signed-off-by: Junio C Hamano <junkio@cox.net>
148 lines
3.5 KiB
C
148 lines
3.5 KiB
C
#include <stdlib.h>
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#include "cache.h"
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#include "commit.h"
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#define PARENT1 1
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#define PARENT2 2
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#define UNINTERESTING 4
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static struct commit *interesting(struct commit_list *list)
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{
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while (list) {
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struct commit *commit = list->item;
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list = list->next;
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if (commit->object.flags & UNINTERESTING)
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continue;
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return commit;
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}
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return NULL;
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}
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/*
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* A pathological example of how this thing works.
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*
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* Suppose we had this commit graph, where chronologically
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* the timestamp on the commit are A <= B <= C <= D <= E <= F
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* and we are trying to figure out the merge base for E and F
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* commits.
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*
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* F
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* / \
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* E A D
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* \ / /
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* B /
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* \ /
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* C
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*
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* First we push E and F to list to be processed. E gets bit 1
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* and F gets bit 2. The list becomes:
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*
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* list=F(2) E(1), result=empty
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*
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* Then we pop F, the newest commit, from the list. Its flag is 2.
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* We scan its parents, mark them reachable from the side that F is
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* reachable from, and push them to the list:
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*
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* list=E(1) D(2) A(2), result=empty
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*
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* Next pop E and do the same.
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*
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* list=D(2) B(1) A(2), result=empty
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*
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* Next pop D and do the same.
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*
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* list=C(2) B(1) A(2), result=empty
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*
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* Next pop C and do the same.
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*
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* list=B(1) A(2), result=empty
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*
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* Now it is B's turn. We mark its parent, C, reachable from B's side,
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* and push it to the list:
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*
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* list=C(3) A(2), result=empty
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*
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* Now pop C and notice it has flags==3. It is placed on the result list,
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* and the list now contains:
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*
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* list=A(2), result=C(3)
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*
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* We pop A and do the same.
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*
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* list=B(3), result=C(3)
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*
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* Next, we pop B and something very interesting happens. It has flags==3
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* so it is also placed on the result list, and its parents are marked
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* uninteresting, retroactively, and placed back on the list:
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*
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* list=C(7), result=C(7) B(3)
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*
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* Now, list does not have any interesting commit. So we find the newest
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* commit from the result list that is not marked uninteresting. Which is
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* commit B.
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*/
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static struct commit *common_ancestor(struct commit *rev1, struct commit *rev2)
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{
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struct commit_list *list = NULL;
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struct commit_list *result = NULL;
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if (rev1 == rev2)
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return rev1;
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parse_commit(rev1);
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parse_commit(rev2);
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rev1->object.flags |= 1;
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rev2->object.flags |= 2;
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insert_by_date(rev1, &list);
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insert_by_date(rev2, &list);
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while (interesting(list)) {
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struct commit *commit = list->item;
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struct commit_list *tmp = list, *parents;
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int flags = commit->object.flags & 7;
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list = list->next;
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free(tmp);
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if (flags == 3) {
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insert_by_date(commit, &result);
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/* Mark children of a found merge uninteresting */
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flags |= UNINTERESTING;
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}
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parents = commit->parents;
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while (parents) {
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struct commit *p = parents->item;
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parents = parents->next;
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if ((p->object.flags & flags) == flags)
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continue;
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parse_commit(p);
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p->object.flags |= flags;
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insert_by_date(p, &list);
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}
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}
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return interesting(result);
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}
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int main(int argc, char **argv)
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{
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struct commit *rev1, *rev2, *ret;
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unsigned char rev1key[20], rev2key[20];
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if (argc != 3 ||
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get_sha1(argv[1], rev1key) ||
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get_sha1(argv[2], rev2key)) {
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usage("git-merge-base <commit-id> <commit-id>");
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}
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rev1 = lookup_commit_reference(rev1key);
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rev2 = lookup_commit_reference(rev2key);
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if (!rev1 || !rev2)
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return 1;
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ret = common_ancestor(rev1, rev2);
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if (!ret)
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return 1;
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printf("%s\n", sha1_to_hex(ret->object.sha1));
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return 0;
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}
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