#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 int show_all = 0;

static int merge_base(struct commit *rev1, struct commit *rev2)
{
        struct commit_list *list = NULL;
        struct commit_list *result = 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 *tmp = list, *parents;
                int flags = commit->object.flags & 7;

                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;

        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];

        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 ||
            get_sha1(argv[1], rev1key) ||
            get_sha1(argv[2], rev2key))
                usage(merge_base_usage);
        rev1 = lookup_commit_reference(rev1key);
        rev2 = lookup_commit_reference(rev2key);
        if (!rev1 || !rev2)
                return 1;
        return merge_base(rev1, rev2);
}