#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 can fail to mark an ancestor
 * of a merge base as UNINTERESTING without the postprocessing phase.
 *
 *                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 end up showing E as an interesting merge base.
 */

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();

        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);
}