#include "cache.h"
#include "commit.h"
#include "diff.h"
#include "revision.h"
#include "bisect.h"

/* bits #0-15 in revision.h */

#define COUNTED         (1u<<16)

/*
 * This is a truly stupid algorithm, but it's only
 * used for bisection, and we just don't care enough.
 *
 * We care just barely enough to avoid recursing for
 * non-merge entries.
 */
static int count_distance(struct commit_list *entry)
{
        int nr = 0;

        while (entry) {
                struct commit *commit = entry->item;
                struct commit_list *p;

                if (commit->object.flags & (UNINTERESTING | COUNTED))
                        break;
                if (!(commit->object.flags & TREESAME))
                        nr++;
                commit->object.flags |= COUNTED;
                p = commit->parents;
                entry = p;
                if (p) {
                        p = p->next;
                        while (p) {
                                nr += count_distance(p);
                                p = p->next;
                        }
                }
        }

        return nr;
}

static void clear_distance(struct commit_list *list)
{
        while (list) {
                struct commit *commit = list->item;
                commit->object.flags &= ~COUNTED;
                list = list->next;
        }
}

#define DEBUG_BISECT 0

static inline int weight(struct commit_list *elem)
{
        return *((int*)(elem->item->util));
}

static inline void weight_set(struct commit_list *elem, int weight)
{
        *((int*)(elem->item->util)) = weight;
}

static int count_interesting_parents(struct commit *commit)
{
        struct commit_list *p;
        int count;

        for (count = 0, p = commit->parents; p; p = p->next) {
                if (p->item->object.flags & UNINTERESTING)
                        continue;
                count++;
        }
        return count;
}

static inline int halfway(struct commit_list *p, int nr)
{
        /*
         * Don't short-cut something we are not going to return!
         */
        if (p->item->object.flags & TREESAME)
                return 0;
        if (DEBUG_BISECT)
                return 0;
        /*
         * 2 and 3 are halfway of 5.
         * 3 is halfway of 6 but 2 and 4 are not.
         */
        switch (2 * weight(p) - nr) {
        case -1: case 0: case 1:
                return 1;
        default:
                return 0;
        }
}

#if !DEBUG_BISECT
#define show_list(a,b,c,d) do { ; } while (0)
#else
static void show_list(const char *debug, int counted, int nr,
                      struct commit_list *list)
{
        struct commit_list *p;

        fprintf(stderr, "%s (%d/%d)\n", debug, counted, nr);

        for (p = list; p; p = p->next) {
                struct commit_list *pp;
                struct commit *commit = p->item;
                unsigned flags = commit->object.flags;
                enum object_type type;
                unsigned long size;
                char *buf = read_sha1_file(commit->object.sha1, &type, &size);
                char *ep, *sp;

                fprintf(stderr, "%c%c%c ",
                        (flags & TREESAME) ? ' ' : 'T',
                        (flags & UNINTERESTING) ? 'U' : ' ',
                        (flags & COUNTED) ? 'C' : ' ');
                if (commit->util)
                        fprintf(stderr, "%3d", weight(p));
                else
                        fprintf(stderr, "---");
                fprintf(stderr, " %.*s", 8, sha1_to_hex(commit->object.sha1));
                for (pp = commit->parents; pp; pp = pp->next)
                        fprintf(stderr, " %.*s", 8,
                                sha1_to_hex(pp->item->object.sha1));

                sp = strstr(buf, "\n\n");
                if (sp) {
                        sp += 2;
                        for (ep = sp; *ep && *ep != '\n'; ep++)
                                ;
                        fprintf(stderr, " %.*s", (int)(ep - sp), sp);
                }
                fprintf(stderr, "\n");
        }
}
#endif /* DEBUG_BISECT */

static struct commit_list *best_bisection(struct commit_list *list, int nr)
{
        struct commit_list *p, *best;
        int best_distance = -1;

        best = list;
        for (p = list; p; p = p->next) {
                int distance;
                unsigned flags = p->item->object.flags;

                if (flags & TREESAME)
                        continue;
                distance = weight(p);
                if (nr - distance < distance)
                        distance = nr - distance;
                if (distance > best_distance) {
                        best = p;
                        best_distance = distance;
                }
        }

        return best;
}

struct commit_dist {
        struct commit *commit;
        int distance;
};

static int compare_commit_dist(const void *a_, const void *b_)
{
        struct commit_dist *a, *b;

        a = (struct commit_dist *)a_;
        b = (struct commit_dist *)b_;
        if (a->distance != b->distance)
                return b->distance - a->distance; /* desc sort */
        return hashcmp(a->commit->object.sha1, b->commit->object.sha1);
}

static struct commit_list *best_bisection_sorted(struct commit_list *list, int nr)
{
        struct commit_list *p;
        struct commit_dist *array = xcalloc(nr, sizeof(*array));
        int cnt, i;

        for (p = list, cnt = 0; p; p = p->next) {
                int distance;
                unsigned flags = p->item->object.flags;

                if (flags & TREESAME)
                        continue;
                distance = weight(p);
                if (nr - distance < distance)
                        distance = nr - distance;
                array[cnt].commit = p->item;
                array[cnt].distance = distance;
                cnt++;
        }
        qsort(array, cnt, sizeof(*array), compare_commit_dist);
        for (p = list, i = 0; i < cnt; i++) {
                struct name_decoration *r = xmalloc(sizeof(*r) + 100);
                struct object *obj = &(array[i].commit->object);

                sprintf(r->name, "dist=%d", array[i].distance);
                r->next = add_decoration(&name_decoration, obj, r);
                p->item = array[i].commit;
                p = p->next;
        }
        if (p)
                p->next = NULL;
        free(array);
        return list;
}

/*
 * zero or positive weight is the number of interesting commits it can
 * reach, including itself.  Especially, weight = 0 means it does not
 * reach any tree-changing commits (e.g. just above uninteresting one
 * but traversal is with pathspec).
 *
 * weight = -1 means it has one parent and its distance is yet to
 * be computed.
 *
 * weight = -2 means it has more than one parent and its distance is
 * unknown.  After running count_distance() first, they will get zero
 * or positive distance.
 */
static struct commit_list *do_find_bisection(struct commit_list *list,
                                             int nr, int *weights,
                                             int find_all)
{
        int n, counted;
        struct commit_list *p;

        counted = 0;

        for (n = 0, p = list; p; p = p->next) {
                struct commit *commit = p->item;
                unsigned flags = commit->object.flags;

                p->item->util = &weights[n++];
                switch (count_interesting_parents(commit)) {
                case 0:
                        if (!(flags & TREESAME)) {
                                weight_set(p, 1);
                                counted++;
                                show_list("bisection 2 count one",
                                          counted, nr, list);
                        }
                        /*
                         * otherwise, it is known not to reach any
                         * tree-changing commit and gets weight 0.
                         */
                        break;
                case 1:
                        weight_set(p, -1);
                        break;
                default:
                        weight_set(p, -2);
                        break;
                }
        }

        show_list("bisection 2 initialize", counted, nr, list);

        /*
         * If you have only one parent in the resulting set
         * then you can reach one commit more than that parent
         * can reach.  So we do not have to run the expensive
         * count_distance() for single strand of pearls.
         *
         * However, if you have more than one parents, you cannot
         * just add their distance and one for yourself, since
         * they usually reach the same ancestor and you would
         * end up counting them twice that way.
         *
         * So we will first count distance of merges the usual
         * way, and then fill the blanks using cheaper algorithm.
         */
        for (p = list; p; p = p->next) {
                if (p->item->object.flags & UNINTERESTING)
                        continue;
                if (weight(p) != -2)
                        continue;
                weight_set(p, count_distance(p));
                clear_distance(list);

                /* Does it happen to be at exactly half-way? */
                if (!find_all && halfway(p, nr))
                        return p;
                counted++;
        }

        show_list("bisection 2 count_distance", counted, nr, list);

        while (counted < nr) {
                for (p = list; p; p = p->next) {
                        struct commit_list *q;
                        unsigned flags = p->item->object.flags;

                        if (0 <= weight(p))
                                continue;
                        for (q = p->item->parents; q; q = q->next) {
                                if (q->item->object.flags & UNINTERESTING)
                                        continue;
                                if (0 <= weight(q))
                                        break;
                        }
                        if (!q)
                                continue;

                        /*
                         * weight for p is unknown but q is known.
                         * add one for p itself if p is to be counted,
                         * otherwise inherit it from q directly.
                         */
                        if (!(flags & TREESAME)) {
                                weight_set(p, weight(q)+1);
                                counted++;
                                show_list("bisection 2 count one",
                                          counted, nr, list);
                        }
                        else
                                weight_set(p, weight(q));

                        /* Does it happen to be at exactly half-way? */
                        if (!find_all && halfway(p, nr))
                                return p;
                }
        }

        show_list("bisection 2 counted all", counted, nr, list);

        if (!find_all)
                return best_bisection(list, nr);
        else
                return best_bisection_sorted(list, nr);
}

struct commit_list *find_bisection(struct commit_list *list,
                                          int *reaches, int *all,
                                          int find_all)
{
        int nr, on_list;
        struct commit_list *p, *best, *next, *last;
        int *weights;

        show_list("bisection 2 entry", 0, 0, list);

        /*
         * Count the number of total and tree-changing items on the
         * list, while reversing the list.
         */
        for (nr = on_list = 0, last = NULL, p = list;
             p;
             p = next) {
                unsigned flags = p->item->object.flags;

                next = p->next;
                if (flags & UNINTERESTING)
                        continue;
                p->next = last;
                last = p;
                if (!(flags & TREESAME))
                        nr++;
                on_list++;
        }
        list = last;
        show_list("bisection 2 sorted", 0, nr, list);

        *all = nr;
        weights = xcalloc(on_list, sizeof(*weights));

        /* Do the real work of finding bisection commit. */
        best = do_find_bisection(list, nr, weights, find_all);
        if (best) {
                if (!find_all)
                        best->next = NULL;
                *reaches = weight(best);
        }
        free(weights);
        return best;
}