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[git/mjg.git] / bisect.c

#include "cache.h"
#include "commit.h"
#include "diff.h"
#include "revision.h"
#include "refs.h"
#include "list-objects.h"
#include "quote.h"
#include "sha1-lookup.h"
#include "run-command.h"
#include "log-tree.h"
#include "bisect.h"
#include "sha1-array.h"
#include "argv-array.h"

static struct sha1_array good_revs;
static struct sha1_array skipped_revs;

static unsigned char *current_bad_sha1;

static const char *argv_checkout[] = {"checkout", "-q", NULL, "--", NULL};
static const char *argv_show_branch[] = {"show-branch", NULL, NULL};
static const char *argv_update_ref[] = {"update-ref", "--no-deref", "BISECT_HEAD", NULL, NULL};

/* Remember to update object flag allocation in object.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);
                const char *subject_start;
                int subject_len;

                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));

                subject_len = find_commit_subject(buf, &subject_start);
                if (subject_len)
                        fprintf(stderr, " %.*s", subject_len, subject_start);
                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++) {
                char buf[100]; /* enough for dist=%d */
                struct object *obj = &(array[i].commit->object);

                snprintf(buf, sizeof(buf), "dist=%d", array[i].distance);
                add_name_decoration(DECORATION_NONE, buf, obj);

                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;
}

static int register_ref(const char *refname, const unsigned char *sha1,
                        int flags, void *cb_data)
{
        if (!strcmp(refname, "bad")) {
                current_bad_sha1 = xmalloc(20);
                hashcpy(current_bad_sha1, sha1);
        } else if (starts_with(refname, "good-")) {
                sha1_array_append(&good_revs, sha1);
        } else if (starts_with(refname, "skip-")) {
                sha1_array_append(&skipped_revs, sha1);
        }

        return 0;
}

static int read_bisect_refs(void)
{
        return for_each_ref_in("refs/bisect/", register_ref, NULL);
}

static void read_bisect_paths(struct argv_array *array)
{
        struct strbuf str = STRBUF_INIT;
        const char *filename = git_path("BISECT_NAMES");
        FILE *fp = fopen(filename, "r");

        if (!fp)
                die_errno("Could not open file '%s'", filename);

        while (strbuf_getline(&str, fp, '\n') != EOF) {
                strbuf_trim(&str);
                if (sq_dequote_to_argv_array(str.buf, array))
                        die("Badly quoted content in file '%s': %s",
                            filename, str.buf);
        }

        strbuf_release(&str);
        fclose(fp);
}

static char *join_sha1_array_hex(struct sha1_array *array, char delim)
{
        struct strbuf joined_hexs = STRBUF_INIT;
        int i;

        for (i = 0; i < array->nr; i++) {
                strbuf_addstr(&joined_hexs, sha1_to_hex(array->sha1[i]));
                if (i + 1 < array->nr)
                        strbuf_addch(&joined_hexs, delim);
        }

        return strbuf_detach(&joined_hexs, NULL);
}

/*
 * In this function, passing a not NULL skipped_first is very special.
 * It means that we want to know if the first commit in the list is
 * skipped because we will want to test a commit away from it if it is
 * indeed skipped.
 * So if the first commit is skipped, we cannot take the shortcut to
 * just "return list" when we find the first non skipped commit, we
 * have to return a fully filtered list.
 *
 * We use (*skipped_first == -1) to mean "it has been found that the
 * first commit is not skipped". In this case *skipped_first is set back
 * to 0 just before the function returns.
 */
struct commit_list *filter_skipped(struct commit_list *list,
                                   struct commit_list **tried,
                                   int show_all,
                                   int *count,
                                   int *skipped_first)
{
        struct commit_list *filtered = NULL, **f = &filtered;

        *tried = NULL;

        if (skipped_first)
                *skipped_first = 0;
        if (count)
                *count = 0;

        if (!skipped_revs.nr)
                return list;

        while (list) {
                struct commit_list *next = list->next;
                list->next = NULL;
                if (0 <= sha1_array_lookup(&skipped_revs,
                                           list->item->object.sha1)) {
                        if (skipped_first && !*skipped_first)
                                *skipped_first = 1;
                        /* Move current to tried list */
                        *tried = list;
                        tried = &list->next;
                } else {
                        if (!show_all) {
                                if (!skipped_first || !*skipped_first)
                                        return list;
                        } else if (skipped_first && !*skipped_first) {
                                /* This means we know it's not skipped */
                                *skipped_first = -1;
                        }
                        /* Move current to filtered list */
                        *f = list;
                        f = &list->next;
                        if (count)
                                (*count)++;
                }
                list = next;
        }

        if (skipped_first && *skipped_first == -1)
                *skipped_first = 0;

        return filtered;
}

#define PRN_MODULO 32768

/*
 * This is a pseudo random number generator based on "man 3 rand".
 * It is not used properly because the seed is the argument and it
 * is increased by one between each call, but that should not matter
 * for this application.
 */
static unsigned get_prn(unsigned count) {
        count = count * 1103515245 + 12345;
        return (count/65536) % PRN_MODULO;
}

/*
 * Custom integer square root from
 * http://en.wikipedia.org/wiki/Integer_square_root
 */
static int sqrti(int val)
{
        float d, x = val;

        if (val == 0)
                return 0;

        do {
                float y = (x + (float)val / x) / 2;
                d = (y > x) ? y - x : x - y;
                x = y;
        } while (d >= 0.5);

        return (int)x;
}

static struct commit_list *skip_away(struct commit_list *list, int count)
{
        struct commit_list *cur, *previous;
        int prn, index, i;

        prn = get_prn(count);
        index = (count * prn / PRN_MODULO) * sqrti(prn) / sqrti(PRN_MODULO);

        cur = list;
        previous = NULL;

        for (i = 0; cur; cur = cur->next, i++) {
                if (i == index) {
                        if (hashcmp(cur->item->object.sha1, current_bad_sha1))
                                return cur;
                        if (previous)
                                return previous;
                        return list;
                }
                previous = cur;
        }

        return list;
}

static struct commit_list *managed_skipped(struct commit_list *list,
                                           struct commit_list **tried)
{
        int count, skipped_first;

        *tried = NULL;

        if (!skipped_revs.nr)
                return list;

        list = filter_skipped(list, tried, 0, &count, &skipped_first);

        if (!skipped_first)
                return list;

        return skip_away(list, count);
}

static void bisect_rev_setup(struct rev_info *revs, const char *prefix,
                             const char *bad_format, const char *good_format,
                             int read_paths)
{
        struct argv_array rev_argv = ARGV_ARRAY_INIT;
        int i;

        init_revisions(revs, prefix);
        revs->abbrev = 0;
        revs->commit_format = CMIT_FMT_UNSPECIFIED;

        /* rev_argv.argv[0] will be ignored by setup_revisions */
        argv_array_push(&rev_argv, "bisect_rev_setup");
        argv_array_pushf(&rev_argv, bad_format, sha1_to_hex(current_bad_sha1));
        for (i = 0; i < good_revs.nr; i++)
                argv_array_pushf(&rev_argv, good_format,
                                 sha1_to_hex(good_revs.sha1[i]));
        argv_array_push(&rev_argv, "--");
        if (read_paths)
                read_bisect_paths(&rev_argv);

        setup_revisions(rev_argv.argc, rev_argv.argv, revs, NULL);
        /* XXX leak rev_argv, as "revs" may still be pointing to it */
}

static void bisect_common(struct rev_info *revs)
{
        if (prepare_revision_walk(revs))
                die("revision walk setup failed");
        if (revs->tree_objects)
                mark_edges_uninteresting(revs, NULL);
}

static void exit_if_skipped_commits(struct commit_list *tried,
                                    const unsigned char *bad)
{
        if (!tried)
                return;

        printf("There are only 'skip'ped commits left to test.\n"
               "The first bad commit could be any of:\n");
        print_commit_list(tried, "%s\n", "%s\n");
        if (bad)
                printf("%s\n", sha1_to_hex(bad));
        printf("We cannot bisect more!\n");
        exit(2);
}

static int is_expected_rev(const unsigned char *sha1)
{
        const char *filename = git_path("BISECT_EXPECTED_REV");
        struct stat st;
        struct strbuf str = STRBUF_INIT;
        FILE *fp;
        int res = 0;

        if (stat(filename, &st) || !S_ISREG(st.st_mode))
                return 0;

        fp = fopen(filename, "r");
        if (!fp)
                return 0;

        if (strbuf_getline(&str, fp, '\n') != EOF)
                res = !strcmp(str.buf, sha1_to_hex(sha1));

        strbuf_release(&str);
        fclose(fp);

        return res;
}

static void mark_expected_rev(char *bisect_rev_hex)
{
        int len = strlen(bisect_rev_hex);
        const char *filename = git_path("BISECT_EXPECTED_REV");
        int fd = open(filename, O_CREAT | O_TRUNC | O_WRONLY, 0600);

        if (fd < 0)
                die_errno("could not create file '%s'", filename);

        bisect_rev_hex[len] = '\n';
        write_or_die(fd, bisect_rev_hex, len + 1);
        bisect_rev_hex[len] = '\0';

        if (close(fd) < 0)
                die("closing file %s: %s", filename, strerror(errno));
}

static int bisect_checkout(char *bisect_rev_hex, int no_checkout)
{

        mark_expected_rev(bisect_rev_hex);

        argv_checkout[2] = bisect_rev_hex;
        if (no_checkout) {
                argv_update_ref[3] = bisect_rev_hex;
                if (run_command_v_opt(argv_update_ref, RUN_GIT_CMD))
                        die("update-ref --no-deref HEAD failed on %s",
                            bisect_rev_hex);
        } else {
                int res;
                res = run_command_v_opt(argv_checkout, RUN_GIT_CMD);
                if (res)
                        exit(res);
        }

        argv_show_branch[1] = bisect_rev_hex;
        return run_command_v_opt(argv_show_branch, RUN_GIT_CMD);
}

static struct commit *get_commit_reference(const unsigned char *sha1)
{
        struct commit *r = lookup_commit_reference(sha1);
        if (!r)
                die("Not a valid commit name %s", sha1_to_hex(sha1));
        return r;
}

static struct commit **get_bad_and_good_commits(int *rev_nr)
{
        int len = 1 + good_revs.nr;
        struct commit **rev = xmalloc(len * sizeof(*rev));
        int i, n = 0;

        rev[n++] = get_commit_reference(current_bad_sha1);
        for (i = 0; i < good_revs.nr; i++)
                rev[n++] = get_commit_reference(good_revs.sha1[i]);
        *rev_nr = n;

        return rev;
}

static void handle_bad_merge_base(void)
{
        if (is_expected_rev(current_bad_sha1)) {
                char *bad_hex = sha1_to_hex(current_bad_sha1);
                char *good_hex = join_sha1_array_hex(&good_revs, ' ');

                fprintf(stderr, "The merge base %s is bad.\n"
                        "This means the bug has been fixed "
                        "between %s and [%s].\n",
                        bad_hex, bad_hex, good_hex);

                exit(3);
        }

        fprintf(stderr, "Some good revs are not ancestor of the bad rev.\n"
                "git bisect cannot work properly in this case.\n"
                "Maybe you mistake good and bad revs?\n");
        exit(1);
}

static void handle_skipped_merge_base(const unsigned char *mb)
{
        char *mb_hex = sha1_to_hex(mb);
        char *bad_hex = sha1_to_hex(current_bad_sha1);
        char *good_hex = join_sha1_array_hex(&good_revs, ' ');

        warning("the merge base between %s and [%s] "
                "must be skipped.\n"
                "So we cannot be sure the first bad commit is "
                "between %s and %s.\n"
                "We continue anyway.",
                bad_hex, good_hex, mb_hex, bad_hex);
        free(good_hex);
}

/*
 * "check_merge_bases" checks that merge bases are not "bad".
 *
 * - If one is "bad", it means the user assumed something wrong
 * and we must exit with a non 0 error code.
 * - If one is "good", that's good, we have nothing to do.
 * - If one is "skipped", we can't know but we should warn.
 * - If we don't know, we should check it out and ask the user to test.
 */
static void check_merge_bases(int no_checkout)
{
        struct commit_list *result;
        int rev_nr;
        struct commit **rev = get_bad_and_good_commits(&rev_nr);

        result = get_merge_bases_many(rev[0], rev_nr - 1, rev + 1);

        for (; result; result = result->next) {
                const unsigned char *mb = result->item->object.sha1;
                if (!hashcmp(mb, current_bad_sha1)) {
                        handle_bad_merge_base();
                } else if (0 <= sha1_array_lookup(&good_revs, mb)) {
                        continue;
                } else if (0 <= sha1_array_lookup(&skipped_revs, mb)) {
                        handle_skipped_merge_base(mb);
                } else {
                        printf("Bisecting: a merge base must be tested\n");
                        exit(bisect_checkout(sha1_to_hex(mb), no_checkout));
                }
        }

        free(rev);
        free_commit_list(result);
}

static int check_ancestors(const char *prefix)
{
        struct rev_info revs;
        struct object_array pending_copy;
        int res;

        bisect_rev_setup(&revs, prefix, "^%s", "%s", 0);

        /* Save pending objects, so they can be cleaned up later. */
        pending_copy = revs.pending;
        revs.leak_pending = 1;

        /*
         * bisect_common calls prepare_revision_walk right away, which
         * (together with .leak_pending = 1) makes us the sole owner of
         * the list of pending objects.
         */
        bisect_common(&revs);
        res = (revs.commits != NULL);

        /* Clean up objects used, as they will be reused. */
        clear_commit_marks_for_object_array(&pending_copy, ALL_REV_FLAGS);
        free(pending_copy.objects);

        return res;
}

/*
 * "check_good_are_ancestors_of_bad" checks that all "good" revs are
 * ancestor of the "bad" rev.
 *
 * If that's not the case, we need to check the merge bases.
 * If a merge base must be tested by the user, its source code will be
 * checked out to be tested by the user and we will exit.
 */
static void check_good_are_ancestors_of_bad(const char *prefix, int no_checkout)
{
        char *filename = git_pathdup("BISECT_ANCESTORS_OK");
        struct stat st;
        int fd;

        if (!current_bad_sha1)
                die("a bad revision is needed");

        /* Check if file BISECT_ANCESTORS_OK exists. */
        if (!stat(filename, &st) && S_ISREG(st.st_mode))
                goto done;

        /* Bisecting with no good rev is ok. */
        if (good_revs.nr == 0)
                goto done;

        /* Check if all good revs are ancestor of the bad rev. */
        if (check_ancestors(prefix))
                check_merge_bases(no_checkout);

        /* Create file BISECT_ANCESTORS_OK. */
        fd = open(filename, O_CREAT | O_TRUNC | O_WRONLY, 0600);
        if (fd < 0)
                warning("could not create file '%s': %s",
                        filename, strerror(errno));
        else
                close(fd);
 done:
        free(filename);
}

/*
 * This does "git diff-tree --pretty COMMIT" without one fork+exec.
 */
static void show_diff_tree(const char *prefix, struct commit *commit)
{
        struct rev_info opt;

        /* diff-tree init */
        init_revisions(&opt, prefix);
        git_config(git_diff_basic_config, NULL); /* no "diff" UI options */
        opt.abbrev = 0;
        opt.diff = 1;

        /* This is what "--pretty" does */
        opt.verbose_header = 1;
        opt.use_terminator = 0;
        opt.commit_format = CMIT_FMT_DEFAULT;

        /* diff-tree init */
        if (!opt.diffopt.output_format)
                opt.diffopt.output_format = DIFF_FORMAT_RAW;

        log_tree_commit(&opt, commit);
}

/*
 * We use the convention that exiting with an exit code 10 means that
 * the bisection process finished successfully.
 * In this case the calling shell script should exit 0.
 *
 * If no_checkout is non-zero, the bisection process does not
 * checkout the trial commit but instead simply updates BISECT_HEAD.
 */
int bisect_next_all(const char *prefix, int no_checkout)
{
        struct rev_info revs;
        struct commit_list *tried;
        int reaches = 0, all = 0, nr, steps;
        const unsigned char *bisect_rev;
        char bisect_rev_hex[41];

        if (read_bisect_refs())
                die("reading bisect refs failed");

        check_good_are_ancestors_of_bad(prefix, no_checkout);

        bisect_rev_setup(&revs, prefix, "%s", "^%s", 1);
        revs.limited = 1;

        bisect_common(&revs);

        revs.commits = find_bisection(revs.commits, &reaches, &all,
                                       !!skipped_revs.nr);
        revs.commits = managed_skipped(revs.commits, &tried);

        if (!revs.commits) {
                /*
                 * We should exit here only if the "bad"
                 * commit is also a "skip" commit.
                 */
                exit_if_skipped_commits(tried, NULL);

                printf("%s was both good and bad\n",
                       sha1_to_hex(current_bad_sha1));
                exit(1);
        }

        if (!all) {
                fprintf(stderr, "No testable commit found.\n"
                        "Maybe you started with bad path parameters?\n");
                exit(4);
        }

        bisect_rev = revs.commits->item->object.sha1;
        memcpy(bisect_rev_hex, sha1_to_hex(bisect_rev), 41);

        if (!hashcmp(bisect_rev, current_bad_sha1)) {
                exit_if_skipped_commits(tried, current_bad_sha1);
                printf("%s is the first bad commit\n", bisect_rev_hex);
                show_diff_tree(prefix, revs.commits->item);
                /* This means the bisection process succeeded. */
                exit(10);
        }

        nr = all - reaches - 1;
        steps = estimate_bisect_steps(all);
        printf("Bisecting: %d revision%s left to test after this "
               "(roughly %d step%s)\n", nr, (nr == 1 ? "" : "s"),
               steps, (steps == 1 ? "" : "s"));

        return bisect_checkout(bisect_rev_hex, no_checkout);
}

static inline int log2i(int n)
{
        int log2 = 0;

        for (; n > 1; n >>= 1)
                log2++;

        return log2;
}

static inline int exp2i(int n)
{
        return 1 << n;
}

/*
 * Estimate the number of bisect steps left (after the current step)
 *
 * For any x between 0 included and 2^n excluded, the probability for
 * n - 1 steps left looks like:
 *
 * P(2^n + x) == (2^n - x) / (2^n + x)
 *
 * and P(2^n + x) < 0.5 means 2^n < 3x
 */
int estimate_bisect_steps(int all)
{
        int n, x, e;

        if (all < 3)
                return 0;

        n = log2i(all);
        e = exp2i(n);
        x = all - e;

        return (e < 3 * x) ? n : n - 1;
}