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 const 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};

/* 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);
		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++) {
		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;
}

static int register_ref(const char *refname, const unsigned char *sha1,
			int flags, void *cb_data)
{
	if (!strcmp(refname, "bad")) {
		current_bad_sha1 = sha1;
	} else if (!prefixcmp(refname, "good-")) {
		sha1_array_append(&good_revs, sha1);
	} else if (!prefixcmp(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 int get_prn(int count) {
	count = count * 1103515245 + 12345;
	return ((unsigned)(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->commits, 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)
{
	int res;

	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 {
		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, 0);

	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;
}