git-rev-list: add --bisect-vars option.

This adds --bisect-vars option to rev-list.  The output is suitable
for `eval` in shell and defines five variables:

 - bisect_rev is the next revision to test.
 - bisect_nr is the expected number of commits to test after
   bisect_rev is tested.
 - bisect_good is the expected number of commits to test
   if bisect_rev turns out to be good.
 - bisect_bad is the expected number of commits to test
   if bisect_rev turns out to be bad.
 - bisect_all is the number of commits we are bisecting right now.

The documentation text was partly stolen from Johannes
Schindelin's patch.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rev-list.txt

@@ -26,6 +26,7 @@ SYNOPSIS
 	     [ [\--objects | \--objects-edge] [ \--unpacked ] ]
 	     [ \--pretty | \--header ]
 	     [ \--bisect ]
+	     [ \--bisect-vars ]
 	     [ \--merge ]
 	     [ \--reverse ]
 	     [ \--walk-reflogs ]
@@ -249,6 +250,18 @@ introduces a regression is thus reduced to a binary search: repeatedly
 generate and test new 'midpoint's until the commit chain is of length
 one.
 
+--bisect-vars::
+
+This calculates the same as `--bisect`, but outputs text ready
+to be eval'ed by the shell. These lines will assign the name of
+the midpoint revision to the variable `bisect_rev`, and the
+expected number of commits to be tested after `bisect_rev` is
+tested to `bisect_nr`, the expected number of commits to be
+tested if `bisect_rev` turns out to be good to `bisect_good`,
+the expected number of commits to be tested if `bisect_rev`
+turns out to be bad to `bisect_bad`, and the number of commits
+we are bisecting right now to `bisect_all`.
+
 --
 
 Commit Ordering

builtin-rev-list.c

@@ -36,7 +36,8 @@ static const char rev_list_usage[] =
 "    --abbrev=nr | --no-abbrev\n"
 "    --abbrev-commit\n"
 "  special purpose:\n"
-"    --bisect"
+"    --bisect\n"
+"    --bisect-vars"
 ;
 
 static struct rev_info revs;
@@ -168,7 +169,8 @@ static void clear_distance(struct commit_list *list)
 	}
 }
 
-static struct commit_list *find_bisection(struct commit_list *list)
+static struct commit_list *find_bisection(struct commit_list *list,
+					  int *reaches, int *all)
 {
 	int nr, closest;
 	struct commit_list *p, *best;
@@ -180,21 +182,23 @@ static struct commit_list *find_bisection(struct commit_list *list)
 			nr++;
 		p = p->next;
 	}
+	*all = nr;
 	closest = 0;
 	best = list;
 
 	for (p = list; p; p = p->next) {
-		int distance;
+		int distance, reach;
 
 		if (revs.prune_fn && !(p->item->object.flags & TREECHANGE))
 			continue;
 
-		distance = count_distance(p);
+		distance = reach = count_distance(p);
 		clear_distance(list);
 		if (nr - distance < distance)
 			distance = nr - distance;
 		if (distance > closest) {
 			best = p;
+			*reaches = reach;
 			closest = distance;
 		}
 	}
@@ -225,6 +229,7 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 	struct commit_list *list;
 	int i;
 	int read_from_stdin = 0;
+	int bisect_show_vars = 0;
 
 	git_config(git_default_config);
 	init_revisions(&revs, prefix);
@@ -247,6 +252,11 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 			bisect_list = 1;
 			continue;
 		}
+		if (!strcmp(arg, "--bisect-vars")) {
+			bisect_list = 1;
+			bisect_show_vars = 1;
+			continue;
+		}
 		if (!strcmp(arg, "--stdin")) {
 			if (read_from_stdin++)
 				die("--stdin given twice?");
@@ -285,8 +295,40 @@ int cmd_rev_list(int argc, const char **argv, const char *prefix)
 	if (revs.tree_objects)
 		mark_edges_uninteresting(revs.commits, &revs, show_edge);
 
-	if (bisect_list)
+	if (bisect_list) {
-		revs.commits = find_bisection(revs.commits);
+		int reaches = reaches, all = all;
+
+		revs.commits = find_bisection(revs.commits,
+					      &reaches, &all);
+		if (bisect_show_vars) {
+			int cnt;
+			if (!revs.commits)
+				return 1;
+			/*
+			 * revs.commits can reach "reaches" commits among
+			 * "all" commits.  If it is good, then there are
+			 * (all-reaches) commits left to be bisected.
+			 * On the other hand, if it is bad, then the set
+			 * to bisect is "reaches".
+			 * A bisect set of size N has (N-1) commits further
+			 * to test, as we already know one bad one.
+			 */
+			cnt = all-reaches;
+			if (cnt < reaches)
+				cnt = reaches;
+			printf("bisect_rev=%s\n"
+			       "bisect_nr=%d\n"
+			       "bisect_good=%d\n"
+			       "bisect_bad=%d\n"
+			       "bisect_all=%d\n",
+			       sha1_to_hex(revs.commits->item->object.sha1),
+			       cnt - 1,
+			       all - reaches - 1,
+			       reaches - 1,
+			       all);
+			return 0;
+		}
+	}
 
 	traverse_commit_list(&revs, show_commit, show_object);