Merge branch 'sg/bisect-approximately-halfway'

"git bisect start/next" in a large span of history spends a lot of
time trying to come up with exactly the half-way point; this can be
optimized by stopping when we see a commit that is close enough to
the half-way point.

* sg/bisect-approximately-halfway:
  bisect: loosen halfway() check for a large number of commits

bisect.c

@@ -103,8 +103,10 @@ static int count_interesting_parents(struct commit *commit, unsigned bisect_flag
 	return count;
 }
 
-static inline int halfway(struct commit_list *p, int nr)
+static inline int approx_halfway(struct commit_list *p, int nr)
 {
+	int diff;
+
 	/*
 	 * Don't short-cut something we are not going to return!
 	 */
@@ -113,13 +115,22 @@ static inline int halfway(struct commit_list *p, int nr)
 	if (DEBUG_BISECT)
 		return 0;
 	/*
-	 * 2 and 3 are halfway of 5.
+	 * For small number of commits 2 and 3 are halfway of 5, and
 	 * 3 is halfway of 6 but 2 and 4 are not.
 	 */
-	switch (2 * weight(p) - nr) {
+	diff = 2 * weight(p) - nr;
+	switch (diff) {
 	case -1: case 0: case 1:
 		return 1;
 	default:
+		/*
+		 * For large number of commits we are not so strict, it's
+		 * good enough if it's within ~0.1% of the halfway point,
+		 * e.g. 5000 is exactly halfway of 10000, but we consider
+		 * the values [4996, 5004] as halfway as well.
+		 */
+		if (abs(diff) < nr / 1024)
+			return 1;
 		return 0;
 	}
 }
@@ -321,8 +332,9 @@ static struct commit_list *do_find_bisection(struct commit_list *list,
 		weight_set(p, count_distance(p));
 		clear_distance(list);
 
-		/* Does it happen to be at exactly half-way? */
-		if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr))
+		/* Does it happen to be at half-way? */
+		if (!(bisect_flags & FIND_BISECTION_ALL) &&
+		      approx_halfway(p, nr))
 			return p;
 		counted++;
 	}
@@ -362,8 +374,9 @@ static struct commit_list *do_find_bisection(struct commit_list *list,
 			else
 				weight_set(p, weight(q));
 
-			/* Does it happen to be at exactly half-way? */
-			if (!(bisect_flags & FIND_BISECTION_ALL) && halfway(p, nr))
+			/* Does it happen to be at half-way? */
+			if (!(bisect_flags & FIND_BISECTION_ALL) &&
+			      approx_halfway(p, nr))
 				return p;
 		}
 	}