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authorChristian Couder <chriscool@tuxfamily.org>2009-06-13 05:21:06 (GMT)
committerJunio C Hamano <gitster@pobox.com>2009-06-13 17:47:34 (GMT)
commitebc9529f0358bdb10192fa27bc75f5d4e452ce90 (patch)
treea96512d00de09a2f767544f390cca87ca7844253 /bisect.c
parenta66037c9755a2beb49bbd915e6f0dd9b1a732925 (diff)
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bisect: use a PRNG with a bias when skipping away from untestable commits
Using a PRNG (pseudo random number generator) with a bias should be better than alternating between 3 fixed ratios. In repositories with many untestable commits it should prevent alternating between areas where many commits are untestable. The bias should favor commits that can give more information, so that the bisection process should not loose much efficiency. HPA suggested to use a PRNG and found that the best bias is to raise a ratio between 0 and 1 given by the PRNG to the power 1.5. An integer square root function is implemented to avoid including <math.h> and linking with -lm. A PRNG function is implemented to get the same number sequence on different machines as suggested by "man 3 rand". Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'bisect.c')
-rw-r--r--bisect.c50
1 files changed, 39 insertions, 11 deletions
diff --git a/bisect.c b/bisect.c
index 6fdff05..095b55e 100644
--- a/bisect.c
+++ b/bisect.c
@@ -585,16 +585,49 @@ struct commit_list *filter_skipped(struct commit_list *list,
return filtered;
}
-static struct commit_list *apply_skip_ratio(struct commit_list *list,
- int count,
- int skip_num, int skip_denom)
+#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.
+ */
+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)
{
- int index, i;
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;
- index = count * skip_num / skip_denom;
for (i = 0; cur; cur = cur->next, i++) {
if (i == index) {
@@ -614,7 +647,6 @@ static struct commit_list *managed_skipped(struct commit_list *list,
struct commit_list **tried)
{
int count, skipped_first;
- int skip_num, skip_denom;
*tried = NULL;
@@ -626,11 +658,7 @@ static struct commit_list *managed_skipped(struct commit_list *list,
if (!skipped_first)
return list;
- /* Use alternatively 1/5, 2/5 and 3/5 as skip ratio. */
- skip_num = count % 3 + 1;
- skip_denom = 5;
-
- return apply_skip_ratio(list, count, skip_num, skip_denom);
+ return skip_away(list, count);
}
static void bisect_rev_setup(struct rev_info *revs, const char *prefix,