Move estimate_bisect_steps to libgit.a

This function is used by bisect.c, part of libgit.a while
estimate_bisect_steps stays in builtin/rev-list.c. Move it to bisect.a
so we won't have undefine reference if a standalone program that uses
libgit.a happens to pull it in.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

1 files changed, 38 insertions, 0 deletions

diff --git a/bisect.c b/bisect.c
index 1aad49b1a6..bd1b7b5dac 100644
--- a/bisect.c
+++ b/bisect.c
@@ -956,3 +956,41 @@ int bisect_next_all(const char *prefix, int no_checkout)
 	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;
+}