From: Juergen Perlinger <perlinger@ntp.org>
Date: Sun, 28 Jun 2020 06:39:28 +0000 (+0200)
Subject: [Bug 3672] fix biased selection in median cut
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=eb509f9a2df1474ce838da4f12f57613c5a76181;p=thirdparty%2Fntp.git

[Bug 3672] fix biased selection in median cut

bk: 5ef83b20C0U4aqQyHOKWkS-wJ8fjLw
---

diff --git a/ChangeLog b/ChangeLog
index eeceaa9f1..e49745ab7 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,6 @@
+---
+* [Bug 3672] fix biased selection in median cut <perlinger@ntp.org>
+
 ---
 (4.2.8p15) 2020/06/23 Released by Harlan Stenn <stenn@ntp.org>
 
diff --git a/ntpd/ntp_refclock.c b/ntpd/ntp_refclock.c
index 872f4a052..33ffb90ce 100644
--- a/ntpd/ntp_refclock.c
+++ b/ntpd/ntp_refclock.c
@@ -583,7 +583,6 @@ refclock_sample(
 {
 	size_t	i, j, k, m, n;
 	double	off[MAXSTAGE];
-	double	offset;
 
 	/*
 	 * Copy the raw offsets and sort into ascending order. Don't do
@@ -601,12 +600,16 @@ refclock_sample(
 	/*
 	 * Reject the furthest from the median of the samples until
 	 * approximately 60 percent of the samples remain.
+	 *
+	 * [Bug 3672] The elimination is now based on the proper
+	 * definition of the median. The true median is not calculated
+	 * directly, though.
 	 */
 	i = 0; j = n;
 	m = n - (n * 4) / 10;
-	while ((j - i) > m) {
-		offset = off[(j + i) / 2];
-		if (off[j - 1] - offset < offset - off[i])
+	while ((k = j - i) > m) {
+		k = (k - 1) >> 1;
+		if ((off[j - 1] - off[j - k - 1]) < (off[i + k] - off[i]))
 			i++;	/* reject low end */
 		else
 			j--;	/* reject high end */