svn commit: r329723 - head/usr.bin/ministat

[Ed Maste]

Author: emaste
Date: Wed Feb 21 15:54:23 2018
New Revision: 329723
URL: https://svnweb.freebsd.org/changeset/base/329723

Log:
  ministat: disallow negative variance / nan Stddev
  
  With all values identical it was possible for Var() to return a negative
  value due to limited floating point precision, resulting in "nan"
  reported as Stddev.
  
  Variance cannot actually be negative, so just return 0.  We can later
  investigate alternate algorithms for calculating variance to reduce the
  effect of catastrophic cancellation here.
  
  Reported by:	Arshan Khanifar <arshankhanifar_gmail.com>
  Approved by:	phk
  Sponsored by:	The FreeBSD Foundation

Modified:
  head/usr.bin/ministat/ministat.c

Modified: head/usr.bin/ministat/ministat.c
==============================================================================
--- head/usr.bin/ministat/ministat.c	Wed Feb 21 15:51:48 2018	(r329722)
+++ head/usr.bin/ministat/ministat.c	Wed Feb 21 15:54:23 2018	(r329723)
@@ -208,6 +208,12 @@ static double
 Var(struct dataset *ds)
 {
 
+	/*
+	 * Due to limited precision it is possible that sy^2/n > syy,
+	 * but variance cannot actually be negative.
+	 */
+	if (ds->syy <= ds->sy * ds->sy / ds->n)
+		return (0);
 	return (ds->syy - ds->sy * ds->sy / ds->n) / (ds->n - 1.0);
 }