--- a/stat.c	Wed Jan 22 14:45:07 2014 +0100
+++ b/stat.c	Wed Jan 22 14:45:22 2014 +0100
@@ -1,3 +1,4 @@
+#include <ansi_c.h>
 #include "stat.h"
 
 void stat_zero(struct stat *s)
@@ -22,22 +23,22 @@
 }
 
 
-void rollmean_zero(struct rollmean *s, int wlen)
+void rollmean_zero(struct rollmean *s)
 {
-	s->wlen = wlen;
 	s->nobs = 0;
 	s->mean = 0.0;
 	s->acc = 0.0;
-	s->prev = 0.0;
+	memset(s->prev, 0, sizeof(s->prev));
 }
 
 
 void rollmean_accumulate(struct rollmean *s, double value)
 {
-	s->acc = s->acc - s->prev + value;
-	if (s->nobs < s->wlen)
-		s->nobs++;
+	s->acc = s->acc + value - s->prev[s->nobs % _ROLLMEAN_WLEN];
+	s->prev[s->nobs % _ROLLMEAN_WLEN] = value;
+	s->nobs++;
+	if (s->nobs < _ROLLMEAN_WLEN)
+		s->mean = s->acc / s->nobs;
 	else
-		s->prev = s->acc;
-	s->mean = s->acc / s->nobs;
+		s->mean = s->acc / _ROLLMEAN_WLEN;
 }