From a2a8e7818cee9efc8e53416ddc1a2e8cfb73dffe Mon Sep 17 00:00:00 2001
From: Vincent Arkesteijn <vincent@arkesteijn.net>
Date: Thu, 27 Apr 2017 15:08:35 +0200
Subject: [PATCH] fixed shifting of filter history in fifth_order()

fifth_order() filters and decimates by two, and therefore always shifts
by two input samples for every output sample. However, for the first
output sample, it shifted the state from the previous call to
fifth_order() by only one input sample, and it never used the last
sample in its input buffer.
---
 rtl_ais.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/rtl_ais.c b/rtl_ais.c
index 9b40435..b341356 100644
--- a/rtl_ais.c
+++ b/rtl_ais.c
@@ -154,12 +154,12 @@ static void fifth_order(int16_t *data, int length, int16_t *hist)
 {
 	int i;
 	int16_t a, b, c, d, e, f;
-	a = hist[1];
-	b = hist[2];
-	c = hist[3];
-	d = hist[4];
-	e = hist[5];
-	f = data[0];
+	a = hist[2];
+	b = hist[3];
+	c = hist[4];
+	d = hist[5];
+	e = data[0];
+	f = data[2];
 	/* a downsample should improve resolution, so don't fully shift */
 	data[0] = (a + (b+e)*5 + (c+d)*10 + f) >> 4;
 	for (i=4; i<length; i+=4) {
@@ -167,8 +167,8 @@ static void fifth_order(int16_t *data, int length, int16_t *hist)
 		b = d;
 		c = e;
 		d = f;
-		e = data[i-2];
-		f = data[i];
+		e = data[i];
+		f = data[i+2];
 		data[i/2] = (a + (b+e)*5 + (c+d)*10 + f) >> 4;
 	}
 	/* archive */