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

@@ -154,21 +154,21 @@ 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) {
 		a = c;
 		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 */