Performance improvement

Author: macaba

source/NSD/GoertzelFilter.cs

@@ -4,39 +4,33 @@ namespace NSD
 {
     public class GoertzelFilter
     {
-        public double coeff;
-        public double sine;
-        public double cosine;
+        private readonly double coeff;
+        private readonly double sine;
+        private readonly double cosine;
 
         public GoertzelFilter(double filterFreq, double sampleFreq)
         {
-            double w = 2 * Math.PI * (filterFreq / sampleFreq);
-            double wr, wi;
-
-            wr = Math.Cos(w);
-            wi = Math.Sin(w);
-            coeff = 2 * wr;
-            cosine = wr;
-            sine = wi;
+            double w = 2.0 * Math.PI * (filterFreq / sampleFreq);
+            cosine = Math.Cos(w);
+            sine = Math.Sin(w);
+            coeff = 2.0 * cosine;
         }
 
         public Complex Process(Span<double> samples)
         {
-            double sprev = 0.0;
-            double sprev2 = 0.0;
-            double s, imag, real;
-            int n;
+            double Q0 = 0.0;
+            double Q1 = 0.0;
+            double Q2 = 0.0;
 
-            for (n = 0; n < samples.Length; n++)
+            for (int n = 0; n < samples.Length; n++)
             {
-                s = samples[n] + coeff * sprev - sprev2;
-                sprev2 = sprev;
-                sprev = s;
+                Q0 = coeff * Q1 - Q2 + samples[n];
+                Q2 = Q1;
+                Q1 = Q0;
             }
 
-            real = sprev * cosine - sprev2;
-            imag = -sprev * sine;
-
+            var real = Q1 * cosine - Q2;
+            var imag = -Q1 * sine;
             return new Complex(real, imag);
         }
     }

source/NSD/Windows.cs

@@ -1,6 +1,4 @@
-using System;
-
-namespace NSD
+namespace NSD
 {
     public static class Windows
     {
@@ -13,11 +11,13 @@ public static Memory<double> HFT95(int width, out double optimumOverlap, out dou
             optimumOverlap = 0.756;
             NENBW = 3.8112;
             Memory<double> window = new double[width];
+            var windowSpan = window.Span;
+            double angleIncrement = 2.0 * Math.PI / width;
             for (int i = 0; i < width; i++)
             {
-                double z = (2.0 * Math.PI * i) / width;
+                double z = angleIncrement * i;
                 double wj = 1 - (1.9383379 * Math.Cos(z)) + (1.3045202 * Math.Cos(2 * z)) - (0.4028270 * Math.Cos(3 * z)) + (0.0350665 * Math.Cos(4 * z));
-                window.Span[i] = wj;
+                windowSpan[i] = wj;
             }
             return window;
         }
@@ -31,11 +31,13 @@ public static Memory<double> HFT90D(int width, out double optimumOverlap, out do
             optimumOverlap = 0.76;
             NENBW = 3.8832;
             Memory<double> window = new double[width];
+            var windowSpan = window.Span;
+            double angleIncrement = 2.0 * Math.PI / width;
             for (int i = 0; i < width; i++)
             {
-                double z = (2.0 * Math.PI * i) / width;
+                double z = angleIncrement * i;
                 double wj = 1 - (1.942604 * Math.Cos(z)) + (1.340318 * Math.Cos(2 * z)) - (0.440811 * Math.Cos(3 * z)) + (0.043097 * Math.Cos(4 * z));
-                window.Span[i] = wj;
+                windowSpan[i] = wj;
             }
 
             return window;
@@ -50,11 +52,13 @@ public static Memory<double> FTNI(int width, out double optimumOverlap, out doub
             optimumOverlap = 0.656;
             NENBW = 2.9656;
             Memory<double> window = new double[width];
+            var windowSpan = window.Span;
+            double angleIncrement = 2.0 * Math.PI / width;
             for (int i = 0; i < width; i++)
             {
-                double z = (2.0 * Math.PI * i) / width;
+                double z = angleIncrement * i;
                 double wj = 0.2810639 - (0.5208972 * Math.Cos(z)) + (0.1980399 * Math.Cos(2 * z));
-                window.Span[i] = wj;
+                windowSpan[i] = wj;
             }
             return window;
         }