resampler: use more accurate bessel function for Kaiser window
diff --git a/src/flowgraph/resampler/KaiserWindow.h b/src/flowgraph/resampler/KaiserWindow.h
new file mode 100644
index 0000000..6a5d086
--- /dev/null
+++ b/src/flowgraph/resampler/KaiserWindow.h
@@ -0,0 +1,64 @@
+/*
+ * Copyright 2019 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef RESAMPLER_KAISER_WINDOW_H
+#define RESAMPLER_KAISER_WINDOW_H
+
+#include "Math.h"
+
+/**
+ * Calculate a Kaiser window centered at 0.
+ */
+class KaiserWindow {
+public:
+ KaiserWindow() {
+ setStopBandAttenuation(90);
+ }
+
+ /**
+ * @param attenuation typical values range from 30 to 90 dB
+ */
+ double setStopBandAttenuation(double attenuation) {
+ double beta = 0.0;
+ if (attenuation > 50) {
+ beta = 0.1102 * (attenuation - 8.7);
+ } else if (attenuation >= 21) {
+ double a21 = attenuation - 21;
+ beta = 0.5842 * pow(a21, 0.4) + (0.07886 * a21);
+ }
+ setBeta(beta);
+ return beta;
+ }
+
+ void setBeta(double beta) {
+ mBeta = beta;
+ mInverseBesselBeta = 1.0 / Math::bessel(beta);
+ }
+
+ /**
+ * @param x ranges from -1.0 to +1.0
+ */
+ double operator()(double x) {
+ double w = mBeta * sqrt(1.0 - (x * x));
+ return Math::bessel(w) * mInverseBesselBeta;
+ }
+
+private:
+ double mBeta = 0.0;
+ double mInverseBesselBeta = 1.0;
+};
+
+#endif //RESAMPLER_KAISER_WINDOW_H
