Implement IIRFilter node for WebAudio.

third_party/WebKit/Source/platform/audio/IIRFilter.h

Index: third_party/WebKit/Source/platform/audio/IIRFilter.h
diff --git a/third_party/WebKit/Source/platform/audio/IIRFilter.h b/third_party/WebKit/Source/platform/audio/IIRFilter.h
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+++ b/third_party/WebKit/Source/platform/audio/IIRFilter.h
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+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef IIRFilter_h
+#define IIRFilter_h
+
+#include "platform/PlatformExport.h"
+#include "platform/audio/AudioArray.h"
+#include "wtf/Vector.h"
+
+namespace blink {
+
+class PLATFORM_EXPORT IIRFilter final {
+public:
+    // The maximum IIR filter order.  This also limits the number of feedforward coefficients.  The
+    // maximum number of coefficients is 20 according to the spec.
+    const static size_t kMaxOrder = 19;
+    IIRFilter(const AudioDoubleArray* feedforwardCoef, const AudioDoubleArray* feedbackCoef);
+    ~IIRFilter();
+
+    void process(const float* sourceP, float* destP, size_t framesToProcess);
+
+    void reset();
+
+    void getFrequencyResponse(int nFrequencies,
+        const float* frequency,
+        float* magResponse,
+        float* phaseResponse);
+
+private:
+    // Filter memory
+    //
+    // For simplicity, we assume |m_xBuffer| and |m_yBuffer| have the same length, and the length is
+    // a power of two.  Since the number of coefficients has a fixed upper length, the size of
+    // xBuffer and yBuffer is fixed. |m_xBuffer| holds the old input values and |m_yBuffer| holds
+    // the old output values needed to compute the new output value.
+    //
+    // m_yBuffer[m_bufferIndex] holds the most recent output value, say, y[n].  Then
+    // m_yBuffer[m_bufferIndex - k] is y[n - k].  Similarly for m_xBuffer.
+    //
+    // To minimize roundoff, these arrays are double's instead of floats.
+    AudioDoubleArray m_xBuffer;
+    AudioDoubleArray m_yBuffer;
+
+    // Index into the xBuffer and yBuffer arrays where the most current x and y values should be
+    // stored.  xBuffer[bufferIndex] corresponds to x[n], the current x input value and
+    // yBuffer[bufferIndex] is where y[n], the current output value.
+    int m_bufferIndex;
+
+    // Coefficients of the IIR filter.  To minimize storage, these point to the arrays given in the
+    // constructor.
+    const AudioDoubleArray* m_feedback;
+    const AudioDoubleArray* m_feedforward;
+};
+
+} // namespace blink
+
+#endif // IIRFilter_h