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+/*
+Copyright 2018 Google Inc. All Rights Reserved.
+
+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 RESONANCE_AUDIO_DSP_BIQUAD_FILTER_H_
+#define RESONANCE_AUDIO_DSP_BIQUAD_FILTER_H_
+
+#include <array>
+
+#include "base/audio_buffer.h"
+#include "base/logging.h"
+
+namespace vraudio {
+
+// Set of transfer function coefficients.
+struct BiquadCoefficients {
+  // Constructor takes as its arguments 6 floats representing the transfer
+  // function coefficients of a biquad filter.
+  BiquadCoefficients(float a0, float a1, float a2, float b0, float b1, float b2)
+      : a({{a0, a1, a2}}), b({{b0, b1, b2}}) {}
+
+  // Default constructor that sets a0 and b0 == 1 so that if the coefficients
+  // are used, the filter is stable and has no effect on input. i.e. input
+  // appears to just pass through.
+  BiquadCoefficients() : BiquadCoefficients(1.0, 0.0, 0.0, 1.0, 0.0, 0.0) {}
+
+  // The denominator quadratic coefficients.
+  std::array<float, 3> a;
+  // The numerator quadratic coefficients.
+  std::array<float, 3> b;
+};
+
+// Class representing a biquad filter as its transfer function coefficients.
+// This class also performs filtering.
+class BiquadFilter {
+ public:
+  // Constructs a BiquadFilter using a BiquadCoefficients struct.
+  //
+  // @param coefficients A BiquadCoefficients to set the internal
+  //     |coefficients_|.
+  // @param frames_per_buffer The number of frames in each data buffer to be
+  //     processed. This is used to set the number of samples to iterate over
+  //     during a change of filter state.
+  BiquadFilter(const BiquadCoefficients& coefficients,
+               size_t frames_per_buffer);
+
+  // Filter method for use with AudioBuffer::Channel.
+  //
+  // @param input_channel An AudioBuffer::Channel of input.
+  // @param output_channel A pointer to an AudioBuffer::Channel for output.
+  void Filter(const AudioBuffer::Channel& input_channel,
+              AudioBuffer::Channel* output_channel);
+
+  // Sets the fiter's coefficents to those passed, with all
+  // coefficients bar a0 scaled by 1/a0.
+  //
+  // @param coefficients A set of BiquadCoefficients.
+  void SetCoefficients(const BiquadCoefficients& coefficients);
+
+  // Sets the target coefficients to be interpolated.
+  //
+  // @param coefficents The BiquadCoefficients we wish to interpolate to over
+  //     samples_to_iterate_over_ samples of the next input buffer.
+  void InterpolateToCoefficients(const BiquadCoefficients& coefficients);
+
+  // Clears the internal state of the filter, except for coefficients.
+  void Clear();
+
+ private:
+  friend class BiquadFilterInterpolateTest;
+
+  // Filters a single sample of input.
+  //
+  // @param input A single input sample from a Planar or Interleaved buffer.
+  // @param delay_line The delay_line to use (important for interpolation).
+  // @param coefficients The biquad coeffients for use in filtering.
+  // @return An output value to be placed in a Planar or Interleaved buffer.
+  float FilterSample(float input, std::array<float, 2>* delay_line,
+                            const BiquadCoefficients& coefficients);
+
+  // If InterpolateToState() has been called to assign new filter coefficients,
+  // this function will be called samples_to_iterate_over_ times within the next
+  // call of the Filter() function to slowly transition to the new coefficients
+  // which were passed to InterpolateToState() previously.
+  void UpdateInterpolate();
+
+  // Filters a single sample of input while transitioning between coefficients.
+  // Performs a linear crossfade over samples_to_iterate_over_ samples.
+  //
+  // @param input A single input sample from a Planar or Interleaved buffer.
+  // @return Output value to be placed in an audio buffer.
+  float InterpolateFilterSample(float input);
+
+  // Stores the memory state of the biquad
+  std::array<float, 2> biquad_delay_line_;
+
+  // Flag that denotes whether or not we are transitioning to another set of
+  // coefficients via interpolation.
+  bool interpolate_flag_;
+
+  // Counter that denotes how far we are into transitioning to another set of
+  // coefficients via interpolation.
+  size_t interpolate_counter_;
+
+  // Stores the memory state of the biquad.
+  std::array<float, 2> old_delay_line_;
+
+  // Number of samples over which to apply the filter coefficients.
+  size_t samples_to_iterate_over_;
+
+  // Value used to crossfade between filter outputs.
+  float fade_scale_;
+
+  // A set of coefficient which are updated as we interpolate between filter
+  // coefficients.
+  BiquadCoefficients old_coefficients_;
+
+  // Represents and maintains the state of the biquad filter in terms of its
+  // transfer function coefficients.
+  BiquadCoefficients coefficients_;
+};
+
+}  // namespace vraudio
+
+#endif  // RESONANCE_AUDIO_DSP_BIQUAD_FILTER_H_