text: /*
   * 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.
   */
  
  #include "PolyphaseResamplerMono.h"
  
  using namespace resampler;
  
  #define MONO  1
  
  PolyphaseResamplerMono::PolyphaseResamplerMono(const MultiChannelResampler::Builder &builder)
          : PolyphaseResampler(builder) {
      assert(builder.getChannelCount() == MONO);
  }
  
  void PolyphaseResamplerMono::writeFrame(const float *frame) {
      // Move cursor before write so that cursor points to last written frame in read.
      if (--mCursor < 0) {
          mCursor = getNumTaps() - 1;
      }
      float *dest = &mX[mCursor * MONO];
      const int offset = mNumTaps * MONO;
      // Write each channel twice so we avoid having to wrap when running the FIR.
      const float sample =  frame[0];
      // Put ordered writes together.
      dest[0] = sample;
      dest[offset] = sample;
  }
  
  void PolyphaseResamplerMono::readFrame(float *frame) {
      // Clear accumulator.
      float sum = 0.0;
  
      // Multiply input times precomputed windowed sinc function.
      const float *coefficients = &mCoefficients[mCoefficientCursor];
      float *xFrame = &mX[mCursor * MONO];
      const int numLoops = mNumTaps >> 2; // n/4
      for (int i = 0; i < numLoops; i++) {
          // Manual loop unrolling, might get converted to SIMD.
          sum += *xFrame++ * *coefficients++;
          sum += *xFrame++ * *coefficients++;
          sum += *xFrame++ * *coefficients++;
          sum += *xFrame++ * *coefficients++;
      }
  
      mCoefficientCursor = (mCoefficientCursor + mNumTaps) % mCoefficients.size();
  
      // Copy accumulator to output.
      frame[0] = sum;
  }
  