text: /*
   *  Copyright (c) 2014 The WebRTC project authors. All Rights Reserved.
   *
   *  Use of this source code is governed by a BSD-style license
   *  that can be found in the LICENSE file in the root of the source
   *  tree. An additional intellectual property rights grant can be found
   *  in the file PATENTS.  All contributing project authors may
   *  be found in the AUTHORS file in the root of the source tree.
   */
  
  #include "common_audio/fir_filter_c.h"
  
  #include <string.h>
  #include <memory>
  
  #include "rtc_base/checks.h"
  
  namespace webrtc {
  
  FIRFilterC::~FIRFilterC() {}
  
  FIRFilterC::FIRFilterC(const float* coefficients, size_t coefficients_length)
      : coefficients_length_(coefficients_length),
        state_length_(coefficients_length - 1),
        coefficients_(new float[coefficients_length_]),
        state_(new float[state_length_]) {
    for (size_t i = 0; i < coefficients_length_; ++i) {
      coefficients_[i] = coefficients[coefficients_length_ - i - 1];
    }
    memset(state_.get(), 0, state_length_ * sizeof(state_[0]));
  }
  
  void FIRFilterC::Filter(const float* in, size_t length, float* out) {
    RTC_DCHECK_GT(length, 0);
  
    // Convolves the input signal |in| with the filter kernel |coefficients_|
    // taking into account the previous state.
    for (size_t i = 0; i < length; ++i) {
      out[i] = 0.f;
      size_t j;
      for (j = 0; state_length_ > i && j < state_length_ - i; ++j) {
        out[i] += state_[i + j] * coefficients_[j];
      }
      for (; j < coefficients_length_; ++j) {
        out[i] += in[j + i - state_length_] * coefficients_[j];
      }
    }
  
    // Update current state.
    if (length >= state_length_) {
      memcpy(state_.get(), &in[length - state_length_],
             state_length_ * sizeof(*in));
    } else {
      memmove(state_.get(), &state_[length],
              (state_length_ - length) * sizeof(state_[0]));
      memcpy(&state_[state_length_ - length], in, length * sizeof(*in));
    }
  }
  
  }  // namespace webrtc
  