cpp/src/utils/resample.cpp

/**
 * Copyright     2013  Pegah Ghahremani
 *               2014  IMSL, PKU-HKUST (author: Wei Shi)
 *               2014  Yanqing Sun, Junjie Wang
 *               2014  Johns Hopkins University (author: Daniel Povey)
 * Copyright     2023  Xiaomi Corporation (authors: Fangjun Kuang)
 *
 * See LICENSE for clarification regarding multiple authors
 *
 * 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 "utils/resample.h"
#include "utils/logger.h"

#include <cassert>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <type_traits>
#include <vector>
#include <memory>

#ifndef M_2PI
#define M_2PI 6.283185307179586476925286766559005
#endif

#ifndef M_PI
#define M_PI 3.1415926535897932384626433832795
#endif

namespace utils {

template <class I>
static I Gcd(I m, I n) {
  // this function is copied from kaldi/src/base/kaldi-math.h
  if (m == 0 || n == 0) {
    if (m == 0 && n == 0) {  // gcd not defined, as all integers are divisors.
      fprintf(stderr, "Undefined GCD since m = 0, n = 0.\n");
      exit(-1);
    }
    return (m == 0 ? (n > 0 ? n : -n) : (m > 0 ? m : -m));
    // return absolute value of whichever is nonzero
  }
  // could use compile-time assertion
  // but involves messing with complex template stuff.
  static_assert(std::is_integral_v<I>);
  while (true) {
    m %= n;
    if (m == 0) return (n > 0 ? n : -n);
    n %= m;
    if (n == 0) return (m > 0 ? m : -m);
  }
}

/// Returns the least common multiple of two integers.  Will
/// crash unless the inputs are positive.
template <class I>
static I Lcm(I m, I n) {
  // This function is copied from kaldi/src/base/kaldi-math.h
  assert(m > 0 && n > 0);
  I gcd = Gcd(m, n);
  return gcd * (m / gcd) * (n / gcd);
}

static float DotProduct(const float *a, const float *b, int32_t n) {
  float sum = 0;
  for (int32_t i = 0; i != n; ++i) {
    sum += a[i] * b[i];
  }
  return sum;
}

LinearResample::LinearResample(int32_t samp_rate_in_hz,
                               int32_t samp_rate_out_hz, float filter_cutoff_hz,
                               int32_t num_zeros)
    : samp_rate_in_(samp_rate_in_hz),
      samp_rate_out_(samp_rate_out_hz),
      filter_cutoff_(filter_cutoff_hz),
      num_zeros_(num_zeros) {
  assert(samp_rate_in_hz > 0.0 && samp_rate_out_hz > 0.0 &&
         filter_cutoff_hz > 0.0 && filter_cutoff_hz * 2 <= samp_rate_in_hz &&
         filter_cutoff_hz * 2 <= samp_rate_out_hz && num_zeros > 0);

  // base_freq is the frequency of the repeating unit, which is the gcd
  // of the input frequencies.
  int32_t base_freq = Gcd(samp_rate_in_, samp_rate_out_);
  input_samples_in_unit_ = samp_rate_in_ / base_freq;
  output_samples_in_unit_ = samp_rate_out_ / base_freq;

  SetIndexesAndWeights();
  Reset();
}

void LinearResample::SetIndexesAndWeights() {
  first_index_.resize(output_samples_in_unit_);
  weights_.resize(output_samples_in_unit_);

  double window_width = num_zeros_ / (2.0 * filter_cutoff_);

  for (int32_t i = 0; i < output_samples_in_unit_; i++) {
    double output_t = i / static_cast<double>(samp_rate_out_);
    double min_t = output_t - window_width, max_t = output_t + window_width;
    // we do ceil on the min and floor on the max, because if we did it
    // the other way around we would unnecessarily include indexes just
    // outside the window, with zero coefficients.  It's possible
    // if the arguments to the ceil and floor expressions are integers
    // (e.g. if filter_cutoff_ has an exact ratio with the sample rates),
    // that we unnecessarily include something with a zero coefficient,
    // but this is only a slight efficiency issue.
    int32_t min_input_index = ceil(min_t * samp_rate_in_),
            max_input_index = floor(max_t * samp_rate_in_),
            num_indices = max_input_index - min_input_index + 1;
    first_index_[i] = min_input_index;
    weights_[i].resize(num_indices);
    for (int32_t j = 0; j < num_indices; j++) {
      int32_t input_index = min_input_index + j;
      double input_t = input_index / static_cast<double>(samp_rate_in_),
             delta_t = input_t - output_t;
      // sign of delta_t doesn't matter.
      weights_[i][j] = FilterFunc(delta_t) / samp_rate_in_;
    }
  }
}

/** Here, t is a time in seconds representing an offset from
    the center of the windowed filter function, and FilterFunction(t)
    returns the windowed filter function, described
    in the header as h(t) = f(t)g(t), evaluated at t.
*/
float LinearResample::FilterFunc(float t) const {
  float window = 0,  // raised-cosine (Hanning) window of width
                     // num_zeros_/2*filter_cutoff_
      filter = 0;    // sinc filter function
  if (std::fabs(t) < num_zeros_ / (2.0 * filter_cutoff_))
    window = 0.5 * (1 + cos(M_2PI * filter_cutoff_ / num_zeros_ * t));
  else
    window = 0.0;  // outside support of window function
  if (t != 0)
    filter = sin(M_2PI * filter_cutoff_ * t) / (M_PI * t);
  else
    filter = 2 * filter_cutoff_;  // limit of the function at t = 0
  return filter * window;
}

void LinearResample::Reset() {
  input_sample_offset_ = 0;
  output_sample_offset_ = 0;
  input_remainder_.resize(0);
}

void LinearResample::Resample(const float *input, int32_t input_dim, bool flush,
                              std::vector<float> *output) {
  int64_t tot_input_samp = input_sample_offset_ + input_dim,
          tot_output_samp = GetNumOutputSamples(tot_input_samp, flush);

  assert(tot_output_samp >= output_sample_offset_);

  output->resize(tot_output_samp - output_sample_offset_);

  // samp_out is the index into the total output signal, not just the part
  // of it we are producing here.
  for (int64_t samp_out = output_sample_offset_; samp_out < tot_output_samp;
       samp_out++) {
    int64_t first_samp_in = 0;
    int32_t samp_out_wrapped = 0;
    GetIndexes(samp_out, &first_samp_in, &samp_out_wrapped);
    const std::vector<float> &weights = weights_[samp_out_wrapped];
    // first_input_index is the first index into "input" that we have a weight
    // for.
    int32_t first_input_index =
        static_cast<int32_t>(first_samp_in - input_sample_offset_);
    float this_output = 0;
    if (first_input_index >= 0 &&
        first_input_index + static_cast<int32_t>(weights.size()) <= input_dim) {
      this_output =
          DotProduct(input + first_input_index, weights.data(), weights.size());
    } else {  // Handle edge cases.
      this_output = 0.0;
      for (int32_t i = 0; i < static_cast<int32_t>(weights.size()); i++) {
        float weight = weights[i];
        int32_t input_index = first_input_index + i;
        if (input_index < 0 &&
            static_cast<int32_t>(input_remainder_.size()) + input_index >= 0) {
          this_output +=
              weight * input_remainder_[input_remainder_.size() + input_index];
        } else if (input_index >= 0 && input_index < input_dim) {
          this_output += weight * input[input_index];
        } else if (input_index >= input_dim) {
          // We're past the end of the input and are adding zero; should only
          // happen if the user specified flush == true, or else we would not
          // be trying to output this sample.
          assert(flush);
        }
      }
    }
    int32_t output_index =
        static_cast<int32_t>(samp_out - output_sample_offset_);
    (*output)[output_index] = this_output;
  }

  if (flush) {
    Reset();  // Reset the internal state.
  } else {
    SetRemainder(input, input_dim);
    input_sample_offset_ = tot_input_samp;
    output_sample_offset_ = tot_output_samp;
  }
}

int64_t LinearResample::GetNumOutputSamples(int64_t input_num_samp,
                                            bool flush) const {
  // For exact computation, we measure time in "ticks" of 1.0 / tick_freq,
  // where tick_freq is the least common multiple of samp_rate_in_ and
  // samp_rate_out_.
  int32_t tick_freq = Lcm(samp_rate_in_, samp_rate_out_);
  int32_t ticks_per_input_period = tick_freq / samp_rate_in_;

  // work out the number of ticks in the time interval
  // [ 0, input_num_samp/samp_rate_in_ ).
  int64_t interval_length_in_ticks = input_num_samp * ticks_per_input_period;
  if (!flush) {
    float window_width = num_zeros_ / (2.0 * filter_cutoff_);
    // To count the window-width in ticks we take the floor.  This
    // is because since we're looking for the largest integer num-out-samp
    // that fits in the interval, which is open on the right, a reduction
    // in interval length of less than a tick will never make a difference.
    // For example, the largest integer in the interval [ 0, 2 ) and the
    // largest integer in the interval [ 0, 2 - 0.9 ) are the same (both one).
    // So when we're subtracting the window-width we can ignore the fractional
    // part.
    int32_t window_width_ticks = std::floor(window_width * tick_freq);
    // The time-period of the output that we can sample gets reduced
    // by the window-width (which is actually the distance from the
    // center to the edge of the windowing function) if we're not
    // "flushing the output".
    interval_length_in_ticks -= window_width_ticks;
  }
  if (interval_length_in_ticks <= 0) return 0;

  int32_t ticks_per_output_period = tick_freq / samp_rate_out_;
  // Get the last output-sample in the closed interval, i.e. replacing [ ) with
  // [ ].  Note: integer division rounds down.  See
  // http://en.wikipedia.org/wiki/Interval_(mathematics) for an explanation of
  // the notation.
  int64_t last_output_samp = interval_length_in_ticks / ticks_per_output_period;
  // We need the last output-sample in the open interval, so if it takes us to
  // the end of the interval exactly, subtract one.
  if (last_output_samp * ticks_per_output_period == interval_length_in_ticks)
    last_output_samp--;

  // First output-sample index is zero, so the number of output samples
  // is the last output-sample plus one.
  int64_t num_output_samp = last_output_samp + 1;
  return num_output_samp;
}

// inline
void LinearResample::GetIndexes(int64_t samp_out, int64_t *first_samp_in,
                                int32_t *samp_out_wrapped) const {
  // A unit is the smallest nonzero amount of time that is an exact
  // multiple of the input and output sample periods.  The unit index
  // is the answer to "which numbered unit we are in".
  int64_t unit_index = samp_out / output_samples_in_unit_;
  // samp_out_wrapped is equal to samp_out % output_samples_in_unit_
  *samp_out_wrapped =
      static_cast<int32_t>(samp_out - unit_index * output_samples_in_unit_);
  *first_samp_in =
      first_index_[*samp_out_wrapped] + unit_index * input_samples_in_unit_;
}

void LinearResample::SetRemainder(const float *input, int32_t input_dim) {
  std::vector<float> old_remainder(input_remainder_);
  // max_remainder_needed is the width of the filter from side to side,
  // measured in input samples.  you might think it should be half that,
  // but you have to consider that you might be wanting to output samples
  // that are "in the past" relative to the beginning of the latest
  // input... anyway, storing more remainder than needed is not harmful.
  int32_t max_remainder_needed =
      std::ceil(samp_rate_in_ * num_zeros_ / filter_cutoff_);
  input_remainder_.resize(max_remainder_needed);
  for (int32_t index = -static_cast<int32_t>(input_remainder_.size());
       index < 0; index++) {
    // we interpret "index" as an offset from the end of "input" and
    // from the end of input_remainder_.
    int32_t input_index = index + input_dim;
    if (input_index >= 0) {
      input_remainder_[index + static_cast<int32_t>(input_remainder_.size())] =
          input[input_index];
    } else if (input_index + static_cast<int32_t>(old_remainder.size()) >= 0) {
      input_remainder_[index + static_cast<int32_t>(input_remainder_.size())] =
          old_remainder[input_index +
                        static_cast<int32_t>(old_remainder.size())];
      // else leave it at zero.
    }
  }
}

std::vector<float> resample(const std::vector<float>& audio_data, int orig_sr, int target_sr) {
    if (orig_sr != target_sr) {
        ALOGD("Audio resample: %d -> %d", orig_sr, target_sr);
        std::vector<float> resampled_data;
        float min_freq = std::min<int32_t>(orig_sr, target_sr);
        float lowpass_cutoff = 0.99 * 0.5 * min_freq;

        int32_t lowpass_filter_width = 6;
        auto resampler = std::make_unique<LinearResample>(
            orig_sr, target_sr, lowpass_cutoff,
            lowpass_filter_width);

        resampler->Resample(audio_data.data(), audio_data.size(), true, &resampled_data);
        return resampled_data;
    } else {
        return audio_data;
    }
}

}  // namespace utils