Basic resampling / audio pitch shifting

resample_interpolation.c

///
// from: https://discourse.libsdl.org/t/audio-pitch-question/4822/5

// resample without interpolation

for (i=0;i<len;i++)
{
  pos=(int)wav_position;
  assert(pos>=0);
  assert(pos<=wav_length);
  my_sample[i]=wav_buffer[pos];
  wav_position=wav_position+speed;
  while (wav_position>=wav_length)
    wav_position=wav_position-wav_length;
}

// resample with linear interpolation

float fract_pos;
float wav_position;

for (i=0;i<len;i++)
{
  pos=(int)wav_position;
  fract_pos=wav_position - pos;
  assert(pos>=0);
  assert(pos<=wav_length);
  my_sample[i]=wav_buffer[pos] + fract_pos*(wav_buffer[pos+1]-wav_buffer[pos]);
  wav_position=wav_position+speed;
  while (wav_position>=wav_length)
    wav_position=wav_position-wav_length;
}