//-------Goertzel function---------------------------------------//     
long int
goertzel (int sample[], long int coeff, int N)
//---------------------------------------------------------------//     
{
//initialize variables to be used in the function
  int Q, Q_prev, Q_prev2, i;
  long prod1, prod2, prod3, power;

  Q_prev = 0;         //set delay element1 Q_prev as zero
  Q_prev2 = 0;         //set delay element2 Q_prev2 as zero
  power = 0;         //set power as zero

  for (i = 0; i < N; i++)   // loop N times and calculate Q, Q_prev, Q_prev2 at each iteration (цикл)
    {
      Q = (sample[i]) + ((coeff * Q_prev) >> 14) - (Q_prev2);   // >>14 used as the coeff was used in Q15 format
      Q_prev2 = Q_prev;      // shuffle delay elements
      Q_prev = Q;
    }

  //calculate the three products used to calculate power (тут рахується вага частоти)
  prod1 = ((long) Q_prev * Q_prev);
  prod2 = ((long) Q_prev2 * Q_prev2);
  prod3 = ((long) Q_prev * coeff) >> 14;
  prod3 = (prod3 * Q_prev2);

  power = ((prod1 + prod2 - prod3)) >> 8;   //calculate power using the three products and scale the result down

  return power;
}

//---------------------------------------------------------------//