// efter Mikael Björnson, Arduinobubblan, 2022-08-31

float slope = 0.0;  // slope variable for the pH adjustment, set to zero at start
float offset;     // offset variable for the pH adjustment

int pHpin = 8;

// storing the values once for multiple use
const float cal_pH[3] = {4.01, 6.86, 9.18};

void setup() 
{
  pinMode(pHpin,INPUT);
}

void loop() 
{
  // if the calibration button is pressed or slope is zero, start calibration function
  if ((slope == 0.0) || (calibration_button_pressed))
  {
    pHcalibration();
  }

  //every 2 seconds, poll the sensor
  read_pH();
}

float read_pH()
{
  return slope * read_raw() + offset;
}

float read_raw()
{
  float avg[10];

  for (int i=0; i<10; i++) // take 10 readings from the sensor during 100 ms. Store them in an array.
  {
    avg[i] = analogRead(pHpin);
    delay(10);
  }

  for (int i=0; i<9; i++) //sorting the readings from low to high
  {
    for (int j=i+1; j<10; j++)
    {
      if (avg[i] > avg[j])
      {
        float temp = avg[i];
        avg[i]   = avg[j];
        avg[j]   = temp;
      }
    }
  }

  float avgVal = 0;
  for (int i=2; i<8; i++) // get the total of the 6 middle values.
  {
    avgVal += avg[i];
  }

  return avgVal / 6;  // we don't even need to divide by 6 if we do it the same way during calibration and measurement
}

void pHcalibration()
{
  //press button when calibration setup is complete for pH cal_pH[0]
  float cal1 = read_raw();

  //press button when calibration setup is complete for pH cal_pH[1]
  float cal2 = read_raw();

  //press button when calibration setup is complete for pH cal_pH[2]
  float cal3 = read_raw();

  // linear least-square fit
  float sumVal = cal1 + cal2 + cal3;

  float sumCal = cal_pH[0] + cal_pH[1] + cal_pH[2];

  float sumValSquare = pow(cal1,2) + pow(cal2,2) + pow(cal3,2);

  float sumProd      = cal1*cal_pH[0] + cal2*cal_pH[1] + cal3*cal_pH[2];

  float slope        = (3*sumProd - sumVal * sumCal) / (3*sumValSquare - pow(sumVal,2));
  float offset       = (sumCal - slope * sumVal) / 3;
}