constexpr static byte pin = 2;
volatile float waterflowAbsolute = 0.0;


auto pulse() -> void;
auto calcWaterflow() -> float;


void setup() {
  Serial.begin(9600);
  attachInterrupt(digitalPinToInterrupt(2), pulse, RISING);

}

void loop() {
  Serial.print("Waterflow l/h: ");
  Serial.println(String(calcWaterflow()));

  delay(1000);
}

// 450 pulses for 1 liter (see product parameters)
auto pulse () -> void
{
  waterflowAbsolute += 1.0 / 450.0;
}

// Calculate Waterflow in relation to one hour
auto calcWaterflow() -> float
{
  static auto timestamp = millis();
  static auto  waterflowOld = waterflowAbsolute;

  if (millis() - timestamp >=  1000)
  {
    timestamp = millis();

    if (waterflowAbsolute >= 0.0)
    {
      auto result = waterflowAbsolute * 3600;
      Serial.println("WaterflowAbs: " + String(waterflowAbsolute));
      waterflowAbsolute = 0;
      return result;
    }

  }

  return 0.0;

}