// Pin definitions
const int STCPPin = 3;          // Pin connected to ST_CP of 74HC595
const int SHCPPin = 4;         // Pin connected to SH_CP of 74HC595 (can be SCK of SPI)
const int DSPin = 2;          // Pin connected to DS of 74HC595 (can be MOSI of SPI)
const int resetButtonPin = 2;    // Pin for the reset button
const int increaseButtonPin = 3; // Pin for the increase button
const int decreaseButtonPin = 4; // Pin for the decrease button
const int pulsePin = 5;          // Interrupt pin for fuel usage pulses

volatile float fuelAmount = 70.0; // Start with 70 liters

const byte digits[10] = {
    0b00111111, // 0
    0b00000110, // 1
    0b01011011, // 2
    0b01001111, // 3
    0b01100110, // 4
    0b01101101, // 5
    0b01111101, // 6
    0b00000111, // 7
    0b01111111, // 8
    0b01101111  // 9
};

void setup()
{
  pinMode(STCPPin, OUTPUT);
  pinMode(SHCPPin, OUTPUT);
  pinMode(DSPin, OUTPUT);
  pinMode(resetButtonPin, INPUT_PULLUP);
  pinMode(increaseButtonPin, INPUT_PULLUP);
  pinMode(decreaseButtonPin, INPUT_PULLUP);

  SPI.begin(); // If using SPI for communication
  attachInterrupt(digitalPinToInterrupt(pulsePin), pulseISR, FALLING);
}

void loop()
{
  if (!digitalRead(resetButtonPin))
  {
    fuelAmount = 70.0;
  }

  if (!digitalRead(increaseButtonPin))
  {
    // Increment to the nearest number divisible by 5, with an upper limit of 70
    float increment = 5.0 - fmod(fuelAmount, 5.0);
    if (increment == 5.0)
    {
      increment = 0; // If already divisible by 5, don't add
    }

    fuelAmount += increment;
    if (fuelAmount > 70.0)
    {
      fuelAmount = 0;
    }
  }

  if (!digitalRead(decreaseButtonPin))
  {
    // Decrement to the nearest number divisible by 5
    fuelAmount -= 1.0;
    if (fuelAmount <= 0.0)
    {
      fuelAmount = 70.0;
    }
  }

  displayFuelAmount(); // Update the display with the current fuel amount
}

void displayFuelAmount()
{
  int fuelInt = (int)(fuelAmount * 100); // Convert fuel amount to integer for display
  for (int i = 0; i < 4; ++i)
  {
    int digit = (fuelInt / (int)pow(10, i)) % 10; // Calculate each digit
    digitalWrite(STCPPin, LOW);
    shiftOut(DSPin, SHCPPin, MSBFIRST, digits[digit]); // Send digit pattern to 74HC595
    digitalWrite(STCPPin, HIGH);
    delay(5); // Small delay to ensure digit is visible
  }
}

void pulseISR()
{
  fuelAmount -= 1.0 / 16.0 / 100.0; // Each pulse represents 1/16 of 1/100th of a liter
  if (fuelAmount < 0.0)
  {
    fuelAmount = 0.0; // Prevent going below 0
  }
}