// Very simple Finite State Machine
// Forum: https://forum.arduino.cc/t/using-finite-state-machine/1114323/
// Tutorial: https://majenko.co.uk/blog/our-blog-1/the-finite-state-machine-26
// This Wokwi project: https://wokwi.com/projects/361809673037018113

const int buttonPin = 2;
const int redLedPin = 12;
const int greenLedPin = 13;

int lastButtonValue = HIGH;       // default HIGH is not pressed.

enum
{
  IDLE,
  RED,
  GREEN,
} state;

void setup() 
{
  pinMode( buttonPin, INPUT_PULLUP);
  pinMode( redLedPin, OUTPUT);
  pinMode( greenLedPin, OUTPUT);
}

void loop() 
{
  // ------------------------------------------------------
  // Gather all the information from buttons and sensors.
  // ------------------------------------------------------
  // The variable 'buttonPressed' is passed on to the Finite State Machine.
  bool buttonPressed = false;        // default: button not pressed
  
  // Checking the event that the button has just been pressed
  // can be done with the finite state machine.
  // I decided to that with a variable.
  int buttonValue = digitalRead( buttonPin);
  if( buttonValue != lastButtonValue) // something changed ?
  {
    if( buttonValue == LOW)          // pressed just now ?
      buttonPressed = true;
    lastButtonValue = buttonValue;   // remember for the next time
  }

  // ------------------------------------------------------
  // Process the information with a Finite State Machine
  // ------------------------------------------------------
  // Changing the leds can be done with a intermediate state,
  // depending on how much needs to be done before going
  // to a new state.
  switch( state)
  {
    case IDLE:
      if( buttonPressed)
      {
        digitalWrite( redLedPin, HIGH);
        state = RED;
      }
      break;
    case RED:
      if( buttonPressed)
      {
        digitalWrite( redLedPin, LOW);
        digitalWrite( greenLedPin, HIGH);
        state = GREEN;
      }
      break;
    case GREEN:
      if( buttonPressed)
      {
        digitalWrite( redLedPin, HIGH);
        digitalWrite( greenLedPin, LOW);
        state = RED;
      }
      break;
  }

  delay(20);   // slow down the sketch, also to avoid button bouncing
}