#define BUTTON_PIN 2
#define LED_PIN 4

//timing
unsigned long lastblink = 0;
const unsigned long interval = 500;

//debouce
unsigned long lastdebounce = 0;
const unsigned long debouncedelay = 50;
int buttonstate = HIGH;
int lastbuttonstate = HIGH;

//led
int ledstate = LOW;


enum SYSTEMSTATE
{
  STATE_BLINKING,
  STATE_OVERIDE
};

SYSTEMSTATE currentstate = STATE_BLINKING;

void setup()
{
  pinMode(LED_PIN, OUTPUT);
  pinMode(BUTTON_PIN, INPUT_PULLUP);
}

void loop()
{
  int now = millis();
  //debounce detect
  int reading = digitalRead(BUTTON_PIN);
  if ( reading != lastbuttonstate)
  {
    lastdebounce = now;
  }

  if ((now - lastdebounce) > debouncedelay)
  {
    if (buttonstate != reading)
    {
      buttonstate = reading;
    }
  }
  lastbuttonstate = reading;

  switch (currentstate)
  {
    case STATE_BLINKING:
      //TIMER EVENT
      if ((now - lastblink) > interval)
      {
        lastblink = now;
        ledstate = ! ledstate;
        digitalWrite(LED_PIN, ledstate);
      }

      //press event
      if (buttonstate == LOW)
      {
        digitalWrite(LED_PIN, LOW);
        currentstate = STATE_OVERIDE;
      }
      break;

    case STATE_OVERIDE:
      //led force off
      digitalWrite(LED_PIN, LOW);

      //release event
      if(buttonstate == HIGH)
      {
        currentstate = STATE_BLINKING;
      }
      break;
  }
}