// https://wokwi.com/projects/372380727802934273
// https://forum.arduino.cc/t/make-an-output-pin-behave-like-a-monostable-multivibrator/1155428/12

// this is meant to be the state diagram in post #12
// the PIR delayTime is set to 2 seconds in the *.json file

# define IR_Pin   2
# define LED_Pin  13

// blink heartbeat-led just to see that we are looping.
# define heartBeatLED_Pin 12

enum {IDLE = 0, LED_ON, LED_OFF, WAIT,};

unsigned long snapshot_PIR_HIGH_detected;
unsigned long snapshot_PIR_went_LOW;

# define LED_ON_TIME  5000
# define NO_OBSTAKLE_WAIT_TIME 2000

void setup() {
  Serial.begin(115200);
  Serial.println("Hello World!\n");

  pinMode(IR_Pin, INPUT);
  pinMode(LED_Pin, OUTPUT);
  pinMode(heartBeatLED_Pin, OUTPUT);
}

byte state = IDLE;
bool PIR_Sensor_HIGH;
unsigned long milliseconds_right_now;

bool ledOn;

void loop() {
  delay(50);

  digitalWrite(heartBeatLED_Pin, (millis() & 512) ? HIGH : LOW);

  // I-P-O-model   I)nputs-P)rocess-O)utput
  // I. INPUTS to the model - time and motion activity
  milliseconds_right_now = millis();

  // when digitalRead(IR_Pin) delivers HIGH
  // (digitalRead(IR_Pin) == HIGH) results in true
  //        HIGH          == HIGH  is true

  // when digitalRead(IR_Pin) delivers LOW
  // (digitalRead(IR_Pin) == HIGH) results in false
  //         LOW          == HIGH  is false
  PIR_Sensor_HIGH = (digitalRead(IR_Pin) == HIGH);

  // P. PROCESS the inputs given the current state and inputs
  theFSM();
  // RIGHT-BELOW-FSM

  // O. OUTPUTS use the process to set the outputs.
  if (ledOn == true) {
    digitalWrite(LED_Pin, HIGH);
  }
  else {
    digitalWrite(LED_Pin, LOW);
  }
  // shortest way to code the above if-else:
  // digitalWrite(LED_Pin, ledOn ? HIGH : LOW);
}

void theFSM() {
  unsigned long time_sincePIR_went_HIGH = 0;
  unsigned long time_sincePIR_went_LOW = 0;

  switch (state) {

    // if variable "state" has value represented by the name "IDLE"
    // mutually exclusive execute code below case IDLE:
    case IDLE :
      if (PIR_Sensor_HIGH) {
        snapshot_PIR_HIGH_detected = milliseconds_right_now;
        Serial.println("A. motion turns LED on");
        state = LED_ON;
      }
      break; // IMMITIATELY jump down to END-OF-SWITCH

    case LED_ON :
      ledOn = true;
      time_sincePIR_went_HIGH = milliseconds_right_now - snapshot_PIR_HIGH_detected;
      if (time_sincePIR_went_HIGH >= LED_ON_TIME) {
        Serial.println("B. time turns LED off");
        ledOn = false;
        state = LED_OFF;
      }
      break; // IMMITIATELY jump down to END-OF-SWITCH

    case LED_OFF :
      if (!PIR_Sensor_HIGH) {
        Serial.println("C. IR signal gone, so");
        snapshot_PIR_went_LOW = milliseconds_right_now;
        state = WAIT;
      }
      break; // IMMITIATELY jump down to END-OF-SWITCH

    case WAIT :
      time_sincePIR_went_LOW = milliseconds_right_now - snapshot_PIR_went_LOW;
      if (PIR_Sensor_HIGH) {
        Serial.println("D. IR signal still or again, so");
        time_sincePIR_went_LOW = milliseconds_right_now;
        state = WAIT;
      }
      else if (time_sincePIR_went_LOW >= NO_OBSTAKLE_WAIT_TIME) {
        Serial.println("E. after a bit more time, we reset");
        //timer = milliseconds_right_now;
        state = IDLE;
      }
      break; // IMMITIATELY jump down to END-OF-SWITCH

  } // END-OF-SWITCH
  // after a break;    code-execution goes on here
  // ==> jump back to RIGHT-BELOW-FSM
}