Simple reaction timer app using switch and interrupts

sketch_reaction_time.ino

// *****************************************
// Reaction timer using interrupt
// *****************************************
#define BUTTON_PIN 2
#define LED_PIN   8

// This must be declared volatile since it is accessed from
// the ISR and from loop()
//https://en.wikipedia.org/wiki/Volatile_(computer_programming)
static volatile int isr_counter = 0 ;

// ISR - execution saves state and jumps here when switch closed
void countBounces( void )
{
  isr_counter ++ ;
}

void setup() {
  Serial.begin( 9600 ) ;
  Serial.println(F("START " __FILE__ " from " __DATE__ "\r\n" ));
  pinMode( BUTTON_PIN, INPUT ) ;
  pinMode( LED_PIN, OUTPUT ) ; 

  // Arduino function that sets up the ISR
  // Note the use of trhe RISING keyword - why?
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), countBounces, RISING ) ;
  Serial.println( "starting") ;
  digitalWrite( LED_PIN, LOW ) ;
}

void loop() {
  // copy isr variable into local -- this requirement may be a bug
  int count = isr_counter ;
  int reaction_time = 0 ;
  isr_counter = 0 ;

  // kill a random amount of time before starting
  long raw_time_value = random() ;
  int delay_milliseconds = map( raw_time_value, 0, RANDOM_MAX, 500, 5000) ;
  delay( delay_milliseconds ) ;

  // Go!
  digitalWrite( LED_PIN, HIGH ) ;
  long start_time = millis ( ) ;

  // Loop waiting for ISR to set isr_counter to non-zero 
  while( !isr_counter ) { ; } 
  // Stop!

  // compute duration
  reaction_time = millis ( ) - start_time ;
  digitalWrite( LED_PIN, LOW ) ;
  
  Serial.println( reaction_time ) ;
  delay( 5000 ) ;
  //Serial.println( count ) ;
}