// https://github.com/lilspikey/arduino_sketches/blob/master/larson/larson.ino

#define PIN_COUNT 8
#define UPDATE_DURATION 30

// Uno (those with the ATmega168 or ATmega328)
// PWM: 3, 5, 6, 9, 10, and 11. Provide 8-bit PWM output with the analogWrite() function
// On the Arduino Mega, it works on pins 2 to 13 and 44 to 46; Provide 8-bit PWM output with the analogWrite() function.
//
int pins[PIN_COUNT] = { 2, 3, 4, 5, 6, 7, 8, 9};
int states[PIN_COUNT];
int current_pin = 0;
int dir = 1;
int update_count = 0;

void setup() {
  for ( int i = 0; i < PIN_COUNT; i++ ) {
    pinMode(pins[i], OUTPUT);
    states[i] = 0;
  }
}

void updatePins() {
  for ( int i = 0; i < PIN_COUNT; i++ ) {
    analogWrite(pins[i], states[i]); // AnalogWrite uses pulse width modulation (PWM), 
                                     // turning a digital pin on and off very quickly, 
                                     // to create a fading effect. 
  }
  delay(6);
}

void decay() {
  for ( int i = 0; i < PIN_COUNT; i++ ) {
    states[i] = (19*states[i]/20);
  }
}

void loop() {
  decay();
  states[current_pin] = 255 * update_count / UPDATE_DURATION;
  updatePins();
  
  update_count++;
  if ( update_count > UPDATE_DURATION ) {
    update_count = 0;
    current_pin += dir;
    if ( current_pin == 0 ) {
      dir = 1;
    }
    else if ( current_pin == (PIN_COUNT-1) ) {
      dir = -1;
    }
  }
}