const int dataPin  = 4;      // DS - data serial
const int latchPin = 8;      // ST_CP - storage register, latch clock pin
const int clockPin = 7;       // SH_CP - shift register clock pin

void updateShiftRegister(unsigned int leds) {
  byte left_out  = lowByte(leds);      // extacts the lower byte, right most byte, from a 16-bit word or unsigned integer
  byte right_out = highByte(leds);     // extracts the higher byte, left most byte, from a 16-bit word or unsigned integer

  digitalWrite(latchPin, LOW);

  // LEDs move from left to right
  shiftOut(dataPin, clockPin, MSBFIRST, right_out);
  shiftOut(dataPin, clockPin, MSBFIRST, left_out);

  digitalWrite(latchPin, HIGH);
}
/* ***************************************************
                     Void Setup
 *************************************************** */
void setup() {
  pinMode(latchPin, OUTPUT);
  pinMode(dataPin,  OUTPUT);
  pinMode(clockPin, OUTPUT);
}
/* ***************************************************
                     Void Loop
 *************************************************** */
void loop() {
  int i=0;
  unsigned int leds = 0B0000000000000000;


  for(i=0; i<=17; i++){
    bitSet(leds, i);
    updateShiftRegister(leds);
    delay(500);
  }

  for(i=0; i<=17; i++){
    bitClear(leds, i);
    updateShiftRegister(leds);
    delay(500);
  }
}