const int tDelay   = 100;
const int dataPin  = 8;
const int latchPin = 9;
const int clockPin = 10;

bool DirectionState = 0;

void updateShiftRegister(unsigned int leds, bool isMSBFIRST = true){
    byte lowLED  = lowByte(leds);
    byte highLED = highByte(leds);

    digitalWrite(latchPin, LOW);
    if (isMSBFIRST == false) {
        shiftOut(dataPin, clockPin, LSBFIRST, lowLED);
        shiftOut(dataPin, clockPin, LSBFIRST, highLED);  
    } else {
        shiftOut(dataPin, clockPin, MSBFIRST, highLED);
        shiftOut(dataPin, clockPin, MSBFIRST, lowLED);
    }
    digitalWrite(latchPin, HIGH);
}

void setup(){
    pinMode(latchPin, OUTPUT);
    pinMode(dataPin,  OUTPUT);  
    pinMode(clockPin, OUTPUT);
}

void loop(){
    unsigned int leds = 0B000000000000000000000000;
    updateShiftRegister(leds);
    delay(tDelay);

    for (int i = 0; i < 24; i++){
        leds = 0B000000000000000000000000;
        bitSet(leds, i);
        updateShiftRegister(leds, DirectionState);
        delay(tDelay);
    }
    DirectionState = !DirectionState;
}