AoC_Day16

package com.adventofcode.day16;

public class DragonChecksum {

    public static void main(String[] args) {
        String input = "01110110101001000";
        int driveLength1 = 272;
        int driveLength2 = 35651584;

        String data1 = driveData(input, driveLength1);
        String checksum1 = driveChecksum(data1);
        System.out.println("First checksum: " + checksum1);

        String data2 = driveData(input, driveLength2);
        String checksum2 = driveChecksum(data2);
        System.out.println("Second checksum: " + checksum2);

    }

    private static String driveChecksum(String input) {
        StringBuilder sb = new StringBuilder();

        for (int i = 0; i < input.length(); i+=2) {
            sb.append(input.charAt(i) == input.charAt(i + 1) ? '1' : '0');
        }

        String checksum = sb.toString();

        if(checksum.length() % 2 == 0) {
            checksum = driveChecksum(checksum);
        }

        return checksum;

    }

    public static String driveData(String a, int driveLength) {

        String b = new StringBuffer(a).reverse().toString();

        b = b.replace('0', 'x');
        b = b.replace('1', '0');
        b = b.replace('x', '1');

        String toReturn = a + '0' + b;

        if(toReturn.length() < driveLength) {
            toReturn = driveData(toReturn, driveLength);
        }

        toReturn = toReturn.substring(0, driveLength);

        return toReturn;

    }

}