Advent of Code 2016 Day 18 /u/Philboyd_Studge

Advent2016_18.java

package snarkbait.reddit;

import java.math.BigInteger;
import java.util.Arrays;

/**
 * @author /u/Philboyd_Studge on 12/18/2016.
 */
public class Advent2016_18 {

    static final int ROWS = 40; // part = 400000



    public static void main(String[] args) {
        String input = "(input here)";
        //String input = ".^^.^.^^^^";
        final int len = input.length();
        BigInteger[] output = new BigInteger[ROWS];

        input = input.replace('.', '0');
        input = input.replace('^', '1');

        System.out.println(input);

        output[0] = new BigInteger(input, 2);


        for (int i = 0; i < ROWS - 1; i++) {
            output[i + 1] = BigInteger.ZERO;

            for (int j = len - 1; j >= 0; j--) {
                boolean left = output[i].testBit(j + 1);
                boolean right = j != 0 && output[i].testBit(j - 1);
                if (left ^ right) output[i + 1] = output[i + 1].setBit(j);
            }
        }

        int count = Arrays.asList(output).stream()
                .mapToInt(BigInteger::bitCount)
                .map(x -> len - x)
                .sum();

        System.out.println(count);
    }


}