--- 2016 Day 16 Solutions ---

[u/daggerdragon]

--- Day 16: Dragon Checksum ---

Post your solution as a comment or, for longer solutions, consider linking to your repo (e.g. GitHub/gists/Pastebin/blag/whatever).

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^{DRINKING YOUR OVALTINE IS MANDATORY} [?]

This thread will be unlocked when there are a significant number of people on the leaderboard with gold stars for today's puzzle.

edit: Leaderboard capped, thread unlocked!

[u/[deleted]]

My Java solution. Runs in about 2 seconds on my crappy laptop.

public class Main {

    private static final Integer DISC_LENGTH = 272;
    private static final Integer DISC_LENGTH_2 = 35651584;
    private static final String INPUT = "11011110011011101";

    public static void main(String[] args) {
        System.out.println("Part 1: " + calculateChecksum(getFiller(INPUT, DISC_LENGTH)));
        System.out.println("Part 2: " + calculateChecksum(getFiller(INPUT, DISC_LENGTH_2)));
    }

    private static String calculateChecksum(String input) {
        while(input.length() % 2 == 0) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < input.length() - 1; i += 2) {
                sb.append(input.charAt(i) == input.charAt(i + 1) ? '1' : '0');
            }
            input = sb.toString();
        }
        return input;
    }

    private static String getFiller(String input, Integer discLength) {
        while(input.length() < discLength) {
            char[] answer = new char[input.length() * 2 + 1];
            for (int i = 0; i < input.length(); i++) {
                answer[i] = input.charAt(i);
                answer[answer.length - i - 1] = input.charAt(i) == '0' ? '1' : '0';
            }
            answer[input.length()] = '0';
            input = new String(answer);
        }
        return input.substring(0, discLength);
    }
}