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Advent of Code 2021 - Day 21
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‎README.md

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@@ -36,3 +36,4 @@ Last year I ran a [cruise log for my 25 days competing in the Advent of Code](ht
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- [The Advent of Code 2021 Day 16, Technical K.O.](https://www.codingnagger.com/2021/12/16/the-advent-of-code-2021-day-16-technical-k-o/)
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- [The Advent of Code 2021 Day 17, Deep Probing](https://www.codingnagger.com/2021/12/20/the-advent-of-code-2021-day-17-deep-probing/)
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- [The Advent of Code 2021 Day 20, Bruteforce McBruteface](https://www.codingnagger.com/2021/12/20/the-advent-of-code-2021-day-20-bruteforce-mcbruteface/)
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- [The Advent of Code 2021 Day 21, Late Night Finish](https://www.codingnagger.com/2021/12/22/the-advent-of-code-2021-day-21-late-night-finish/)

‎src/main/java/com/codingnagger/App.java

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@@ -3,6 +3,7 @@
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import com.codingnagger.days.Day;
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import com.codingnagger.days.Day16;
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import com.codingnagger.days.Day20;
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import com.codingnagger.days.Day21;
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import com.codingnagger.utils.InputLoader;
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import java.io.IOException;
@@ -15,9 +16,9 @@ public class App {
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public static void main(String[] args) throws IOException {
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System.out.println("Advent of Code 2021");
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List<String> input = InputLoader.Load("day20.txt");
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List<String> input = InputLoader.Load("day21.txt");
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Day day = new Day20();
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Day day = new Day21();
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System.out.println("Part 1:");
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System.out.println(day.partOne(input));

‎src/main/java/com/codingnagger/days/Day21.java

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package com.codingnagger.days;
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import java.math.BigInteger;
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import java.util.HashMap;
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import java.util.List;
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import java.util.Map;
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public class Day21 implements Day {
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public static final int PART_2VICTORY = 21;
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@Override
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public String partOne(List<String> input) {
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int[] diceRoll = new int[] {1,2,3};
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int p1Score = 0, p2Score = 0;
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int p1Position = Integer.parseInt(input.get(0).split("starting position: ")[1]);
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int p2Position = Integer.parseInt(input.get(1).split("starting position: ")[1]);
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boolean p1Turn = true;
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int turns = 0;
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while (p1Score < 1000 && p2Score < 1000) {
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if (p1Turn) {
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p1Position = endPosition(p1Position, diceRoll);
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p1Score += p1Position;
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} else {
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p2Position = endPosition(p2Position, diceRoll);
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p2Score += p2Position;
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}
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diceRoll[0] = roll(diceRoll[0]);
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diceRoll[1] = roll(diceRoll[1]);
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diceRoll[2] = roll(diceRoll[2]);
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turns += 3;
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p1Turn = !p1Turn;
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}
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return String.valueOf(turns * Math.min(p1Score, p2Score));
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}
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private int roll(int value) {
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if (value + 3 > 100) {
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return value + 3 - 100;
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}
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return value + 3;
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}
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private int endPosition(int position, int[] diceRoll) {
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return ((position-1 + diceRoll[0] + diceRoll[1] + diceRoll[2]) % 10) + 1;
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}
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private int endPosition(int position, int diceRoll) {
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return ((position-1 + diceRoll) % 10) + 1;
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}
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private static final Map<String, UniverseResult> UNIVERSE_RESULTS = new HashMap<>();
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@Override
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public String partTwo(List<String> input) {
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int p1Position = Integer.parseInt(input.get(0).split("starting position: ")[1]);
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int p2Position = Integer.parseInt(input.get(1).split("starting position: ")[1]);
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UniverseResult result = playGame(0, 0, p1Position, p2Position, 0, 0);
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return String.valueOf(Math.max(result.getP1Wins().longValue(), result.getP2Wins().longValue()));
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}
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public UniverseResult playGame(int p1Score, int p2Score, int p1Position, int p2Position, int turn, int throwSum) {
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boolean isP1Turn = turn < 3;
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boolean lastP1Throw = turn == 2;
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boolean lastP2Throw = turn == 5;
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String cacheKey = String.format("%d-%d-%d-%d-%d-%d", p1Position, p1Score, p2Position, p2Score, turn, throwSum);
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UniverseResult res;
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if (p1Score >= PART_2VICTORY) {
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res = new UniverseResult(BigInteger.ONE, BigInteger.ZERO);
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} else if (p2Score >= PART_2VICTORY) {
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res = new UniverseResult(BigInteger.ZERO, BigInteger.ONE);
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} else {
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if (UNIVERSE_RESULTS.containsKey(cacheKey)) {
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return UNIVERSE_RESULTS.get(cacheKey);
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}
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int nextTurn = (turn + 1) % 6;
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BigInteger p1Wins = BigInteger.ZERO;
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BigInteger p2Wins = BigInteger.ZERO;
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for (int roll = 1; roll <= 3; roll++) {
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UniverseResult result = playGame(
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isP1Turn ? p1Score + (lastP1Throw ? endPosition(p1Position, roll) : 0) : p1Score,
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!isP1Turn ? p2Score + (lastP2Throw ? endPosition(p2Position, roll) : 0) : p2Score,
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isP1Turn ? endPosition(p1Position, roll) : p1Position,
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!isP1Turn ? endPosition(p2Position, roll) : p2Position,
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nextTurn,
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turn % 3 == 0 ? 0 : throwSum + roll
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);
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p1Wins = p1Wins.add(result.getP1Wins());
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p2Wins = p2Wins.add(result.getP2Wins());
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}
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res = new UniverseResult(p1Wins, p2Wins);
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}
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UNIVERSE_RESULTS.put(cacheKey, res);
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return res;
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}
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class UniverseResult {
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private final BigInteger p1Wins;
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private final BigInteger p2Wins;
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public UniverseResult(BigInteger p1Wins, BigInteger p2Wins) {
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this.p1Wins = p1Wins;
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this.p2Wins = p2Wins;
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}
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public BigInteger getP1Wins() {
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return p1Wins;
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}
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public BigInteger getP2Wins() {
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return p2Wins;
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}
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}
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}

‎src/main/resources/day21.txt

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Player 1 starting position: 2
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Player 2 starting position: 7

‎src/test/java/com/codingnagger/days/Day21Test.java

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package com.codingnagger.days;
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import com.codingnagger.utils.InputLoader;
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import org.junit.jupiter.api.Test;
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import java.util.List;
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import static org.assertj.core.api.Assertions.assertThat;
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public class Day21Test {
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private static final List<String> INPUT = InputLoader.LoadTest("day21.txt");
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private static final Day DAY = new Day21();
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@Test
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public void partOne_shoudlYieldCorrectResult() {
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String result = DAY.partOne(INPUT);
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assertThat(result).isEqualTo("739785");
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}
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@Test
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public void partTwo_shoudlYieldCorrectResult() {
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String result = DAY.partTwo(INPUT);
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assertThat(result).isEqualTo("444356092776315"); // test answer too low, real input answer too high 343042929017714
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}
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}

‎src/test/resources/day21.txt

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Player 1 starting position: 4
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Player 2 starting position: 8

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