Add year 2021 day21

Author: linl33

year2021/src/main/java/dev/linl33/adventofcode/year2021/Day21.java

@@ -0,0 +1,104 @@
+package dev.linl33.adventofcode.year2021;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+public class Day21 extends AdventSolution2021<Integer, Long> {
+  private static final int P1_WINNING_SCORE = 1000;
+  private static final int P2_WINNING_SCORE = 21;
+  private static final int[] MOVES_TO_U = new int[] { 0, 0, 0, 1, 3, 6, 7, 6, 3, 1 };
+
+  public static void main(String[] args) {
+    new Day21().runAndPrintAll();
+  }
+
+  @Override
+  public Integer part1(@NotNull BufferedReader reader) throws IOException {
+    var p1Line = reader.readLine();
+    var p2Line = reader.readLine();
+
+    var pos = new int[] {
+        Integer.parseInt(p1Line, 28, p1Line.length(), 10),
+        Integer.parseInt(p2Line, 28, p2Line.length(), 10),
+    };
+
+    var scores = new int[2];
+
+    var moves = -3;
+    var player = 0;
+    while (scores[0] < P1_WINNING_SCORE && scores[1] < P1_WINNING_SCORE) {
+      moves += 9;
+      movePlayer(pos, scores, player, moves);
+
+      player = 1 - player;
+    }
+
+    return scores[player] * ((moves + 3) / 9) * 3;
+  }
+
+  @Override
+  public Long part2(@NotNull BufferedReader reader) throws IOException {
+    var p1Line = reader.readLine();
+    var p2Line = reader.readLine();
+
+    var nullPos = new int[] {
+        Integer.parseInt(p1Line, 28, p1Line.length(), 10),
+        Integer.parseInt(p2Line, 28, p2Line.length(), 10),
+    };
+
+    var universes = new long[2];
+
+    // TODO: make this run faster
+    var stack = new ArrayDeque<List<Integer>>();
+    for (int i = 9; i >= 3; i--) {
+      stack.push(List.of(i));
+    }
+
+    while (!stack.isEmpty()) {
+      var next = stack.pop();
+
+      var pos = Arrays.copyOf(nullPos, 2);
+      var scores = new int[2];
+      var player = 0;
+
+      for (int moves : next) {
+        movePlayer(pos, scores, player, moves);
+        player = 1 - player;
+      }
+
+      if (scores[0] >= P2_WINNING_SCORE || scores[1] >= P2_WINNING_SCORE) {
+        var uTally = 1L;
+
+        for (int moves : next) {
+          uTally *= MOVES_TO_U[moves];
+        }
+
+        if (scores[0] >= P2_WINNING_SCORE) {
+          universes[0] += uTally;
+        } else {
+          universes[1] += uTally;
+        }
+      } else {
+        for (int i = 9; i >= 3; i--) {
+          var nextMoves = new ArrayList<>(next);
+          nextMoves.add(i);
+          stack.push(nextMoves);
+        }
+      }
+    }
+
+    return Math.max(universes[0], universes[1]);
+  }
+
+  private static void movePlayer(@NotNull int[] pawnPositions, @NotNull int[] scores, int player, int moves) {
+    var newPos = ((pawnPositions[player] + moves - 1) % 10) + 1;
+    pawnPositions[player] = newPos;
+    scores[player] += newPos;
+  }
+}

year2021/src/main/resources/dev/linl33/adventofcode/year2021/day21

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+Player 1 starting position: 8
+Player 2 starting position: 10

year2021/src/main/resources/dev/linl33/adventofcode/year2021/day21test1

@@ -0,0 +1,2 @@
+Player 1 starting position: 4
+Player 2 starting position: 8

year2021/src/test/java/dev/linl33/adventofcode/year2021/test/Day21Test.java

@@ -0,0 +1,31 @@
+package dev.linl33.adventofcode.year2021.test;
+
+import dev.linl33.adventofcode.lib.solution.AdventSolution;
+import dev.linl33.adventofcode.testlib.AdventSolutionTest;
+import dev.linl33.adventofcode.year2021.Day20;
+import dev.linl33.adventofcode.year2021.Day21;
+
+import java.util.Map;
+
+class Day21Test implements AdventSolutionTest<Integer, Long> {
+  @Override
+  public AdventSolution<Integer, Long> newSolutionInstance() {
+    return new Day21();
+  }
+
+  @Override
+  public Map<Object, Integer> getPart1Cases() {
+    return Map.of(
+        newSolutionInstance().getPart1Resource(), 605070,
+        "day21test1", 739785
+    );
+  }
+
+  @Override
+  public Map<Object, Long> getPart2Cases() {
+    return Map.of(
+        newSolutionInstance().getPart2Resource(), 218433063958910L,
+        "day21test1", 444356092776315L
+    );
+  }
+}