Add 2021 solutions

src/2021/Day21.cpp

@@ -0,0 +1,107 @@
+#include "common/aoc.h"
+
+namespace y2021::day21
+{
+
+  int Mod(int i)
+  {
+    return (i - 1) % 10 + 1;
+  }
+
+  std::pair<int64_t, int64_t> Roll(int position1, int points1, int position2, int points2, bool player1, int depth)
+  {
+    if (points2 >= 21)
+      return {0, 1};
+    else if (points1 >= 21)
+      return {1, 0};
+
+    // 3: 111
+    // 4: 211 121 112
+    // 5: 221 212 122 311 131 113
+    // 6: 222 321 312 213 231 123 132
+    // 7: 223 232 323 133 331 313
+    // 8: 233 323 322
+    // 9: 333
+    std::map<int, int> outcomes{{3, 1}, {4, 3}, {5, 6}, {6, 7}, {7, 6}, {8, 3}, {9, 1}};
+
+    std::pair<int64_t, int64_t> wins{0, 0};
+    int initialPos = 0;
+    int initialPoints = 0;
+    if (player1)
+    {
+      initialPos = position1;
+      initialPoints = points1;
+    }
+    else
+    {
+      initialPos = position2;
+      initialPoints = points2;
+    }
+    for (auto [value, times] : outcomes)
+    {
+      if (player1)
+      {
+        position1 = initialPos;
+        points1 = initialPoints;
+        position1 = Mod(position1 + value);
+        points1 += position1;
+      }
+      else
+      {
+        position2 = initialPos;
+        points2 = initialPoints;
+        position2 = Mod(position2 + value);
+        points2 += position2;
+      }
+      auto subwins = Roll(position1, points1, position2, points2, !player1, depth + 1);
+      wins.first += times * subwins.first;
+      wins.second += times * subwins.second;
+    }
+    return wins;
+  }
+
+  REGISTER_DAY(2021, Day21, std::vector<int>, int64_t);
+
+  REGISTER_TEST_EXAMPLE(2021, Day21, ExampleInput, 1, 739785);
+  REGISTER_TEST(2021, Day21, Input, 1, 503478);
+  REGISTER_TEST_EXAMPLE(2021, Day21, ExampleInput, 2, 444356092776315);
+  REGISTER_TEST(2021, Day21, Input, 2, 716241959649754);
+
+  READ_INPUT(input)
+  {
+    std::vector<int> players;
+    int i;
+    std::string str;
+    while (getline(input, str))
+    {
+      std::stringstream ss{str};
+      ss >> "Player" >> i >> "starting position:" >> i;
+      players.emplace_back(i);
+    }
+    return players;
+  }
+
+  OUTPUT1(input)
+  {
+    std::vector<int> positions = input;
+    std::vector<int> points;
+    points.resize(input.size());
+    for (int i = 1; i < 10000; i += 3)
+    {
+      int index = (i - 1) % 2;
+      positions[index] = Mod(positions[index] + 3 * (i + 1));
+      points[index] += positions[index];
+      if (points[index] >= 1000)
+      {
+        return ((int64_t)i + 2ll) * (int64_t)points[(index + 1) % 2];
+      }
+    }
+    return 0;
+  }
+
+  OUTPUT2(input)
+  {
+    std::pair<int64_t, int64_t> wins = Roll(input[0], 0, input[1], 0, true, 0);
+    return std::max(wins.first, wins.second);
+  }
+}