Add 2021 day 21 puzzles

2021/puzzle-21-01.cc

@@ -0,0 +1,82 @@
+#include <iostream>
+#include <numeric>
+#include <regex>
+#include <string>
+#include <vector>
+
+using UInt = unsigned;
+
+struct PlayerState
+{
+  constexpr PlayerState(UInt initial_pos) : pos_(initial_pos) {}
+
+  [[nodiscard]] constexpr auto score() const noexcept -> UInt { return score_; }
+  [[nodiscard]] constexpr auto pos() const noexcept -> UInt { return pos_; }
+
+  constexpr void move(UInt amount) noexcept
+  {
+    amount %= 10;
+    pos_ += amount;
+    if (pos_ > 10) {
+      pos_ -= 10;
+    }
+    score_ += pos_;
+  }
+
+  UInt pos_;
+  UInt score_{0};
+};
+
+struct Die
+{
+  [[nodiscard]] constexpr auto roll_count() const noexcept -> UInt { return roll_count_; }
+
+  [[nodiscard]] constexpr auto roll() noexcept -> UInt
+  {
+    ++roll_count_;
+    ++current_;
+    if (current_ == 101) {
+      current_ = 1;
+    }
+    return current_;
+  }
+
+private:
+  UInt current_{0};
+  UInt roll_count_{0};
+};
+
+auto main() -> int
+{
+  std::string line;
+  std::regex re{"Player (\\d) starting position: (\\d+)"};
+
+  std::vector<PlayerState> players;
+  while (std::getline(std::cin, line) && !line.empty()) {
+    std::smatch m;
+    if (!std::regex_search(line, m, re)) {
+      std::cerr << "Unable to match: " << line << "\n";
+      return 1;
+    }
+    players.push_back(PlayerState(std::stol(m.str(2))));
+  }
+
+  Die die;
+  bool cont{true};
+  while (cont) {
+    for (auto& player : players) {
+      UInt move = die.roll() + die.roll() + die.roll();
+      player.move(move);
+      if (player.score() >= 1'000) {
+        cont = false;
+        break;
+      }
+    }
+  }
+
+  auto r{
+    std::accumulate(players.begin(), players.end(), UInt{die.roll_count()},
+                    [](auto a, auto const& p) { return a * (p.score() < 1000 ? p.score() : 1); })};
+  std::cout << "Multiplied scores " << r << '\n';
+  return 0;
+}