2023 Day 10

2023/puzzle-10-01.cc

@@ -0,0 +1,135 @@
+#include <cassert>
+#include <cstdint>
+#include <cstdlib>
+#include <iostream>
+#include <string>
+#include <utility>
+#include <vector>
+
+using Int = std::int64_t;
+using NumVec = std::vector<Int>;
+using Pos = std::pair<std::size_t, std::size_t>; // (x, y) pair
+
+[[nodiscard]] constexpr auto operator==(Pos const& lhs, Pos const& rhs) -> bool
+{
+  return lhs.first == rhs.first && lhs.second == rhs.second;
+}
+
+struct Grid
+{
+  void push_line(std::string const& line)
+  {
+    if (auto x{line.find('S')}; x != std::string::npos) {
+      assert(animal_.first == std::string::npos);
+      animal_.first = x;
+      animal_.second = grid_.size();
+    }
+    grid_.push_back(line);
+  }
+
+  [[nodiscard]] auto animal_pos() const -> Pos { return animal_; }
+
+  [[nodiscard]] auto width() const -> std::size_t { return grid_.front().size(); }
+  [[nodiscard]] auto height() const -> std::size_t { return grid_.size(); }
+
+  [[nodiscard]] auto walk_pipes(Pos const& start, Pos pos) const -> std::pair<Pos, std::size_t>
+  {
+    auto last{start};
+    std::size_t steps{0};
+    while (true) {
+      auto const [x, y] = pos;
+      auto const c{grid_[y][x]};
+      if (c == '.' || c == 'S') { break; }
+      if (start == pos) { break; }
+
+      if (c == '|' || c == 'L' || c == 'J') {
+        if (Pos const next_pos{x, y - 1}; y > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '|' || c == 'F' || c == '7') {
+        if (Pos const next_pos{x, y + 1}; y < height() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == '7' || c == 'J') {
+        if (Pos const next_pos{x - 1, y}; x > 0 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      if (c == '-' || c == 'L' || c == 'F') {
+        if (Pos const next_pos{x + 1, y}; x < width() - 1 && next_pos != last) {
+          last = pos;
+          pos = next_pos;
+          ++steps;
+          continue;
+        }
+      }
+
+      std::abort();
+    }
+
+    return std::make_pair(pos, steps);
+  }
+
+private:
+  std::vector<std::string> grid_;
+  Pos animal_{std::string::npos, std::string::npos};
+};
+
+auto main() -> int try {
+  std::string line;
+  Grid grid;
+
+  while (std::getline(std::cin, line)) {
+    grid.push_line(line);
+  }
+
+  auto animal = grid.animal_pos();
+
+  if (animal.second > 0) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first, animal.second - 1});
+    if (end_pos == animal) {
+      std::cout << "Started going north ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.first > 0) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first - 1, animal.second});
+    if (end_pos == animal) {
+      std::cout << "Started going west ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.second < grid.height() - 1) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first, animal.second + 1});
+    if (end_pos == animal) {
+      std::cout << "Started going south ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  if (animal.first < grid.width() - 1) {
+    auto [end_pos, length] = grid.walk_pipes(animal, {animal.first + 1, animal.second});
+    if (end_pos == animal) {
+      std::cout << "Started going east ended back at animal: " << length / 2 + 1 << '\n';
+    }
+  }
+
+  return EXIT_SUCCESS;
+}
+catch (...) {
+  std::cerr << "Uncaught exception.\n";
+  return EXIT_FAILURE;
+}