Initial commit

2020/puzzle-24-02.cc

@@ -0,0 +1,120 @@
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <iostream>
+#include <list>
+#include <map>
+#include <regex>
+#include <set>
+#include <string>
+#include <tuple>
+#include <vector>
+
+using Coord = long;
+using Position = std::pair<Coord, Coord>;
+struct State {
+  bool black_ = false;
+  int neighbours_ = 0;
+};
+using Tiles = std::map<Position, State>;
+
+struct Tiler {
+  void flip_tile(std::string const &s) {
+    Coord x = 0;
+    Coord y = 0;
+    auto it = s.begin();
+    while (it != s.end()) {
+      Coord dx = 2;
+      if (*it == 's' || *it == 'n') {
+        y += (*it == 'n') - (*it == 's');
+        ++it;
+        dx = 1;
+        assert(it != s.end());
+      }
+
+      if (*it == 'e') {
+        x += dx;
+      } else if (*it == 'w') {
+        x -= dx;
+      } else {
+        assert(false);
+      }
+      ++it;
+    }
+
+    // Insert tile to be flipped - if we fail remove it.
+    flip_tile(x, y);
+  }
+
+  Tiler next_state() const {
+    Tiler next(*this);
+    for (auto const &kv : tiles_) {
+      if (kv.second.black_ &&
+          (kv.second.neighbours_ == 0 || kv.second.neighbours_ > 2)) {
+        next.flip_tile(kv.first.first, kv.first.second);
+      } else if (!kv.second.black_ && kv.second.neighbours_ == 2) {
+        next.flip_tile(kv.first.first, kv.first.second);
+      }
+    }
+
+    return next;
+  }
+
+  std::size_t black_count() const noexcept {
+    std::size_t count = 0;
+    for (auto const &kv : tiles_) {
+      count += kv.second.black_;
+    }
+
+    return count;
+  }
+
+private:
+  void set_neighbours(Coord x, Coord y, int delta) {
+    set_neighbour(x + 2, y, delta);
+    set_neighbour(x + 1, y - 1, delta);
+    set_neighbour(x - 1, y - 1, delta);
+    set_neighbour(x - 2, y, delta);
+    set_neighbour(x - 1, y + 1, delta);
+    set_neighbour(x + 1, y + 1, delta);
+  }
+
+  void set_neighbour(Coord x, Coord y, int delta) {
+    auto [it, success] = tiles_.insert({{x, y}, {false, delta}});
+    if (!success) {
+      it->second.neighbours_ += delta;
+    }
+    assert(it->second.neighbours_ >= 0);
+  }
+
+  void flip_tile(Coord x, Coord y) {
+    auto [it, success] = tiles_.insert({{x, y}, {true, 0}});
+    if (!success) {
+      it->second.black_ = !it->second.black_;
+    }
+    if (it->second.black_) {
+      set_neighbours(x, y, 1);
+    } else {
+      set_neighbours(x, y, -1);
+    }
+  }
+
+private:
+  Tiles tiles_;
+};
+
+int main(void) {
+  Tiler tiler;
+  std::string line;
+  while (std::getline(std::cin, line)) {
+    tiler.flip_tile(line);
+  }
+
+  std::cout << "Day 0: " << tiler.black_count() << "\n";
+  for (unsigned i = 1; i < 101; ++i) {
+    tiler = tiler.next_state();
+    std::cout << "Day " << i << ": " << tiler.black_count() << "\n";
+  }
+
+  return 0;
+}