diff --git a/2019/18.cpp b/2019/18.cpp
index b4cdfbf..beb9bed 100644
--- a/2019/18.cpp
+++ b/2019/18.cpp
@@ -18,6 +18,8 @@
 #include <aocpp/Startup.hpp>
 #include <aocpp/Coord.hpp>
 
+using namespace aocpp;
+
 // lowercase key
 // uppercase door
 // start at @
@@ -26,8 +28,8 @@
 
 namespace {
 
-using namespace aocpp;
-using Map = std::vector<std::string>;
+Coord(* const directions[4])(Coord) = {Up, Down, Left, Right};
+
 using Features = std::map<char, Coord>;
 using Doors = std::bitset<26>;
 using Distances = std::map<char,std::map<char, std::pair<std::int64_t, Doors>>>;
@@ -36,22 +38,29 @@ auto SetKey(Doors& doors, char key) {
   return doors.set(std::toupper(key) - 'A');
 }
 
-auto ReadMap(std::istream & in) -> Map {
-  Map result;
-  std::string line;
-  while (std::getline(in, line)) {
-    result.emplace_back(std::move(line));
+struct Map {
+  std::vector<std::string> rows;
+
+  auto operator[](Coord c)       -> char& { return rows[c.y][c.x]; }
+  auto operator[](Coord c) const -> char  { return rows[c.y][c.x]; }
+
+  static auto Parse(std::istream & in) -> Map {
+    Map result;
+    std::string line;
+    while (std::getline(in, line)) {
+      result.rows.emplace_back(std::move(line));
+    }
+    return {result};
   }
-  return result;
-}
+};
 
 auto FindFeatures(Map const& map) -> Features {
-  std::map<char,Coord> features;
-  for (std::int64_t y = 0; y < map.size(); y++) {
-    for (std::int64_t x = 0; x < map[y].size(); x++) {
-      auto const c = map[y][x];
+  Features features;
+  for (std::int64_t y = 0; y < map.rows.size(); y++) {
+    for (std::int64_t x = 0; x < map.rows[y].size(); x++) {
+      auto const c = map[{x,y}];
       if ('#' != c && '.' != c) {
-        features[c] = Coord{x,y};
+        features[c] = {x,y};
       }
     }
   }
@@ -59,15 +68,15 @@ auto FindFeatures(Map const& map) -> Features {
 }
 
 auto FindDistancesFrom(
-  Map const& map, Features const& features,
+  Map const& map,
+  Features const& features,
   Distances & distances,
-  char start_letter,
-  Coord start
+  char const start_letter,
+  Coord const start
 ) {
   std::map<char, std::pair<std::int64_t, Doors>> result;
 
-  std::deque<std::tuple<std::int64_t, Coord, Doors>> todo
-    {{0, start, {}}};
+  std::deque<std::tuple<std::int64_t, Coord, Doors>> todo {{0, start, {}}};
 
   std::set<Coord> seen;
 
@@ -77,7 +86,7 @@ auto FindDistancesFrom(
     // Don't visit the same coordinate twice
     if (!seen.insert(here).second) continue;
 
-    auto const c = map[here.y][here.x];
+    auto const c = map[here];
     if (c == '#') continue; // avoid walls
 
     // success, we've found a key, record the path
@@ -92,7 +101,7 @@ auto FindDistancesFrom(
     }
 
     // Visit all neighbors
-    for (auto && fn : {Up,Down,Left,Right}) {
+    for (auto const fn : directions) {
       todo.emplace_back(steps+1, fn(here), doors);
     }
   }
@@ -100,15 +109,13 @@ auto FindDistancesFrom(
   return result;
 }
 
-auto FindDistances(Map const& map, Features const& features) -> Distances {
+auto FindDistances(Map const& map, Features const& features) {
   Distances distances;
-
-  for (auto && [start_letter, start_coord] : features) {
+  for (auto const [start_letter, start_coord] : features) {
     if (!std::isupper(start_letter)) {
       FindDistancesFrom(map, features, distances, start_letter, start_coord);
     }
   }
-
   return distances;
 }
 
@@ -121,11 +128,12 @@ auto SolveMaze(
   using Visited = std::pair<std::string, unsigned long long>;
   std::set<Visited> seen;
 
-  using QElt = std::tuple<std::int64_t, std::string, Doors>;
-  struct Cmp { auto operator()(QElt const& x, QElt const& y) -> bool {
-    return std::get<0>(x) > std::get<0>(y);
-  }};
-  std::priority_queue<QElt, std::vector<QElt>, Cmp> todo;
+  // Priority queue returning lowest path cost states first.
+  using PqElt = std::tuple<std::int64_t, std::string, Doors>;
+  using PqCmp = decltype([](PqElt const& x, PqElt const& y) {
+                return std::get<0>(x) > std::get<0>(y); });
+  std::priority_queue<PqElt, std::vector<PqElt>, PqCmp> todo;
+
   todo.emplace(0, initial_locations, Doors());
 
   while(!todo.empty()) {
@@ -138,10 +146,10 @@ auto SolveMaze(
     if (seen.emplace(locations, keys.to_ullong()).second) {
 
       for (auto& location : locations) {
-        auto save = location;
+        auto const save = location;
 
-        for (auto [next, costneed] : distances.at(location)) {
-          auto [cost, need] = costneed;
+        for (auto const [next, costneed] : distances.at(location)) {
+          auto const [cost, need] = costneed;
           if ((need & ~keys).none()) { // no missing keys
             location = next;
             auto keys_ = keys; SetKey(keys_, next);
@@ -156,30 +164,28 @@ auto SolveMaze(
   throw std::runtime_error{"no solution to part 1"};
 }
 
-auto UpdateMap(Map& map, Features & features) {
-  auto start = features['@'];
-  map[start.y][start.x] = '#';
-  map[start.y][start.x+1] = '#';
-  map[start.y][start.x-1] = '#';
-  map[start.y+1][start.x] = '#';
-  map[start.y-1][start.x] = '#';
+// Part 2 instructs us to update the map splitting it into 4 quadrants
+auto Part2(Map& map, Features & features) {
+  auto const start = features['@'];
+  for (auto const fn : directions) {
+    map[fn(start)] = '#';
+  }
   features.erase('@');
-  features['^'] = Up(Left(start));
-  features['&'] = Down(Left(start));
-  features['*'] = Up(Right(start));
+  features['^'] =   Up(Left (start));
+  features['&'] = Down(Left (start));
+  features['*'] =   Up(Right(start));
   features['$'] = Down(Right(start));
 }
 
 } // namespace
 
 auto main(int argc, char** argv) -> int {
-  auto map = ReadMap(Startup(argc, argv));
+  auto map = Map::Parse(Startup(argc, argv));
   auto features = FindFeatures(map);
   auto distances = FindDistances(map, features);
-
   std::cout << "Part 1: " << SolveMaze(distances, "@") << std::endl;
-  
-  UpdateMap(map, features);
+
+  Part2(map, features);
   distances = FindDistances(map, features);
   std::cout << "Part 2: " << SolveMaze(distances, "^&*$") << std::endl;
 }