From 6865e3594e2fe89cfbbe0a94bcc19314c90c5cb2 Mon Sep 17 00:00:00 2001
From: Eric Mertens <emertens@galois.com>
Date: Thu, 10 Nov 2022 17:46:30 -0800
Subject: [PATCH] 18

---
 2019/18.cpp | 111 +++++++++++++++++++++++++---------------------------
 1 file changed, 54 insertions(+), 57 deletions(-)

diff --git a/2019/18.cpp b/2019/18.cpp
index 14db007..b4cdfbf 100644
--- a/2019/18.cpp
+++ b/2019/18.cpp
@@ -32,6 +32,10 @@ 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>>>;
 
+auto SetKey(Doors& doors, char key) {
+  return doors.set(std::toupper(key) - 'A');
+}
+
 auto ReadMap(std::istream & in) -> Map {
   Map result;
   std::string line;
@@ -45,11 +49,9 @@ 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 c = map[y][x];
-      switch (c) {
-        default: features[c] = Coord{x,y};
-        case '#':
-        case '.': {}
+      auto const c = map[y][x];
+      if ('#' != c && '.' != c) {
+        features[c] = Coord{x,y};
       }
     }
   }
@@ -59,32 +61,39 @@ auto FindFeatures(Map const& map) -> Features {
 auto FindDistancesFrom(
   Map const& map, Features const& features,
   Distances & distances,
-  char start_letter, Coord start
+  char start_letter,
+  Coord start
 ) {
   std::map<char, std::pair<std::int64_t, Doors>> result;
-  std::set<Coord> seen;
+
   std::deque<std::tuple<std::int64_t, Coord, Doors>> todo
     {{0, start, {}}};
 
-  while (!todo.empty()) {
+  std::set<Coord> seen;
+
+  for (; !todo.empty(); todo.pop_front()) {
     auto [steps, here, doors] = todo.front();
-    todo.pop_front();
-    if (seen.insert(here).second) {
-      auto c = map[here.y][here.x];
-      if (c == '#') {
-        continue;
-      }
-      if (c != start_letter && std::islower(c)) {
+
+    // Don't visit the same coordinate twice
+    if (!seen.insert(here).second) continue;
+
+    auto const c = map[here.y][here.x];
+    if (c == '#') continue; // avoid walls
+
+    // success, we've found a key, record the path
+    if (c != start_letter && std::islower(c)) {
         distances[start_letter][c] = {steps, doors};
-        continue;
-      }
-      if (std::isupper(c)) {
-        doors.set(c - 'A');
-      }
-      for (auto && fn : {Up,Down,Left,Right}) {
-        auto here_ = fn(here);
-        todo.push_back({steps+1, here_, doors});
-      }
+        continue; // don't walk beyond the key
+    }
+
+    // Note any keys we encounter on our journey
+    if (std::isupper(c)) {
+      SetKey(doors, c);
+    }
+
+    // Visit all neighbors
+    for (auto && fn : {Up,Down,Left,Right}) {
+      todo.emplace_back(steps+1, fn(here), doors);
     }
   }
 
@@ -103,53 +112,41 @@ auto FindDistances(Map const& map, Features const& features) -> Distances {
   return distances;
 }
 
-auto HasKey(Doors doors, char key) {
-  return doors.test(std::toupper(key) - 'A');
-}
-
-auto SetKey(Doors doors, char key) {
-  return doors.set(std::toupper(key) - 'A');
-}
-
-template <class Elt, class Compare>
-auto MakePQueue(Compare const& compare)
-  -> std::priority_queue<Elt, std::vector<Elt>, Compare>
-{
-  return {compare};
-}
-
 auto SolveMaze(
   Distances const& distances,
-  std::set<char> const initial_locations
+  std::string const initial_locations
 ) -> std::int64_t
 {
   // Track current positions and current set of keys in easy to compare form
-  using Visited = std::pair<std::set<char>, unsigned long long>;
+  using Visited = std::pair<std::string, unsigned long long>;
   std::set<Visited> seen;
 
-  auto todo = MakePQueue<std::tuple<std::int64_t, std::set<char>, Doors>>(
-    [](auto const& x, auto const& y) {
-      return std::get<0>(x) > std::get<0>(y);
-    }
-  );
-
+  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;
   todo.emplace(0, initial_locations, Doors());
 
-  while (!todo.empty()) {
+  while(!todo.empty()) {
     auto [steps, locations, keys] = todo.top();
     todo.pop();
 
     if (keys.all()) { return steps; }
 
-    if (seen.insert({locations,keys.to_ullong()}).second) {
-      for (auto location : locations) {
-        auto locations1 = locations; locations1.erase(location);
+    std::sort(locations.begin(), locations.end());
+    if (seen.emplace(locations, keys.to_ullong()).second) {
 
-        for (auto && [next, costneed] : distances.at(location)) {
+      for (auto& location : locations) {
+        auto save = location;
+
+        for (auto [next, costneed] : distances.at(location)) {
           auto [cost, need] = costneed;
-          if ((need & ~keys).none()) {
-            auto locations2 = locations1; locations2.insert(next);
-            todo.emplace(steps + cost, locations2, SetKey(keys, next));
+          if ((need & ~keys).none()) { // no missing keys
+            location = next;
+            auto keys_ = keys; SetKey(keys_, next);
+            todo.emplace(steps + cost, locations, keys_);
+            location = save;
           }
         }
       }
@@ -180,9 +177,9 @@ auto main(int argc, char** argv) -> int {
   auto features = FindFeatures(map);
   auto distances = FindDistances(map, features);
 
-  std::cout << "Part 1: " << SolveMaze(distances, {'@'}) << std::endl;
+  std::cout << "Part 1: " << SolveMaze(distances, "@") << std::endl;
   
   UpdateMap(map, features);
   distances = FindDistances(map, features);
-  std::cout << "Part 2: " << SolveMaze(distances, {'^', '&', '*', '$'}) << std::endl;
+  std::cout << "Part 2: " << SolveMaze(distances, "^&*$") << std::endl;
 }