Day 2022-16 tidied up.

2022/puzzle-16-02.cc

@@ -6,15 +6,13 @@
 #include <iostream>
 #include <list>
 #include <map>
-#include <numeric>
 #include <regex>
 #include <set>
 #include <stdexcept>
 #include <utility>
 #include <vector>
 
-using Int = std::int64_t;
-using UInt = std::uint64_t;
+using UInt = std::uint32_t;
 
 using namespace std::string_literals;
 
@@ -59,18 +57,26 @@ struct State
   ValveID id_[2]{ValveID("AA"s), ValveID("AA"s)};
   UInt next_time_[2]{26, 26};
   UInt total_rate_{0};
-  std::vector<char> open_{std::vector<char>(ValveID::max(), false)};
+  std::vector<char> open_{std::vector<char>(ValveID::max(), 0)};
 
   auto operator<=>(State const& rhs) const noexcept -> std::strong_ordering
   {
-    for (unsigned i = 0; i < 2; ++i) {
-      if (id_[i] != rhs.id_[i]) {
-        return id_[i] <=> rhs.id_[i];
-      }
-      if (next_time_[i] != rhs.next_time_[i]) {
-        return next_time_[i] <=> rhs.next_time_[i];
-      }
+    /* For comparisons - we don't care whether the human or elephant is at each site - only that one
+     * of them is.  So we compare the max and min
+     */
+    auto lm1{std::max(id_[0], id_[1])};
+    auto rm1{std::max(rhs.id_[0], rhs.id_[1])};
+    auto lm2{std::min(id_[0], id_[1])};
+    auto rm2{std::min(rhs.id_[0], rhs.id_[1])};
+    if (lm1 != rm1) {
+      return lm1 <=> rm1;
     }
+    if (lm2 != rm2) {
+      return lm2 <=> rm2;
+    }
+
+    /* We do not care about the time left or the score for sorting.  */
+
     for (UInt i{0}; i < open_.size(); ++i) {
       if (open_[i] != rhs.open_[i]) {
         return open_[i] <=> rhs.open_[i];
@@ -85,9 +91,12 @@ auto main() -> int
   std::string line;
   std::regex const re{
     "Valve ([A-Z][A-Z]) has flow rate=(\\d+); tunnels? leads? to valves? ([A-Z, ]+)"};
+
+  /* The edges vector will end up so that edges[a][b] will contain the number of seconds it will
+   * take to go from a to b, or 0 if there are no routes, or it is not worth stopping at b.
+   */
   std::vector<std::vector<UInt>> edges(ValveID::max(), std::vector<UInt>(ValveID::max(), 0));
   std::vector<UInt> rates(ValveID::max(), 0);
-  UInt size{0};
 
   while (std::getline(std::cin, line)) {
     std::smatch m;
@@ -110,6 +119,9 @@ auto main() -> int
     }
   }
 
+  /* edges currently contains all routes that are one step away.  Now update it so that all possible
+   * moves are possible.
+   */
   bool changed{true};
   while (changed) {
     changed = false;
@@ -127,12 +139,12 @@ auto main() -> int
     }
   }
 
-  // Turning default state on.
+  // Moves from `a` to `a` are worthless.
   for (UInt id{0}; id < ValveID::max(); ++id) {
     edges[id][id] = 0;
   }
 
-  // If rate at a site is zero then change the cost to zero.
+  // If rate at a site is zero then change the cost to zero, as there is no point stopping here.
   for (UInt e1{0}; e1 < ValveID::max(); ++e1) {
     for (UInt e2{0}; e2 < ValveID::max(); ++e2) {
       if (rates[e2] == 0) {
@@ -156,15 +168,18 @@ auto main() -> int
         break;
       }
 
+      std::list<State> next_states;
       for (auto it{current_states.begin()}; it != current_states.end();) {
+        // Is it the current time to modify this item?
         if (it->next_time_[mover] != time_left) {
           ++it;
           continue;
         }
 
-        ValveID from{it->id_[mover]};
+        ValveID const from{it->id_[mover]};
 
         for (UInt to{0}; to < ValveID::max(); ++to) {
+          // Determine were to move to.
           if (edges[from][to] == 0) {
             continue;
           }
@@ -177,12 +192,14 @@ auto main() -> int
 
           State next_state{*it};
           next_state.id_[mover] = ValveID(to);
-          next_state.open_[to] = true;
-          auto timestep = edges[from][to] + 1;
-          next_state.next_time_[mover] -= timestep;
+          next_state.open_[to] = 1;
+          auto const time_step{edges[from][to] + 1};
+          next_state.next_time_[mover] -= time_step;
           next_state.total_rate_ += (next_state.next_time_[mover]) * rates[to];
           best_rate = std::max(best_rate, next_state.total_rate_);
 
+          // Ensure that this is going to make things better: Either we've never visited the state
+          // before.  Or if we have then this version has a higher score.
           auto vit = visited_states.find(next_state);
           if (vit != visited_states.end() && vit->total_rate_ >= next_state.total_rate_) {
             continue;
@@ -191,10 +208,11 @@ auto main() -> int
             visited_states.erase(vit);
           }
           visited_states.insert(next_state);
-          current_states.push_back(next_state);
+          next_states.push_back(next_state);
         }
         it = current_states.erase(it);
       }
+      current_states.splice(current_states.end(), next_states);
     }
   }