2023 Day 16 Part 2 optimisation

We reduce the number of items we put on the list of beams to analyse.
Saving memory allocations list modifications.

2023/puzzle-16-02.cc

@@ -126,11 +126,7 @@ private:
     while (!beams.empty()) {
       auto beam{beams.front()};
       beams.pop_front();
-      if (!loc_valid(beam.loc_)) { continue; }
-      // Insert into hash of energised locations.  If that fails we've done this before and don't
-      // need to do it again.
-      if (auto [it, success] = energised_.insert(beam); !success) { continue; }
-
+      while (loc_valid(beam.loc_) && energised_.insert(beam).second) {
         switch (grid_[beam.loc_.second][beam.loc_.first]) {
         case '.':
           break;
@@ -169,9 +165,8 @@ private:
         default:
           std::abort();
         }
-
         beam.loc_ = beam.loc_ + beam.dir_;
-      beams.push_back(beam);
+      }
     }
   }