Use optional integer to store exchange partner (#455)

* Formatting

* Use optional exchange instead of double

This uses an optional integer to represent exchange MPI rank.

src/move.cpp

@@ -878,7 +878,7 @@ void ParallelTempering::_move(Change& change)
     }
     partner->generate(mpi.world, slump);
     if (partner->rank.has_value()) {
-        exchangeState(change);        
+        exchangeState(change);
     }
 }
 
@@ -921,7 +921,7 @@ void ParallelTempering::_accept([[maybe_unused]] Change& change)
 void ParallelTempering::_reject([[maybe_unused]] Change& change)
 {
     acceptance_map[partner->getPair(mpi.world)] += 0.0;
-    exchange = -1.0;
+    exchange = std::nullopt;
     writeToFileStream();
 }
 
@@ -941,8 +941,7 @@ void ParallelTempering::_from_json(const json& j)
 void ParallelTempering::writeToFileStream() const
 {
     if (stream) {
-        // file to disk?:
-        *stream << fmt::format("{:d} {:.6E}\n", number_of_attempted_moves , exchange);
+        *stream << fmt::format("{:d} {}\n", number_of_attempted_moves, exchange.value_or(-1));
     }
 }
 

src/move.h

@@ -576,7 +576,7 @@ class ParallelTempering : public Move
     Geometry::VolumeMethod volume_scaling_method =
         Geometry::VolumeMethod::ISOTROPIC;                               //!< How to scale volumes
     std::map<MPI::Partner::PartnerPair, Average<double>> acceptance_map; //!< Exchange statistics
-    
+
     Random slump; // static instance of Random (shared for all in ParallelTempering)
     void _to_json(json& j) const override;
     void _from_json(const json& j) override;
@@ -588,11 +588,11 @@ class ParallelTempering : public Move
     double exchangeEnergy(double energy_change); //!< Exchange energy with partner
     void exchangeState(Change& change);          //!< Exchange positions, charges, volume etc.
     void exchangeGroupSizes(Space::GroupVector& groups, int partner_rank);
-    
-    std::string filename;                           //file name for exchange statistics
-    std::unique_ptr<std::ostream> stream;    //log exchange statistics in file
-    double exchange;  // if no exchange, this is 0              
-    void writeToFileStream() const; //!< Write exchange statistics to file
+
+    std::string filename;                 //!< File name for exchange statistics
+    std::unique_ptr<std::ostream> stream; //!< Log exchange statistics in file
+    std::optional<int> exchange;          //!< Rank of exchange partner, if any
+    void writeToFileStream() const;       //!< Write exchange statistics to file
 
   public:
     explicit ParallelTempering(Space& spc, const MPI::Controller& mpi);