Move write_tallies to C++

include/openmc/tallies/tally.h

@@ -2,6 +2,7 @@
 #define OPENMC_TALLIES_TALLY_H
 
 #include "openmc/constants.h"
+#include "openmc/tallies/filter.h"
 #include "openmc/tallies/trigger.h"
 
 #include "pugixml.hpp"
@@ -21,6 +22,8 @@ class Tally {
 public:
   Tally() {}
 
+  void init_from_xml(pugi::xml_node node);
+
   void set_scores(pugi::xml_node node);
 
   void set_scores(std::vector<std::string> scores);
@@ -50,6 +53,8 @@ public:
 
   int id_; //!< user-defined identifier
 
+  std::string name_; //!< user-defined name
+
   int type_ {TALLY_VOLUME}; //!< volume, surface current
 
   //! Event type that contributes to this tally
@@ -93,6 +98,41 @@ private:
   int32_t n_filter_bins_ {0};
 };
 
+//==============================================================================
+//! An iterator over all combinations of a tally's matching filter bins.
+//
+//! This iterator handles two distinct tasks.  First, it maps the N-dimensional
+//! space created by the indices of N filters onto a 1D sequence.  In other
+//! words, it provides a single number that uniquely identifies a combination of
+//! bins for many filters.  Second, it handles the task of finding each all
+//! valid combinations of filter bins given that each filter can have 1 or 2 or
+//! many bins that are valid for the current tally event.
+//==============================================================================
+
+class FilterBinIter
+{
+public:
+  FilterBinIter(const Tally& tally, Particle* p);
+
+  FilterBinIter(const Tally& tally, bool end);
+
+  bool operator==(const FilterBinIter& other) const
+  {return index_ == other.index_;}
+
+  bool operator!=(const FilterBinIter& other) const
+  {return !(*this == other);}
+
+  FilterBinIter& operator++();
+
+  int index_ {1};
+  double weight_ {1.};
+
+private:
+  void compute_index_weight();
+
+  const Tally& tally_;
+};
+
 //==============================================================================
 // Global variable declarations
 //==============================================================================