SortByPointers.h

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#ifndef LARCOREALG_COREUTILS_SORTBYPOINTER_H
#define LARCOREALG_COREUTILS_SORTBYPOINTER_H

// C/C++ standard libraries
#include <vector>
#include <algorithm> // std::transform(), std::sort()
#include <memory> // std::addressof()
#include <iterator> // std::back_inserter()
#include <utility> // std::move()
#include <type_traits> // std::add_pointer_t


namespace util {
  
  namespace details {
    
    template <typename Coll, typename PtrColl>
    struct MoveFromPointersImpl;
    
  } // namespace details
  
  
  //----------------------------------------------------------------------------
  template <typename Coll>
  auto makePointerVector(Coll& coll);
  
  
  //----------------------------------------------------------------------------
  template <typename Coll, typename PtrColl>
  void MoveFromPointers(Coll& dest, PtrColl& src)
    { details::MoveFromPointersImpl<Coll, PtrColl>::move(dest, src); }
  
  
  //----------------------------------------------------------------------------
  template <typename Coll, typename Sorter>
  void SortByPointers(Coll& coll, Sorter sorter);
  
  
  //----------------------------------------------------------------------------
  
} // namespace util


//------------------------------------------------------------------------------
//---  Template implementation
//------------------------------------------------------------------------------
template <typename Coll>
auto util::makePointerVector(Coll& coll) {
  
  using coll_t = Coll;
  using value_type = typename coll_t::value_type;
  using pointer_type = std::add_pointer_t<value_type>;
  using ptr_coll_t = std::vector<pointer_type>;
  
  auto const n = coll.size();
  
  //
  // create the collection of pointers to data
  //
  ptr_coll_t ptrs;
  ptrs.reserve(n);
  std::transform(coll.begin(), coll.end(), std::back_inserter(ptrs),
    [](auto& obj){ return &obj; });
  
  return ptrs;
  
} // util::makePointerVector()


//------------------------------------------------------------------------------
template <typename Coll, typename Sorter>
void util::SortByPointers(Coll& coll, Sorter sorter) {
  
  using coll_t = Coll;
  
  //
  // create the collection of pointers to data
  //
  auto ptrs = makePointerVector(coll);
  
  //
  // delegate the sorting by pointers
  //
  sorter(ptrs);
  
  //
  // create a sorted collection moving the content from the original one
  //
  coll_t sorted;
  MoveFromPointers(sorted, ptrs);
  
  //
  // replace the old container with the new one
  //
  coll = std::move(sorted);
  
} // util::SortByPointers()


//------------------------------------------------------------------------------
namespace util {
  namespace details {
    
    template <typename Coll, typename PtrColl>
    void moveFromPointersImplBase(Coll& dest, PtrColl& src)
      { for (auto&& ptr: src) dest.push_back(std::move(*ptr)); }
    
    
    template <typename Coll, typename PtrColl>
    struct MoveFromPointersImpl {
      static void move(Coll& dest, PtrColl& src)
        {
          dest.clear();
          moveFromPointersImplBase(dest, src);
        }
    }; // struct MoveFromPointersImpl
    
    
    template <typename Data, typename PtrColl>
    struct MoveFromPointersImpl<std::vector<Data>, PtrColl> {
      static void move(std::vector<Data>& dest, PtrColl& src)
        {
          dest.clear();
          dest.reserve(src.size());
          moveFromPointersImplBase(dest, src);
        }
    }; // struct MoveFromPointersImpl
    
  } // namespace details
} // namespace util


//------------------------------------------------------------------------------

#endif // LARCOREALG_COREUTILS_SORTBYPOINTER_H