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 //----------------------------------*-C++-*----------------------------------//
 // Copyright 2023-2024 UT-Battelle, LLC, and other Celeritas developers.
 // See the top-level COPYRIGHT file for details.
 // SPDX-License-Identifier: (Apache-2.0 OR MIT)
 //---------------------------------------------------------------------------//
 //! \file corecel/data/DedupeCollectionBuilder.hh
 //---------------------------------------------------------------------------//
 #pragma once
 
 #include <algorithm>
 #include <cstddef>
 #include <unordered_set>
 
 #include "corecel/math/HashUtils.hh"
 
 #include "Collection.hh"
 
 namespace celeritas
 {
 //---------------------------------------------------------------------------//
 /*!
  * Build collections, returning the same ID for reused data spans.
  */
 template<class T, class I = ItemId<T>>
 class DedupeCollectionBuilder
 {
   public:
     //!@{
     //! \name Type aliases
     using CollectionT = Collection<T, Ownership::value, MemSpace::host, I>;
     using value_type = T;
     using size_type = typename CollectionT::size_type;
     using ItemIdT = typename CollectionT::ItemIdT;
     using ItemRangeT = typename CollectionT::ItemRangeT;
     //!@}
 
   public:
     // Construct from a collection
     explicit DedupeCollectionBuilder(CollectionT* collection);
 
     //// ACCESSORS ////
 
     //! Number of elements in the collection
     size_type size() const { return this->storage().size(); }
 
     //! Get the size as an ID type
     ItemIdT size_id() const { return ItemIdT{this->size()}; }
 
     //// MUTATORS ////
 
     // Reserve space and hash table space
     inline void reserve(std::size_t count);
 
     // Extend with a series of elements, returning the range inserted
     template<class InputIterator>
     inline ItemRangeT insert_back(InputIterator first, InputIterator last);
 
     // Extend with a series of elements from an initializer list
     inline ItemRangeT insert_back(std::initializer_list<value_type> init);
 
     // Append a single element
     inline ItemIdT push_back(value_type const& element);
 
   private:
     //// CLASSES ////
 
     using StorageT = typename CollectionT::StorageT;
 
     struct HashRange
     {
         StorageT* storage{nullptr};
         std::size_t operator()(ItemRangeT) const;
     };
     struct EqualRange
     {
         StorageT* storage{nullptr};
         bool operator()(ItemRangeT const&, ItemRangeT const&) const;
     };
 
     //// DATA ////
 
     CollectionT* col_;
     std::unordered_set<ItemRangeT, HashRange, EqualRange> ranges_;
 
     //// FUNCTIONS ////
 
     //!@{
     //! Access storage
     CELER_FORCEINLINE StorageT& storage() { return col_->storage(); }
     CELER_FORCEINLINE StorageT const& storage() const
     {
         return col_->storage();
     }
     //!@}
 };
 
 //---------------------------------------------------------------------------//
 // INLINE DEFINITIONS
 //---------------------------------------------------------------------------//
 /*!
  * Construct from a collection.
  */
 template<class T, class I>
 DedupeCollectionBuilder<T, I>::DedupeCollectionBuilder(CollectionT* collection)
     : col_{collection}
     , ranges_{0, HashRange{&this->storage()}, EqualRange{&this->storage()}}
 {
     CELER_EXPECT(col_);
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Reserve space for elements.
  */
 template<class T, class I>
 void DedupeCollectionBuilder<T, I>::reserve(std::size_t count)
 {
     this->storage().reserve(count);
     ranges_.reserve(count);
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Insert the given list of elements at the end of the allocation.
  */
 template<class T, class I>
 template<class InputIterator>
 auto DedupeCollectionBuilder<T, I>::insert_back(InputIterator first,
                                                 InputIterator last) -> ItemRangeT
 {
     auto& s = this->storage();
 
     auto start = this->size_id();
     s.insert(s.end(), first, last);
     ItemRangeT result{start, this->size_id()};
     auto [iter, inserted] = ranges_.insert(result);
     if (!inserted)
     {
         // Roll back the change by erasing the last elements
         s.erase(s.begin() + start.unchecked_get(), s.end());
         // Return existing range
         return *iter;
     }
     return result;
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Insert the given list of elements at the end of the allocation.
  */
 template<class T, class I>
 auto DedupeCollectionBuilder<T, I>::insert_back(std::initializer_list<T> init)
     -> ItemRangeT
 {
     return this->insert_back(init.begin(), init.end());
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Add a new element to the end of the allocation.
  */
 template<class T, class I>
 auto DedupeCollectionBuilder<T, I>::push_back(T const& el) -> ItemIdT
 {
     auto result = this->size_id();
     this->storage().push_back(el);
     return result;
 }
 
 //---------------------------------------------------------------------------//
 // NESTED CLASSES
 //---------------------------------------------------------------------------//
 /*!
  * Compute a hash of the data underlying a range.
  *
  * By default, this uses the std::hash specialization in \c HashUtils.
  */
 template<class T, class I>
 std::size_t
 DedupeCollectionBuilder<T, I>::HashRange::operator()(ItemRangeT r) const
 {
     CELER_EXPECT(*r.end() <= storage->size());
 
     // Hash data pointed to by range
     Span<T const> data{storage->data() + r.begin()->unchecked_get(), r.size()};
     return std::hash<decltype(data)>{}(data);
 }
 
 //---------------------------------------------------------------------------//
 /*!
  * Compare as equal the data referenced by two ranges.
  */
 template<class T, class I>
 bool DedupeCollectionBuilder<T, I>::EqualRange::operator()(
     ItemRangeT const& a, ItemRangeT const& b) const
 {
     CELER_EXPECT(*a.end() <= storage->size());
     CELER_EXPECT(*b.end() <= storage->size());
 
     auto values_equal = [&s = *storage](ItemIdT left, ItemIdT right) {
         return s[left.unchecked_get()] == s[right.unchecked_get()];
     };
     return std::equal(a.begin(), a.end(), b.begin(), b.end(), values_equal);
 }
 
 //---------------------------------------------------------------------------//
 }  // namespace celeritas

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