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 // This file is part of the ACTS project.
 //
 // Copyright (C) 2016 CERN for the benefit of the ACTS project
 //
 // This Source Code Form is subject to the terms of the Mozilla Public
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
 #pragma once
 
 #include "Acts/Utilities/ContainerConcepts.hpp"
 #include "Acts/Utilities/TypeTraits.hpp"
 
 #include <span>
 
 namespace Acts {
 
 template <typename Container, typename Value, typename Index, bool ReadOnly>
 class ContainerSubset {
  public:
   using container_type = const_if_t<ReadOnly, Container>;
   using value_type = Value;
   using index_type = Index;
   static constexpr bool read_only = ReadOnly;
   using index_subset_type = std::span<const index_type>;
 
   class Iterator {
    public:
     using subset_iterator = index_subset_type::iterator;
 
     using value_type = Value;
     using difference_type = std::ptrdiff_t;
     using pointer = void;
     using reference = void;
 
     using iterator_category = std::random_access_iterator_tag;
     using iterator_concept = std::random_access_iterator_tag;
 
     constexpr Iterator() noexcept = default;
     constexpr Iterator(container_type &container,
                        subset_iterator iterator) noexcept
         : m_container(&container), m_iterator(iterator) {}
 
     constexpr value_type operator*() const {
       static_assert(
           ContainerHasAt<Container> || ContainerHasArrayAccess<Container>,
           "Container must support at() or operator[] for indexing");
       constexpr bool HasArrayAccess = ContainerHasArrayAccess<Container>;
 
       if constexpr (HasArrayAccess) {
         return (*m_container)[*m_iterator];
       } else {
         return m_container->at(*m_iterator);
       }
     }
     constexpr value_type operator[](difference_type n) const {
       static_assert(
           ContainerHasAt<Container> || ContainerHasArrayAccess<Container>,
           "Container must support at() or operator[] for indexing");
       constexpr bool HasArrayAccess = ContainerHasArrayAccess<Container>;
 
       if constexpr (HasArrayAccess) {
         return (*m_container)[m_iterator[n]];
       } else {
         return m_container->at(m_iterator[n]);
       }
     }
 
     constexpr Iterator &operator++() noexcept {
       ++m_iterator;
       return *this;
     }
     constexpr Iterator operator++(int) noexcept {
       auto tmp = *this;
       ++(*this);
       return tmp;
     }
     constexpr Iterator &operator--() noexcept {
       --m_iterator;
       return *this;
     }
     constexpr Iterator operator--(int) noexcept {
       auto tmp = *this;
       --(*this);
       return tmp;
     }
 
     constexpr Iterator &operator+=(difference_type n) noexcept {
       m_iterator += n;
       return *this;
     }
     constexpr Iterator &operator-=(difference_type n) noexcept {
       m_iterator -= n;
       return *this;
     }
 
    private:
     container_type *m_container{};
     subset_iterator m_iterator{};
 
     friend constexpr Iterator operator+(Iterator it,
                                         difference_type n) noexcept {
       return it += n;
     }
 
     friend constexpr Iterator operator+(difference_type n,
                                         Iterator it) noexcept {
       return it += n;
     }
 
     friend constexpr Iterator operator-(Iterator it,
                                         difference_type n) noexcept {
       return it -= n;
     }
 
     friend constexpr difference_type operator-(const Iterator &lhs,
                                                const Iterator &rhs) noexcept {
       return lhs.m_iterator - rhs.m_iterator;
     }
 
     friend constexpr auto operator<=>(const Iterator &a,
                                       const Iterator &b) noexcept {
       if (a.m_iterator < b.m_iterator) {
         return std::strong_ordering::less;
       }
       if (a.m_iterator > b.m_iterator) {
         return std::strong_ordering::greater;
       }
       return std::strong_ordering::equal;
     }
     friend constexpr bool operator==(const Iterator &a,
                                      const Iterator &b) noexcept {
       return a.m_iterator == b.m_iterator;
     }
   };
   using iterator = Iterator;
 
   constexpr ContainerSubset(container_type &container,
                             const index_subset_type &subset) noexcept
       : m_container(&container), m_subset(subset) {}
   template <bool OtherReadOnly>
   explicit constexpr ContainerSubset(
       const ContainerSubset<Container, Value, Index, OtherReadOnly>
           &other) noexcept
     requires(ReadOnly && !OtherReadOnly)
       : m_container(&other.container()), m_subset(other.subset()) {}
 
   constexpr ContainerSubset<Container, Value, Index, true> asConst()
       const noexcept
     requires(!ReadOnly)
   {
     return {*m_container, m_subset};
   }
 
   constexpr container_type &container() const noexcept { return *m_container; }
   constexpr const index_subset_type &subset() const noexcept {
     return m_subset;
   }
 
   constexpr std::size_t size() const noexcept { return m_subset.size(); }
   constexpr bool empty() const noexcept { return size() == 0; }
 
   constexpr auto front() const noexcept {
     return container()[m_subset.front()];
   }
   constexpr auto back() const noexcept { return container()[m_subset.back()]; }
 
   constexpr iterator begin() const noexcept {
     return iterator(*m_container, m_subset.begin());
   }
   constexpr iterator end() const noexcept {
     return iterator(*m_container, m_subset.end());
   }
 
  private:
   container_type *m_container{};
   index_subset_type m_subset{};
 };
 
 }  // namespace Acts

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