indexaccess.hh

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// -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
// vi: set et ts=4 sw=2 sts=2:
 
// SPDX-FileCopyrightText: Copyright © DUNE Project contributors, see file AUTHORS.md
// SPDX-License-Identifier: LicenseRef-GPL-2.0-only-with-DUNE-exception OR LGPL-3.0-or-later
 
#ifndef DUNE_FUNCTIONS_COMMON_INDEX_ACCESS_HH
#define DUNE_FUNCTIONS_COMMON_INDEX_ACCESS_HH
 
 
#include <utility>
#include <type_traits>
 
#include <dune/common/typetraits.hh>
#include <dune/common/concept.hh>
#include <dune/common/hybridutilities.hh>
 
#include <dune/functions/common/utility.hh>
 
 
 
namespace Dune {
namespace Functions {
 
 
namespace Imp {
 
namespace Concept {
 
template<class size_type>
struct HasDynamicIndexAccess
{
  template<class C>
  auto require(C&& c) -> decltype(
    c[std::declval<size_type>()]
  );
};
 
struct HasStaticIndexAccess
{
  template<class C>
  auto require(C&& c) -> decltype(
    c[Dune::Indices::_0]
  );
};
 
} // namespace Concept
 
} // namespace Imp
 
 
 
template<class C, class I, class F,
  std::enable_if_t< Dune::models<Imp::Concept::HasDynamicIndexAccess<I>, C>(), int> = 0>
auto hybridIndexAccess(C&& c, const I& i, F&& f)
  -> decltype(f(c[i]))
{
  return f(c[i]);
}
 
template<class C, class I, class F,
  std::enable_if_t< not Dune::models<Imp::Concept::HasDynamicIndexAccess<I>, C>(), int> = 0>
decltype(auto) hybridIndexAccess(C&& c, const I& i, F&& f)
{
  using Size = decltype(Hybrid::size(c));
  return Hybrid::switchCases(std::make_index_sequence<Size::value>(), i,
      [&](const auto& ii) -> decltype(auto){
        return f(c[ii]);
      }, [&]() -> decltype(auto){
        return f(c[Dune::Indices::_0]);
      });
}
 
 
namespace Imp {
 
  template<class Index, std::size_t offset=1>
  class ShiftedDynamicMultiIndex
  {
  public:
    ShiftedDynamicMultiIndex(const Index& index) :
      index_(index)
    {}
 
    std::size_t operator[](std::size_t position) const
    {
      if (position<size())
        return index_[position+offset];
      else
        return 0;
    }
 
    ShiftedDynamicMultiIndex<Index, offset+1> pop() const
    {
      return {index_};
    }
 
    std::size_t size() const
    {
      if (offset < index_.size())
        return index_.size() - offset;
      else
        return 0;
    }
 
    const Index& originalIndex() const
    {
      return index_;
    }
 
  private:
    const Index& index_;
  };
 
  template<class Index, std::size_t offset=1>
  class ShiftedStaticMultiIndex
  {
  public:
    ShiftedStaticMultiIndex(const Index& index) :
      index_(index)
    {}
 
    template<std::size_t i>
    auto operator[](Dune::index_constant<i>) const
    {
      if constexpr (i<size()) {
        return index_[Dune::index_constant<i+offset>{}];
      } else {
        return Dune::index_constant<0>{};
      }
    }
 
    ShiftedStaticMultiIndex<Index, offset+1> pop() const
    {
      return {index_};
    }
 
    static constexpr std::size_t size()
    {
      auto fullSize = decltype(Hybrid::size(std::declval<Index>()))::value;
      if (offset < fullSize)
        return fullSize - offset;
      else
        return 0;
    }
 
  private:
    const Index& index_;
  };
 
  template<std::size_t offset, class Index>
  ShiftedDynamicMultiIndex<Index, offset> shiftedDynamicMultiIndex(const Index& index)
  {
    return {index};
  }
 
  template<std::size_t offset, class Index, std::size_t oldOffset>
  ShiftedDynamicMultiIndex<Index, offset+oldOffset> shiftedDynamicMultiIndex(const ShiftedDynamicMultiIndex<Index, oldOffset>& index)
  {
    return {index.originalIndex()};
  }
 
  template<std::size_t offset, class Index>
  ShiftedStaticMultiIndex<Index, offset> shiftedStaticMultiIndex(const Index& index)
  {
    return {index};
  }
 
} // namespace Imp
 
 
 
 
namespace Imp {
 
template<class Result, class Index>
struct MultiIndexResolver
{
  MultiIndexResolver(const Index& index) :
    index_(index)
  {}
 
  template<class C,
    std::enable_if_t<not std::is_convertible_v<C&, Result>, int> = 0>
  Result operator()(C&& c)
  {
    auto&& subIndex = Imp::shiftedDynamicMultiIndex<1>(index_);
    auto&& subIndexResolver = MultiIndexResolver<Result, decltype(subIndex)>(subIndex);
    return (Result)(hybridIndexAccess(c, index_[Dune::Indices::_0], subIndexResolver));
  }
 
  template<class C,
    std::enable_if_t<std::is_convertible_v<C&, Result>, int> = 0>
  Result operator()(C&& c)
  {
    return (Result)(std::forward<C>(c));
  }
 
  const Index& index_;
};
 
} // namespace Imp
 
 
 
template<class Result, class C, class MultiIndex>
Result hybridMultiIndexAccess(C&& c, const MultiIndex& index)
{
 
  Imp::MultiIndexResolver<Result, MultiIndex> multiIndexResolver(index);
  return multiIndexResolver(c);
}
 
 
 
 
 
 
namespace Imp {
 
  template<class C, class MultiIndex, class IsFinal>
  constexpr decltype(auto) resolveDynamicMultiIndex(C&& c, const MultiIndex& multiIndex, const IsFinal& isFinal)
  {
    constexpr bool stopRecursion = decltype(isFinal(c))::value;
    constexpr bool hasDynamicAccess = requires { c[0]; };
 
    // If c is already considered final simply return it,
    // else resolve the next multiIndex entry.
    if constexpr (stopRecursion)
    {
      assert(multiIndex.size() == 0);
      // There are two possibilities for the result:
      // If c is an l-value reference, then C is of the form C=RawC& and we return an l-value reference.
      // If c is an r-value reference, then C is of the form C=RawC and we return a copy of c by value.
      // Notice that we should neither return plain c nor std::forward<C>(c) here.
      // The former would bind a returned r-value reference to an r-value reference (which is an l-value!).
      // The latter would deduce C&& as return value and thus return an r-value reference to the temporary
      // which is gone after the function call.
      return static_cast<C>(c);
    }
    else
    {
      // Split multiIndex into first entry and remaining ones.
      auto i = multiIndex[0];
      auto tail = multiIndex.pop();
 
      // Resolve first multiIndex entry by c[multiIndex[0]] and
      // continue resolving with the remaining remaining ones.
      // If c has a dynamic operator[] this is straight forward.
      // Else the dynamic multiIndex[0] has to be translated into
      // a static one using hybridIndexAccess.
      if constexpr (hasDynamicAccess)
        return Imp::resolveDynamicMultiIndex(c[i], tail, isFinal);
      else
      {
        // auto indexRange = range(Hybrid::size(id(c)));
        auto indexRange = typename decltype(range(Hybrid::size(c)))::integer_sequence();
        return Hybrid::switchCases(indexRange, i, [&](auto static_i) -> decltype(auto){
          // Do rescursion with static version of i
          return Imp::resolveDynamicMultiIndex(c[static_i], tail, isFinal);
        }, [&]() -> decltype(auto){
          // As fallback we use c[0] this is needed, because there must be one branch that matches.
          return Imp::resolveDynamicMultiIndex(c[Dune::Indices::_0], tail, isFinal);
        });
      }
    }
  }
 
  template<class C, class MultiIndex>
  constexpr decltype(auto) resolveStaticMultiIndex(C&& c, const MultiIndex& multiIndex)
  {
    constexpr bool stopRecursion = (decltype(Hybrid::size(multiIndex))::value == 0);
    // Cf. resolveDynamicMultiIndex() for the explanation of the static_cast used in the first branch.
    if constexpr (stopRecursion)
      return static_cast<C>(c);
    else
      return Imp::resolveStaticMultiIndex(c[multiIndex[Dune::Indices::_0]], multiIndex.pop());
  }
 
} // namespace Imp
 
 
 
template<class C, class MultiIndex, class IsFinal>
constexpr decltype(auto) resolveDynamicMultiIndex(C&& c, const MultiIndex& multiIndex, const IsFinal& isFinal)
{
  return Imp::resolveDynamicMultiIndex(std::forward<C>(c), Imp::shiftedDynamicMultiIndex<0>(multiIndex), isFinal);
}
 
template<class C, class MultiIndex>
constexpr decltype(auto) resolveDynamicMultiIndex(C&& c, const MultiIndex& multiIndex)
{
  auto hasNoIndexAccess = [](auto&& cc ) { return std::bool_constant<not requires { cc[Dune::Indices::_0]; }>{}; };
  return Imp::resolveDynamicMultiIndex(std::forward<C>(c), Imp::shiftedDynamicMultiIndex<0>(multiIndex), hasNoIndexAccess);
}
 
template<class C, class MultiIndex>
constexpr decltype(auto) resolveStaticMultiIndex(C&& c, const MultiIndex& multiIndex)
{
  return Imp::resolveStaticMultiIndex(std::forward<C>(c), Imp::shiftedStaticMultiIndex<0>(multiIndex));
}
 
 
 
} // namespace Dune::Functions
} // namespace Dune
 
 
 
#endif // DUNE_FUNCTIONS_COMMON_INDEX_ACCESS_HH