MappedRangeView.hpp 13.2 KB
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#pragma once

#include <iterator>
#include <type_traits>

#include <amdis/common/TypeTraits.hpp>

namespace AMDiS
{
  // NOTE: this is a backport of the dune-common TransformedRangeView with added size() function.

  namespace Impl
  {
    // Helper class to mimic a pointer for proxy objects.
    // This is needed to implement operator-> on an iterator
    // using proxy-values. It stores the proxy value but
    // provides operator-> like a pointer.
    template <class ProxyType>
    class PointerProxy
    {
    public:
      PointerProxy(ProxyType&& p)
        : p_(p)
      {}

      ProxyType* operator->()
      {
        return &p_;
      }

      ProxyType p_;
    };

    // An iterator transforming a wrapped iterator using
    // an unary function. It inherits the iterator-category
    // of the underlying iterator.
    template <class I, class F, class C = typename std::iterator_traits<I>::iterator_category>
    class MappedRangeIterator;

    template <class I, class F>
    class MappedRangeIterator<I,F,std::forward_iterator_tag>
    {
    public:
      using iterator_category = std::forward_iterator_tag;
      using reference = decltype(std::declval<F>()(*(std::declval<I>())));
      using value_type = std::decay_t<reference>;
      using pointer = PointerProxy<value_type>;

      // If we later want to allow standalone MappedRangeIterators,
      // we could customize the FunctionPointer to be a default-constructible,
      // copy-assignable type storing a function but acting like a pointer
      // to function.
      using FunctionPointer = const F*;

      constexpr MappedRangeIterator(const I& it, FunctionPointer f) noexcept
        : it_(it)
        , f_(f)
      {}

      // Explicitly initialize members. Using a plain
      //
      //   constexpr MappedRangeIterator() noexcept {}
      //
      // would default-initialize the members while
      //
      //   constexpr MappedRangeIterator() noexcept : it_(), f_() {}
      //
      // leads to value-initialization. This is a case where
      // both are really different. If it_ is a raw pointer (i.e. POD)
      // then default-initialization leaves it uninitialized while
      // value-initialization zero-initializes it.
      constexpr MappedRangeIterator() noexcept
        : it_()
        , f_()
      {}

      // Dereferencing returns a value created by the function
      constexpr reference operator*() const noexcept
      {
        return (*f_)(*it_);
      }

      // Dereferencing returns a value created by the function
      pointer operator->() const noexcept
      {
        return (*f_)(*it_);
      }

      constexpr MappedRangeIterator& operator=(MappedRangeIterator const&) = default;

      constexpr bool operator==(const MappedRangeIterator& other) const noexcept
      {
        return (it_ == other.it_);
      }

      constexpr bool operator!=(const MappedRangeIterator& other) const noexcept
      {
        return (it_ != other.it_);
      }

      MappedRangeIterator& operator++() noexcept
      {
        ++it_;
        return *this;
      }

      MappedRangeIterator operator++(int) noexcept
      {
        MappedRangeIterator copy(*this);
        ++(*this);
        return copy;
      }

    protected:
      I it_;
      FunctionPointer f_;
    };


    template <class I, class F>
    class MappedRangeIterator<I,F,std::bidirectional_iterator_tag>
        : public MappedRangeIterator<I,F,std::forward_iterator_tag>
    {
    protected:
      using Base = MappedRangeIterator<I,F,std::forward_iterator_tag>;
      using Base::it_;
      using Base::f_;
    public:
      using iterator_category = std::bidirectional_iterator_tag;
      using reference = typename Base::reference;
      using value_type = typename Base::value_type;
      using pointer = typename Base::pointer;

      using FunctionPointer = typename Base::FunctionPointer;

      // inheriting constructor
      using Base::Base;

      // Member functions of the forward_iterator that need
      // to be redefined because the base class methods return a
      // forward_iterator.
      constexpr MappedRangeIterator& operator=(MappedRangeIterator const&) = default;

      MappedRangeIterator& operator++() noexcept
      {
        ++it_;
        return *this;
      }

      MappedRangeIterator operator++(int) noexcept
      {
        MappedRangeIterator copy(*this);
        ++(*this);
        return copy;
      }

      // Additional member functions of bidirectional_iterator
      MappedRangeIterator& operator--() noexcept
      {
        --(this->it_);
        return *this;
      }

      MappedRangeIterator operator--(int) noexcept
      {
        MappedRangeIterator copy(*this);
        --(*this);
        return copy;
      }
    };


    template <class I, class F>
    class MappedRangeIterator<I,F,std::random_access_iterator_tag>
        : public MappedRangeIterator<I,F,std::bidirectional_iterator_tag>
    {
    protected:
      using Base = MappedRangeIterator<I,F,std::bidirectional_iterator_tag>;
      using Base::it_;
      using Base::f_;
    public:
      using iterator_category = std::random_access_iterator_tag;
      using reference = typename Base::reference;
      using value_type = typename Base::value_type;
      using pointer = typename Base::pointer;
      using difference_type = typename std::iterator_traits<I>::difference_type;

      using FunctionPointer = typename Base::FunctionPointer;

      // inheriting constructor
      using Base::Base;

      // Member functions of the forward_iterator that need
      // to be redefined because the base class methods return a
      // forward_iterator.
      constexpr MappedRangeIterator& operator=(MappedRangeIterator const&) = default;

      MappedRangeIterator& operator++() noexcept
      {
        ++it_;
        return *this;
      }

      MappedRangeIterator operator++(int) noexcept
      {
        MappedRangeIterator copy(*this);
        ++(*this);
        return copy;
      }

      // Member functions of the bidirectional_iterator that need
      // to be redefined because the base class methods return a
      // bidirectional_iterator.
      MappedRangeIterator& operator--() noexcept
      {
        --(this->it_);
        return *this;
      }

      MappedRangeIterator operator--(int) noexcept
      {
        MappedRangeIterator copy(*this);
        --(*this);
        return copy;
      }

      // Additional member functions of random_access_iterator
      MappedRangeIterator& operator+=(difference_type n) noexcept
      {
        it_ += n;
        return *this;
      }

      MappedRangeIterator& operator-=(difference_type n) noexcept
      {
        it_ -= n;
        return *this;
      }

      bool operator<(const MappedRangeIterator& other) noexcept
      {
        return it_<other.it_;
      }

      bool operator<=(const MappedRangeIterator& other) noexcept
      {
        return it_<=other.it_;
      }

      bool operator>(const MappedRangeIterator& other) noexcept
      {
        return it_>other.it_;
      }

      bool operator>=(const MappedRangeIterator& other) noexcept
      {
        return it_>=other.it_;
      }

      reference operator[](difference_type n) noexcept
      {
        return (*f_)(*(it_+n));
      }

      friend
      MappedRangeIterator operator+(const MappedRangeIterator& it, difference_type n) noexcept
      {
        return MappedRangeIterator(it.it_+n, it.f_);
      }

      friend
      MappedRangeIterator operator+(difference_type n, const MappedRangeIterator& it) noexcept
      {
        return MappedRangeIterator(n+it.it_, it.f_);
      }

      friend
      MappedRangeIterator operator-(const MappedRangeIterator& it, difference_type n) noexcept
      {
        return MappedRangeIterator(it.it_-n, it.f_);
      }

      friend
      difference_type operator-(const MappedRangeIterator& first, const MappedRangeIterator& second) noexcept
      {
        return first.it_-second.it_;
      }
    };


  } // namespace Impl


  /**
   * \brief A range transforming the values of another range on-the-fly
   *
   * This behaves like a range providing `begin()` and `end()`.
   * The iterators over this range internally iterate over
   * the wrapped range. When dereferencing the iterator,
   * the value is transformed on-the-fly using a given
   * transformation function leaving the underlying range
   * unchanged.
   *
   * The transformation may either return temorary values
   * or l-value references. In the former case the range behaves
   * like a proxy-container. In the latter case it forwards these
   * references allowing, e.g., to sort a subset of some container
   * by applying a transformation to an index-range for those values.
   *
   * The iterators of the MappedRangeView have the same
   * iterator_category as the ones of the wrapped container.
   *
   * If range is given as r-value, then the returned MappedRangeView
   * stores it by value, if range is given as (const) l-value, then the
   * MappedRangeView stores it by (const) reference.
   *
   * If R is a value type, then the MappedRangeView stores the wrapped range by value,
   * if R is a reference type, then the MappedRangeView stores the wrapped range by reference.
   *
   * \tparam R Underlying range.
   * \tparam F Unary function used to transform the values in the underlying range.
   **/
  template <class R, class F>
  class MappedRangeView
  {
    using  RawConstIterator = TYPEOF(std::declval<const R>().begin());
    using  RawIterator = TYPEOF(std::declval<R>().begin());

  public:

    /**
     * \brief Iterator type
     *
     * This inherits the iterator_category of the iterators
     * of the underlying range.
     */
    using const_iterator = Impl::MappedRangeIterator<RawConstIterator, F>;

    using iterator = Impl::MappedRangeIterator<RawIterator, F>;

    /**
     * \brief Construct from range and function
     */
    template <class RR>
    constexpr MappedRangeView(RR&& rawRange, F const& f) noexcept
      : rawRange_(FWD(rawRange))
      , f_(f)
    {}

    /**
     * \brief Obtain a iterator to the first element
     *
     * The life time of the returned iterator is bound to
     * the life time of the range since it only contains a
     * pointer to the transformation function stored
     * in the range.
     */
    constexpr const_iterator begin() const noexcept
    {
      return const_iterator(rawRange_.begin(), &f_);
    }

    constexpr iterator begin() noexcept
    {
      return iterator(rawRange_.begin(), &f_);
    }

    /**
     * \brief Return the number of elements in the range, if availble.
     *
     * Note, this function is only availble if the underlying raw range
     * knows its size and provides a function `size()`.
     */
    template <class RR = R, class = decltype(std::declval<RR>().size())>
    constexpr auto size() const noexcept
    {
      return rawRange_.size();
    }

    /// Provide element access for random-accessible ranges
    template <class RR = R, class = decltype(std::declval<RR>().operator[](std::size_t(0)))>
    decltype(auto) operator[](std::size_t i) const
    {
      return f_(rawRange_[i]);
    }

    /**
     * \brief Checks whether the range is empty
     */
    constexpr bool empty() const noexcept
    {
      return rawRange_.begin() == rawRange_.end();
    }

    /**
     * \brief Obtain a iterator past the last element
     *
     * The life time of the returned iterator is bound to
     * the life time of the range since it only contains a
     * pointer to the transformation function stored
     * in the range.
     */
    constexpr const_iterator end() const noexcept
    {
      return const_iterator(rawRange_.end(), &f_);
    }

    constexpr iterator end() noexcept
    {
      return iterator(rawRange_.end(), &f_);
    }

  private:
    R rawRange_;
    F f_;
  };


  /**
   * \brief Create a MappedRangeView
   *
   * \param range The range the transform
   * \param f Unary function that should the applied to the entries of the range.
   *
   * This behaves like a range providing `begin()` and `end()`.
   * The iterators over this range internally iterate over
   * the wrapped range. When dereferencing the iterator,
   * the value is transformed on-the-fly using a given
   * transformation function leaving the underlying range
   * unchanged.
   *
   * The transformation may either return temporary values
   * or l-value references. In the former case the range behaves
   * like a proxy-container. In the latter case it forwards these
   * references allowing, e.g., to sort a subset of some container
   * by applying a transformation to an index-range for those values.
   *
   * The iterators of the MappedRangeView have the same
   * iterator_category as the ones of the wrapped container.
   *
   * If range is an r-value, then the MappedRangeView stores it by value,
   * if range is an l-value, then the MappedRangeView stores it by reference.
   **/
  template <class R, class F>
  auto mappedRangeView(R&& range, F const& f)
  {
    return MappedRangeView<R, F>(FWD(range), f);
  }


  template <class Iter, class F>
  auto mappedIterator(Iter it, F const* f)
  {
    using iterator = Impl::MappedRangeIterator<Iter, F>;
    return iterator(it, f);
  }

  template <class ConstIter, class F>
  auto mappedConstIterator(ConstIter it, F const* f)
  {
    using const_iterator = Impl::MappedRangeIterator<ConstIter, F>;
    return const_iterator(it, f);
  }

} // end namespace AMDiS