iterator_impl.h

// -*- C++ -*-
//===-- iterator_impl.h ---------------------------------------------------===//
//
// Copyright (C) Intel Corporation
//
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
// This file incorporates work covered by the following copyright and permission
// notice:
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
//
//===----------------------------------------------------------------------===//

#ifndef _ONEDPL_ITERATOR_IMPL_H
#define _ONEDPL_ITERATOR_IMPL_H

#include <iterator>
#include <tuple>
#include <cassert>
#include <type_traits>

#include "onedpl_config.h"
#include "utils.h"
#include "tuple_impl.h"

namespace oneapi
{
namespace dpl
{
namespace __internal
{
template <size_t _Np>
struct __tuple_util
{
    template <typename _TupleType, typename _DifferenceType>
    static void
    __increment(_TupleType& __it, _DifferenceType __forward)
    {
        ::std::get<_Np - 1>(__it) = ::std::get<_Np - 1>(__it) + __forward;
        __tuple_util<_Np - 1>::__increment(__it, __forward);
    }
    template <typename _TupleType>
    static void
    __pre_increment(_TupleType& __it)
    {
        ++::std::get<_Np - 1>(__it);
        __tuple_util<_Np - 1>::__pre_increment(__it);
    }
};

template <>
struct __tuple_util<0>
{
    template <typename _TupleType, typename _DifferenceType>
    static void
    __increment(_TupleType&, _DifferenceType)
    {
    }
    template <typename _TupleType>
    static void
    __pre_increment(_TupleType&)
    {
    }
};

template <typename _TupleReturnType>
struct __make_references
{
    template <typename _TupleType, ::std::size_t... _Ip>
    _TupleReturnType
    operator()(const _TupleType& __t, ::std::index_sequence<_Ip...>)
    {
        return _TupleReturnType(*::std::get<_Ip>(__t)...);
    }
};

template <typename _Iter, typename = void>
struct __is_legacy_passed_directly : std::false_type
{
};

template <typename _Iter>
struct __is_legacy_passed_directly<_Iter, ::std::enable_if_t<_Iter::is_passed_directly::value>> : std::true_type
{
};

template <typename _T>
struct __is_reversed_indirectly_device_accessible_it;

template <typename T>
constexpr auto is_onedpl_indirectly_device_accessible_iterator(T)
    -> std::disjunction<
#if _ONEDPL_BACKEND_SYCL
        oneapi::dpl::__internal::__is_known_usm_vector_iter<std::decay_t<T>>,             // USM vector iterator
#endif // _ONEDPL_BACKEND_SYCL
        std::is_pointer<std::decay_t<T>>,                                                 // USM pointer
        oneapi::dpl::__internal::__is_legacy_passed_directly<std::decay_t<T>>,            // legacy passed directly iter
        oneapi::dpl::__internal::__is_reversed_indirectly_device_accessible_it<std::decay_t<T>>>; // reverse iterator

struct __is_onedpl_indirectly_device_accessible_iterator_fn
{
    template <typename T>
    constexpr auto
    operator()(const T& t) const -> decltype(is_onedpl_indirectly_device_accessible_iterator(t));
};

inline constexpr __is_onedpl_indirectly_device_accessible_iterator_fn __is_onedpl_indirectly_device_accessible_iterator;

//zip_iterator version for forward iterator
//== and != comparison is performed only on the first element of the tuple
//
//zip_forward_iterator is implemented as an internal class and should remain so. Users should never encounter
//this class or be returned a type of its value_type, reference, etc as the tuple-like type used internally
//is variable dependent on the C++ standard library version and could cause an inconsistent ABI due to resulting
//layout changes of this class.
template <typename... _Types>
class zip_forward_iterator
{
    template <typename... _Ts>
    using __tuple_t =
#if _ONEDPL_CAN_USE_STD_TUPLE_PROXY_ITERATOR
        ::std::tuple<_Ts...>;
#else
        oneapi::dpl::__internal::tuple<_Ts...>;
#endif

    static const ::std::size_t __num_types = sizeof...(_Types);
    typedef __tuple_t<_Types...> __it_types;

  public:
    typedef ::std::make_signed_t<::std::size_t> difference_type;
    typedef __tuple_t<typename ::std::iterator_traits<_Types>::value_type...> value_type;
    typedef __tuple_t<typename ::std::iterator_traits<_Types>::reference...> reference;
    typedef __tuple_t<typename ::std::iterator_traits<_Types>::pointer...> pointer;
    typedef ::std::forward_iterator_tag iterator_category;

    zip_forward_iterator() : __my_it_() {}
    explicit zip_forward_iterator(_Types... __args) : __my_it_(__tuple_t<_Types...>{__args...}) {}

    // On windows, this requires clause is necessary so that concepts in MSVC STL do not detect the iterator as
    // dereferenceable when a source iterator is a sycl_iterator, which is a supported type.
    reference
    operator*() const _ONEDPL_CPP20_REQUIRES(std::indirectly_readable<_Types> &&...)
    {
        return __make_references<reference>()(__my_it_, ::std::make_index_sequence<__num_types>());
    }

    zip_forward_iterator&
    operator++()
    {
        __tuple_util<__num_types>::__pre_increment(__my_it_);
        return *this;
    }
    zip_forward_iterator
    operator++(int)
    {
        zip_forward_iterator __it(*this);
        ++(*this);
        return __it;
    }

    bool
    operator==(const zip_forward_iterator& __it) const
    {
        return ::std::get<0>(__my_it_) == ::std::get<0>(__it.__my_it_);
    }
    bool
    operator!=(const zip_forward_iterator& __it) const
    {
        return !(*this == __it);
    }

    __it_types
    base() const
    {
        return __my_it_;
    }

  private:
    __it_types __my_it_;
};

} // namespace __internal
} // namespace dpl
} // namespace oneapi

namespace oneapi
{
namespace dpl
{

template <typename T>
struct is_indirectly_device_accessible
    : decltype(oneapi::dpl::__internal::__is_onedpl_indirectly_device_accessible_iterator(std::declval<T>()))
{
    static_assert(
        std::is_same_v<decltype(decltype(oneapi::dpl::__internal::__is_onedpl_indirectly_device_accessible_iterator(
                           std::declval<T>()))::value),
                       const bool>,
        "Return type of is_onedpl_indirectly_device_accessible_iterator does not have the characteristics of a "
        "bool_constant");
};

template <typename T>
inline constexpr bool is_indirectly_device_accessible_v = is_indirectly_device_accessible<T>::value;

template <typename _Ip>
class counting_iterator
{
    static_assert(::std::is_integral_v<_Ip>, "Cannot instantiate counting_iterator with a non-integer type");

  public:
    typedef ::std::make_signed_t<_Ip> difference_type;
    typedef _Ip value_type;
    typedef const _Ip* pointer;
    // There is no storage behind the iterator, so we return a value instead of reference.
    typedef _Ip reference;
    typedef ::std::random_access_iterator_tag iterator_category;

    counting_iterator() : __my_counter_() {}
    explicit counting_iterator(_Ip __init) : __my_counter_(__init) {}

    reference operator*() const { return __my_counter_; }
    reference operator[](difference_type __i) const { return *(*this + __i); }

    difference_type
    operator-(const counting_iterator& __it) const
    {
        return __my_counter_ - __it.__my_counter_;
    }

    counting_iterator&
    operator+=(difference_type __forward)
    {
        __my_counter_ += __forward;
        return *this;
    }
    counting_iterator&
    operator-=(difference_type __backward)
    {
        return *this += -__backward;
    }
    counting_iterator&
    operator++()
    {
        return *this += 1;
    }
    counting_iterator&
    operator--()
    {
        return *this -= 1;
    }

    counting_iterator
    operator++(int)
    {
        counting_iterator __it(*this);
        ++(*this);
        return __it;
    }
    counting_iterator
    operator--(int)
    {
        counting_iterator __it(*this);
        --(*this);
        return __it;
    }

    counting_iterator
    operator-(difference_type __backward) const
    {
        return counting_iterator(__my_counter_ - __backward);
    }
    counting_iterator
    operator+(difference_type __forward) const
    {
        return counting_iterator(__my_counter_ + __forward);
    }
    friend counting_iterator
    operator+(difference_type __forward, const counting_iterator __it)
    {
        return __it + __forward;
    }

    bool
    operator==(const counting_iterator& __it) const
    {
        return *this - __it == 0;
    }
    bool
    operator!=(const counting_iterator& __it) const
    {
        return !(*this == __it);
    }
    bool
    operator<(const counting_iterator& __it) const
    {
        return *this - __it < 0;
    }
    bool
    operator>(const counting_iterator& __it) const
    {
        return __it < *this;
    }
    bool
    operator<=(const counting_iterator& __it) const
    {
        return !(*this > __it);
    }
    bool
    operator>=(const counting_iterator& __it) const
    {
        return !(*this < __it);
    }

    friend std::true_type
    is_onedpl_indirectly_device_accessible_iterator(const counting_iterator&);

  private:
    _Ip __my_counter_;
};

template <typename... _Types>
class zip_iterator
{
    static_assert(sizeof...(_Types) > 0, "Cannot instantiate zip_iterator with empty template parameter pack");
    static const ::std::size_t __num_types = sizeof...(_Types);
    typedef oneapi::dpl::__internal::tuple<_Types...> __it_types;

  public:
    typedef ::std::make_signed_t<::std::size_t> difference_type;
    typedef oneapi::dpl::__internal::tuple<typename ::std::iterator_traits<_Types>::value_type...> value_type;
    typedef oneapi::dpl::__internal::tuple<typename ::std::iterator_traits<_Types>::reference...> reference;
    typedef ::std::tuple<typename ::std::iterator_traits<_Types>::pointer...> pointer;
    typedef ::std::random_access_iterator_tag iterator_category;
    using is_zip = ::std::true_type;

    zip_iterator() : __my_it_() {}
    explicit zip_iterator(_Types... __args) : __my_it_(::std::make_tuple(__args...)) {}
    explicit zip_iterator(std::tuple<_Types...> __arg) : __my_it_(__arg) {}

    // On windows, this requires clause is necessary so that concepts in MSVC STL do not detect the iterator as
    // dereferenceable when a source iterator is a sycl_iterator, which is a supported type.
    reference
    operator*() const _ONEDPL_CPP20_REQUIRES(std::indirectly_readable<_Types> &&...)
    {
        return oneapi::dpl::__internal::__make_references<reference>()(__my_it_,
                                                                       ::std::make_index_sequence<__num_types>());
    }

    reference operator[](difference_type __i) const { return *(*this + __i); }

    difference_type
    operator-(const zip_iterator& __it) const
    {
        return ::std::get<0>(__my_it_) - ::std::get<0>(__it.__my_it_);
    }

    zip_iterator&
    operator+=(difference_type __forward)
    {
        oneapi::dpl::__internal::__tuple_util<__num_types>::__increment(__my_it_, __forward);
        return *this;
    }
    zip_iterator&
    operator-=(difference_type __backward)
    {
        return *this += -__backward;
    }
    zip_iterator&
    operator++()
    {
        return *this += 1;
    }
    zip_iterator&
    operator--()
    {
        return *this -= 1;
    }

    zip_iterator
    operator++(int)
    {
        zip_iterator __it(*this);
        ++(*this);
        return __it;
    }
    zip_iterator
    operator--(int)
    {
        zip_iterator __it(*this);
        --(*this);
        return __it;
    }

    zip_iterator
    operator-(difference_type __backward) const
    {
        zip_iterator __it(*this);
        return __it -= __backward;
    }
    zip_iterator
    operator+(difference_type __forward) const
    {
        zip_iterator __it(*this);
        return __it += __forward;
    }
    friend zip_iterator
    operator+(difference_type __forward, const zip_iterator& __it)
    {
        return __it + __forward;
    }

    bool
    operator==(const zip_iterator& __it) const
    {
        return *this - __it == 0;
    }
    __it_types
    base() const
    {
        return __my_it_;
    }

    bool
    operator!=(const zip_iterator& __it) const
    {
        return !(*this == __it);
    }
    bool
    operator<(const zip_iterator& __it) const
    {
        return *this - __it < 0;
    }
    bool
    operator>(const zip_iterator& __it) const
    {
        return __it < *this;
    }
    bool
    operator<=(const zip_iterator& __it) const
    {
        return !(*this > __it);
    }
    bool
    operator>=(const zip_iterator& __it) const
    {
        return !(*this < __it);
    }

    friend auto
    is_onedpl_indirectly_device_accessible_iterator(const zip_iterator&)
        -> std::conjunction<oneapi::dpl::is_indirectly_device_accessible<_Types>...>;

  private:
    __it_types __my_it_;
};

template <typename... _Tp>
zip_iterator<_Tp...>
make_zip_iterator(_Tp... __args)
{
    return zip_iterator<_Tp...>(__args...);
}

template <typename... _Tp>
zip_iterator<_Tp...>
make_zip_iterator(std::tuple<_Tp...> __arg)
{
    return zip_iterator<_Tp...>(__arg);
}

template <typename _Iter, typename _UnaryFunc>
class transform_iterator
{
  private:
    _Iter __my_it_;
    _UnaryFunc __my_unary_func_;

    static_assert(std::is_invocable_v<const std::decay_t<_UnaryFunc>, typename std::iterator_traits<_Iter>::reference>,
                  "_UnaryFunc does not have a const-qualified call operator which accepts the reference type of the "
                  "base iterator as argument.");

  public:
    typedef typename ::std::iterator_traits<_Iter>::difference_type difference_type;
    typedef decltype(__my_unary_func_(::std::declval<typename ::std::iterator_traits<_Iter>::reference>())) reference;
    typedef ::std::remove_reference_t<reference> value_type;
    typedef typename ::std::iterator_traits<_Iter>::pointer pointer;
    typedef typename ::std::iterator_traits<_Iter>::iterator_category iterator_category;

    //default constructor will only be present if both the unary functor and iterator are default constructible
    transform_iterator() = default;

    //only enable this constructor if the unary functor is default constructible
    template <typename _UnaryFuncLocal = _UnaryFunc,
              std::enable_if_t<std::is_default_constructible_v<_UnaryFuncLocal>, int> = 0>
    transform_iterator(_Iter __it) : __my_it_(std::move(__it))
    {
    }

    transform_iterator(_Iter __it, _UnaryFunc __unary_func)
        : __my_it_(std::move(__it)), __my_unary_func_(std::move(__unary_func))
    {
    }

    transform_iterator(const transform_iterator&) = default;
    transform_iterator&
    operator=(const transform_iterator& __input)
    {
        __my_it_ = __input.__my_it_;

        // If copy assignment is available, copy the functor, otherwise skip it.
        // For non-copy assignable functors, this copy assignment operator departs from the sycl 2020 specification
        // requirement of device copyable types for copy assignment to be the same as a bitwise copy of the object.
        // TODO: Explore (ABI breaking) change to use std::optional or similar and using copy constructor to implement
        //       copy assignment to better comply with SYCL 2020 specification.
        if constexpr (std::is_copy_assignable_v<_UnaryFunc>)
        {
            __my_unary_func_ = __input.__my_unary_func_;
        }
        return *this;
    }

    // On windows, this requires clause is necessary so that concepts in MSVC STL do not detect the iterator as
    // dereferenceable when the source iterator is a sycl_iterator, which is a supported type.
    reference
    operator*() const _ONEDPL_CPP20_REQUIRES(std::indirectly_readable<_Iter>)
    {
        return __my_unary_func_(*__my_it_);
    }
    reference operator[](difference_type __i) const { return *(*this + __i); }
    transform_iterator&
    operator++()
    {
        ++__my_it_;
        return *this;
    }
    transform_iterator&
    operator--()
    {
        --__my_it_;
        return *this;
    }
    transform_iterator
    operator++(int)
    {
        transform_iterator __it(*this);
        ++(*this);
        return __it;
    }
    transform_iterator
    operator--(int)
    {
        transform_iterator __it(*this);
        --(*this);
        return __it;
    }
    transform_iterator
    operator+(difference_type __forward) const
    {
        return {__my_it_ + __forward, __my_unary_func_};
    }
    transform_iterator
    operator-(difference_type __backward) const
    {
        return {__my_it_ - __backward, __my_unary_func_};
    }
    transform_iterator&
    operator+=(difference_type __forward)
    {
        __my_it_ += __forward;
        return *this;
    }
    transform_iterator&
    operator-=(difference_type __backward)
    {
        __my_it_ -= __backward;
        return *this;
    }
    friend transform_iterator
    operator+(difference_type __forward, const transform_iterator& __it)
    {
        return __it + __forward;
    }
    difference_type
    operator-(const transform_iterator& __it) const
    {
        return __my_it_ - __it.__my_it_;
    }
    bool
    operator==(const transform_iterator& __it) const
    {
        return __my_it_ == __it.__my_it_;
    }
    bool
    operator!=(const transform_iterator& __it) const
    {
        return !(*this == __it);
    }
    bool
    operator<(const transform_iterator& __it) const
    {
        return *this - __it < 0;
    }
    bool
    operator>(const transform_iterator& __it) const
    {
        return __it < *this;
    }
    bool
    operator<=(const transform_iterator& __it) const
    {
        return !(*this > __it);
    }
    bool
    operator>=(const transform_iterator& __it) const
    {
        return !(*this < __it);
    }

    _Iter
    base() const
    {
        return __my_it_;
    }
    _UnaryFunc
    functor() const
    {
        return __my_unary_func_;
    }
    friend auto
    is_onedpl_indirectly_device_accessible_iterator(const transform_iterator&)
        -> oneapi::dpl::is_indirectly_device_accessible<_Iter>;
};

template <typename _Iter, typename _UnaryFunc>
transform_iterator<_Iter, _UnaryFunc>
make_transform_iterator(_Iter __it, _UnaryFunc __unary_func)
{
    return transform_iterator<_Iter, _UnaryFunc>(__it, __unary_func);
}

namespace __internal
{

//functor concept
struct __functor_concept
{
    template <typename _T>
    static auto
    test(int) -> decltype(::std::declval<_T>()(0), ::std::true_type{});

    template <typename _T>
    static auto
    test(...) -> ::std::false_type;
};

template <typename _T>
inline constexpr bool __is_functor = decltype(__functor_concept::test<_T>(0))::value;

} // namespace __internal

template <typename SourceIterator, typename _Permutation>
class permutation_iterator
{
  public:
    typedef std::conditional_t<
        !__internal::__is_functor<_Permutation>, _Permutation,
        transform_iterator<counting_iterator<typename ::std::iterator_traits<SourceIterator>::difference_type>,
                           _Permutation>>
        IndexMap;
    typedef typename ::std::iterator_traits<SourceIterator>::difference_type difference_type;
    typedef typename ::std::iterator_traits<SourceIterator>::value_type value_type;
    typedef typename ::std::iterator_traits<SourceIterator>::pointer pointer;
    typedef typename ::std::iterator_traits<SourceIterator>::reference reference;
    typedef SourceIterator base_type;
    typedef ::std::random_access_iterator_tag iterator_category;
    typedef ::std::true_type is_permutation;

    permutation_iterator() = default;

    template <typename _T = _Permutation, ::std::enable_if_t<!__internal::__is_functor<_T>, int> = 0>
    permutation_iterator(SourceIterator input1, _Permutation input2) : my_source_it(input1), my_index(input2)
    {
    }

    template <typename _T = _Permutation, ::std::enable_if_t<__internal::__is_functor<_T>, int> = 0>
    permutation_iterator(SourceIterator input1, _Permutation __f, difference_type __idx = 0)
        : my_source_it(input1), my_index(counting_iterator<difference_type>(__idx), __f)
    {
    }

  private:
    template <typename _T = _Permutation, ::std::enable_if_t<__internal::__is_functor<_T>, int> = 0>
    permutation_iterator(SourceIterator input1, IndexMap input2) : my_source_it(input1), my_index(input2)
    {
    }

  public:
    SourceIterator
    base() const
    {
        return my_source_it;
    }

    auto
    map() const
    {
        if constexpr (__internal::__is_functor<_Permutation>)
            return my_index.functor();
        else
            return my_index;
    }

    // On windows, this requires clause is necessary so that concepts in MSVC STL do not detect the iterator as
    // dereferenceable when the source or map iterator is a sycl_iterator, which is a supported type for both.
    reference
    operator*() const
        _ONEDPL_CPP20_REQUIRES(std::indirectly_readable<SourceIterator> && std::indirectly_readable<IndexMap>)
    {
        return my_source_it[*my_index];
    }

    reference operator[](difference_type __i) const { return *(*this + __i); }

    permutation_iterator&
    operator++()
    {
        ++my_index;
        return *this;
    }

    permutation_iterator
    operator++(int)
    {
        permutation_iterator it(*this);
        ++(*this);
        return it;
    }

    permutation_iterator&
    operator--()
    {
        --my_index;
        return *this;
    }

    permutation_iterator
    operator--(int)
    {
        permutation_iterator it(*this);
        --(*this);
        return it;
    }

    permutation_iterator
    operator+(difference_type forward) const
    {
        return permutation_iterator(my_source_it, my_index + forward);
    }

    permutation_iterator
    operator-(difference_type backward) const
    {
        return permutation_iterator(my_source_it, my_index - backward);
    }

    permutation_iterator&
    operator+=(difference_type forward)
    {
        my_index += forward;
        return *this;
    }

    permutation_iterator&
    operator-=(difference_type forward)
    {
        my_index -= forward;
        return *this;
    }

    difference_type
    operator-(const permutation_iterator& it) const
    {
        return my_index - it.my_index;
    }

    friend permutation_iterator
    operator+(difference_type forward, const permutation_iterator& it)
    {
        return permutation_iterator(it.my_source_it, it.my_index + forward);
    }

    bool
    operator==(const permutation_iterator& it) const
    {
        return *this - it == 0;
    }
    bool
    operator!=(const permutation_iterator& it) const
    {
        return !(*this == it);
    }
    bool
    operator<(const permutation_iterator& it) const
    {
        return *this - it < 0;
    }
    bool
    operator>(const permutation_iterator& it) const
    {
        return it < *this;
    }
    bool
    operator<=(const permutation_iterator& it) const
    {
        return !(*this > it);
    }
    bool
    operator>=(const permutation_iterator& it) const
    {
        return !(*this < it);
    }

    friend auto
    is_onedpl_indirectly_device_accessible_iterator(const permutation_iterator&)
        -> std::conjunction<oneapi::dpl::is_indirectly_device_accessible<SourceIterator>,
                            oneapi::dpl::is_indirectly_device_accessible<
                                typename oneapi::dpl::permutation_iterator<SourceIterator, _Permutation>::IndexMap>>;

  private:
    SourceIterator my_source_it;
    IndexMap my_index;
};

template <typename SourceIterator, typename IndexMap, typename... StartIndex>
permutation_iterator<SourceIterator, IndexMap>
make_permutation_iterator(SourceIterator source, IndexMap map, StartIndex... idx)
{
    return permutation_iterator<SourceIterator, IndexMap>(source, map, idx...);
}

namespace internal
{
// Copyable implementation of ignore to allow creation of temporary buffers using the type.
struct ignore_copyable
{
    template <typename T>
    ignore_copyable&
    operator=(T&&)
    {
        return *this;
    }

    template <typename T>
    const ignore_copyable&
    operator=(T&&) const
    {
        return *this;
    }

    bool
    operator==(const ignore_copyable&) const
    {
        return true;
    }

    bool
    operator!=(const ignore_copyable& other) const
    {
        return !(*this == other);
    }
};

inline constexpr ignore_copyable ignore{};
} // namespace internal

class discard_iterator
{
  public:
    typedef ::std::ptrdiff_t difference_type;
    typedef internal::ignore_copyable value_type;
    typedef void* pointer;
    typedef value_type reference;
    typedef ::std::random_access_iterator_tag iterator_category;
    using is_discard = ::std::true_type;

    discard_iterator() : __my_position_() {}
    explicit discard_iterator(difference_type __init) : __my_position_(__init) {}

    reference operator*() const { return internal::ignore; }
    reference operator[](difference_type) const { return internal::ignore; }

    // GCC Bug 66297: constexpr non-static member functions of non-literal types
#if __GNUC__ && _ONEDPL_GCC_VERSION < 70200 && !(__INTEL_COMPILER || __clang__)
#    define _ONEDPL_CONSTEXPR_FIX
#else
#    define _ONEDPL_CONSTEXPR_FIX constexpr
#endif

    _ONEDPL_CONSTEXPR_FIX difference_type
    operator-(const discard_iterator& __it) const
    {
        return __my_position_ - __it.__my_position_;
    }

    _ONEDPL_CONSTEXPR_FIX bool
    operator==(const discard_iterator& __it) const
    {
        return __my_position_ == __it.__my_position_;
    }
    _ONEDPL_CONSTEXPR_FIX bool
    operator!=(const discard_iterator& __it) const
    {
        return !(*this == __it);
    }
#undef _ONEDPL_CONSTEXPR_FIX

    bool
    operator<(const discard_iterator& __it) const
    {
        return *this - __it < 0;
    }
    bool
    operator>(const discard_iterator& __it) const
    {
        return __it < *this;
    }

    discard_iterator&
    operator++()
    {
        return *this += 1;
    }
    discard_iterator&
    operator--()
    {
        return *this -= 1;
    }
    discard_iterator
    operator++(int)
    {
        discard_iterator __it(*this);
        ++(*this);
        return __it;
    }
    discard_iterator
    operator--(int)
    {
        discard_iterator __it(*this);
        --(*this);
        return __it;
    }
    discard_iterator&
    operator+=(difference_type __forward)
    {
        __my_position_ += __forward;
        return *this;
    }
    discard_iterator&
    operator-=(difference_type __backward)
    {
        *this += -__backward;
        return *this;
    }

    discard_iterator
    operator+(difference_type __forward) const
    {
        return discard_iterator(__my_position_ + __forward);
    }
    discard_iterator
    operator-(difference_type __backward) const
    {
        return discard_iterator(__my_position_ - __backward);
    }
    friend discard_iterator
    operator+(difference_type __forward, const discard_iterator& __it)
    {
        return __it + __forward;
    }
    bool
    operator<=(const discard_iterator& __it) const
    {
        return !(*this > __it);
    }
    bool
    operator>=(const discard_iterator& __it) const
    {
        return !(*this < __it);
    }

    friend std::true_type
    is_onedpl_indirectly_device_accessible_iterator(const discard_iterator&);

  private:
    difference_type __my_position_;
};

} // namespace dpl
} // namespace oneapi

namespace oneapi
{
namespace dpl
{
namespace __internal
{
template <typename _T>
struct __is_reversed_indirectly_device_accessible_it : std::false_type
{
};

template <typename _BaseIter>
struct __is_reversed_indirectly_device_accessible_it<std::reverse_iterator<_BaseIter>>
    : oneapi::dpl::is_indirectly_device_accessible<_BaseIter>
{
};

struct make_zipiterator_functor
{
    template <typename... Args>
    auto
    operator()(Args&&... args) const -> decltype(oneapi::dpl::make_zip_iterator(::std::forward<Args>(args)...))
    {
        return oneapi::dpl::make_zip_iterator(::std::forward<Args>(args)...);
    }
};

// The functions are required because
// after applying a functor to each element of a tuple
// we may need to get a zip iterator

template <typename F, template <typename...> class TBig, typename... T, typename... RestTuples>
auto
map_zip(F f, TBig<T...> in, RestTuples... rest)
    -> decltype(map_tuple_impl(make_zipiterator_functor{}, ::std::make_index_sequence<sizeof...(T)>(), in, rest...))
{
    return map_tuple_impl(make_zipiterator_functor{}, f, ::std::make_index_sequence<sizeof...(T)>(), in, rest...);
}

} // namespace __internal
} // namespace dpl
} // namespace oneapi

#endif // _ONEDPL_ITERATOR_IMPL_H