// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// SPDX-FileCopyrightText: Copyright Contributors to the Kokkos project

#ifndef KOKKOS_STD_ALGORITHMS_TRANSFORM_REDUCE_IMPL_HPP
#define KOKKOS_STD_ALGORITHMS_TRANSFORM_REDUCE_IMPL_HPP

#include <Kokkos_Macros.hpp>
#ifdef KOKKOS_ENABLE_EXPERIMENTAL_CXX20_MODULES
import kokkos.core;
#else
#include <Kokkos_Core.hpp>
#endif
#include "Kokkos_Constraints.hpp"
#include "Kokkos_HelperPredicates.hpp"
#include <string>

namespace Kokkos {
namespace Experimental {
namespace Impl {

template <class ValueType>
struct StdTranformReduceDefaultBinaryTransformFunctor {
  KOKKOS_FUNCTION
  constexpr ValueType operator()(const ValueType& a, const ValueType& b) const {
    return (a * b);
  }
};

template <class ValueType>
struct StdTranformReduceDefaultJoinFunctor {
  KOKKOS_FUNCTION
  constexpr ValueType operator()(const ValueType& a, const ValueType& b) const {
    return a + b;
  }
};

template <class IteratorType, class ReducerType, class TransformType>
struct StdTransformReduceSingleIntervalFunctor {
  using red_value_type = typename ReducerType::value_type;
  using index_type     = typename IteratorType::difference_type;

  const IteratorType m_first;
  const ReducerType m_reducer;
  const TransformType m_transform;

  KOKKOS_FUNCTION
  void operator()(const index_type i, red_value_type& red_value) const {
    auto tmp_wrapped_value = red_value_type{m_transform(m_first[i]), false};
    if (red_value.is_initial) {
      red_value = tmp_wrapped_value;
    } else {
      m_reducer.join(red_value, tmp_wrapped_value);
    }
  }

  KOKKOS_FUNCTION
  StdTransformReduceSingleIntervalFunctor(IteratorType first,
                                          ReducerType reducer,
                                          TransformType transform)
      : m_first(std::move(first)),
        m_reducer(std::move(reducer)),
        m_transform(std::move(transform)) {}
};

template <class IndexType, class IteratorType1, class IteratorType2,
          class ReducerType, class TransformType>
struct StdTransformReduceTwoIntervalsFunctor {
  using red_value_type = typename ReducerType::value_type;

  const IteratorType1 m_first1;
  const IteratorType2 m_first2;
  const ReducerType m_reducer;
  const TransformType m_transform;

  KOKKOS_FUNCTION
  void operator()(const IndexType i, red_value_type& red_value) const {
    auto tmp_wrapped_value =
        red_value_type{m_transform(m_first1[i], m_first2[i]), false};

    if (red_value.is_initial) {
      red_value = tmp_wrapped_value;
    } else {
      m_reducer.join(red_value, tmp_wrapped_value);
    }
  }

  KOKKOS_FUNCTION
  StdTransformReduceTwoIntervalsFunctor(IteratorType1 first1,
                                        IteratorType2 first2,
                                        ReducerType reducer,
                                        TransformType transform)
      : m_first1(std::move(first1)),
        m_first2(std::move(first2)),
        m_reducer(std::move(reducer)),
        m_transform(std::move(transform)) {}
};

//------------------------------
//
// impl functions
//
//------------------------------

//
// exespace impl
//

template <class ExecutionSpace, class IteratorType, class ValueType,
          class JoinerType, class UnaryTransformerType>
ValueType transform_reduce_custom_functors_exespace_impl(
    const std::string& label, const ExecutionSpace& ex, IteratorType first,
    IteratorType last, ValueType init_reduction_value, JoinerType joiner,
    UnaryTransformerType transformer) {
  // checks
  Impl::static_assert_random_access_and_accessible(ex, first);
  Impl::expect_valid_range(first, last);

  if (first == last) {
    // init is returned, unmodified
    return init_reduction_value;
  }

  // aliases
  using reducer_type =
      ReducerWithArbitraryJoinerNoNeutralElement<ValueType, JoinerType>;
  using functor_type =
      StdTransformReduceSingleIntervalFunctor<IteratorType, reducer_type,
                                              UnaryTransformerType>;
  using reduction_value_type = typename reducer_type::value_type;

  // run
  reduction_value_type result;
  reducer_type reducer(result, joiner);
  const auto num_elements = Kokkos::Experimental::distance(first, last);
  ::Kokkos::parallel_reduce(label,
                            RangePolicy<ExecutionSpace>(ex, 0, num_elements),
                            functor_type(first, reducer, transformer), reducer);

  // fence not needed since reducing into scalar

  // as per standard, transform is not applied to the init value
  // https://en.cppreference.com/w/cpp/algorithm/transform_reduce
  return joiner(result.val, init_reduction_value);
}

template <class ExecutionSpace, class IteratorType1, class IteratorType2,
          class ValueType, class JoinerType, class BinaryTransformerType>
ValueType transform_reduce_custom_functors_exespace_impl(
    const std::string& label, const ExecutionSpace& ex, IteratorType1 first1,
    IteratorType1 last1, IteratorType2 first2, ValueType init_reduction_value,
    JoinerType joiner, BinaryTransformerType transformer) {
  // checks
  Impl::static_assert_random_access_and_accessible(ex, first1, first2);
  Impl::static_assert_iterators_have_matching_difference_type(first1, first2);
  Impl::expect_valid_range(first1, last1);

  if (first1 == last1) {
    // init is returned, unmodified
    return init_reduction_value;
  }

  // aliases
  using index_type = typename IteratorType1::difference_type;
  using reducer_type =
      ReducerWithArbitraryJoinerNoNeutralElement<ValueType, JoinerType>;
  using functor_type =
      StdTransformReduceTwoIntervalsFunctor<index_type, IteratorType1,
                                            IteratorType2, reducer_type,
                                            BinaryTransformerType>;
  using reduction_value_type = typename reducer_type::value_type;

  // run
  reduction_value_type result;
  reducer_type reducer(result, joiner);

  const auto num_elements = Kokkos::Experimental::distance(first1, last1);
  ::Kokkos::parallel_reduce(
      label, RangePolicy<ExecutionSpace>(ex, 0, num_elements),
      functor_type(first1, first2, reducer, transformer), reducer);

  // fence not needed since reducing into scalar
  return joiner(result.val, init_reduction_value);
}

template <class ExecutionSpace, class IteratorType1, class IteratorType2,
          class ValueType>
ValueType transform_reduce_default_functors_exespace_impl(
    const std::string& label, const ExecutionSpace& ex, IteratorType1 first1,
    IteratorType1 last1, IteratorType2 first2, ValueType init_reduction_value) {
  // checks
  Impl::static_assert_random_access_and_accessible(ex, first1, first2);
  Impl::static_assert_iterators_have_matching_difference_type(first1, first2);
  Impl::expect_valid_range(first1, last1);

  // aliases
  using transformer_type =
      Impl::StdTranformReduceDefaultBinaryTransformFunctor<ValueType>;
  using joiner_type = Impl::StdTranformReduceDefaultJoinFunctor<ValueType>;

  return transform_reduce_custom_functors_exespace_impl(
      label, ex, first1, last1, first2, std::move(init_reduction_value),
      joiner_type(), transformer_type());
}

//
// team impl
//

template <class TeamHandleType, class IteratorType, class ValueType,
          class JoinerType, class UnaryTransformerType>
KOKKOS_FUNCTION ValueType transform_reduce_custom_functors_team_impl(
    const TeamHandleType& teamHandle, IteratorType first, IteratorType last,
    ValueType init_reduction_value, JoinerType joiner,
    UnaryTransformerType transformer) {
  // checks
  Impl::static_assert_random_access_and_accessible(teamHandle, first);
  Impl::expect_valid_range(first, last);

  if (first == last) {
    // init is returned, unmodified
    return init_reduction_value;
  }

  // aliases
  using reducer_type =
      ReducerWithArbitraryJoinerNoNeutralElement<ValueType, JoinerType>;
  using functor_type =
      StdTransformReduceSingleIntervalFunctor<IteratorType, reducer_type,
                                              UnaryTransformerType>;
  using reduction_value_type = typename reducer_type::value_type;

  // run
  reduction_value_type result;
  reducer_type reducer(result, joiner);
  const auto num_elements = Kokkos::Experimental::distance(first, last);
  ::Kokkos::parallel_reduce(TeamThreadRange(teamHandle, 0, num_elements),
                            functor_type(first, reducer, transformer), reducer);

  teamHandle.team_barrier();

  // as per standard, transform is not applied to the init value
  // https://en.cppreference.com/w/cpp/algorithm/transform_reduce
  return joiner(result.val, init_reduction_value);
}

template <class TeamHandleType, class IteratorType1, class IteratorType2,
          class ValueType, class JoinerType, class BinaryTransformerType>
KOKKOS_FUNCTION ValueType transform_reduce_custom_functors_team_impl(
    const TeamHandleType& teamHandle, IteratorType1 first1, IteratorType1 last1,
    IteratorType2 first2, ValueType init_reduction_value, JoinerType joiner,
    BinaryTransformerType transformer) {
  // checks
  Impl::static_assert_random_access_and_accessible(teamHandle, first1, first2);
  Impl::static_assert_iterators_have_matching_difference_type(first1, first2);
  Impl::expect_valid_range(first1, last1);

  if (first1 == last1) {
    // init is returned, unmodified
    return init_reduction_value;
  }

  // aliases
  using index_type = typename IteratorType1::difference_type;
  using reducer_type =
      ReducerWithArbitraryJoinerNoNeutralElement<ValueType, JoinerType>;
  using functor_type =
      StdTransformReduceTwoIntervalsFunctor<index_type, IteratorType1,
                                            IteratorType2, reducer_type,
                                            BinaryTransformerType>;
  using reduction_value_type = typename reducer_type::value_type;

  // run
  reduction_value_type result;
  reducer_type reducer(result, joiner);

  const auto num_elements = Kokkos::Experimental::distance(first1, last1);
  ::Kokkos::parallel_reduce(TeamThreadRange(teamHandle, 0, num_elements),
                            functor_type(first1, first2, reducer, transformer),
                            reducer);

  teamHandle.team_barrier();

  return joiner(result.val, init_reduction_value);
}

template <class TeamHandleType, class IteratorType1, class IteratorType2,
          class ValueType>
KOKKOS_FUNCTION ValueType transform_reduce_default_functors_team_impl(
    const TeamHandleType& teamHandle, IteratorType1 first1, IteratorType1 last1,
    IteratorType2 first2, ValueType init_reduction_value) {
  // checks
  Impl::static_assert_random_access_and_accessible(teamHandle, first1, first2);
  Impl::static_assert_iterators_have_matching_difference_type(first1, first2);
  Impl::expect_valid_range(first1, last1);

  // aliases
  using transformer_type =
      Impl::StdTranformReduceDefaultBinaryTransformFunctor<ValueType>;
  using joiner_type = Impl::StdTranformReduceDefaultJoinFunctor<ValueType>;

  return transform_reduce_custom_functors_team_impl(
      teamHandle, first1, last1, first2, std::move(init_reduction_value),
      joiner_type(), transformer_type());
}

}  // namespace Impl
}  // namespace Experimental
}  // namespace Kokkos

#endif
