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

#ifndef KOKKOS_STD_ALGORITHMS_UNIQUE_COPY_IMPL_HPP
#define KOKKOS_STD_ALGORITHMS_UNIQUE_COPY_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 "Kokkos_MustUseKokkosSingleInTeam.hpp"
#include "Kokkos_CopyCopyN.hpp"
#include <std_algorithms/Kokkos_Distance.hpp>
#include <string>

namespace Kokkos {
namespace Experimental {
namespace Impl {

template <class InputIt, class OutputIt, class BinaryPredicateType>
struct StdUniqueCopyFunctor {
  using index_type = typename InputIt::difference_type;
  InputIt m_first_from;
  InputIt m_last_from;
  OutputIt m_first_dest;
  BinaryPredicateType m_pred;

  KOKKOS_FUNCTION
  StdUniqueCopyFunctor(InputIt first_from, InputIt last_from,
                       OutputIt first_dest, BinaryPredicateType pred)
      : m_first_from(std::move(first_from)),
        m_last_from(std::move(last_from)),
        m_first_dest(std::move(first_dest)),
        m_pred(std::move(pred)) {}

  KOKKOS_FUNCTION
  void operator()(const index_type i, std::size_t& update,
                  const bool final_pass) const {
    const auto& val_i   = m_first_from[i];
    const auto& val_ip1 = m_first_from[i + 1];

    if (final_pass) {
      if (!m_pred(val_i, val_ip1)) {
        m_first_dest[update] = val_i;
      }
    }

    if (!m_pred(val_i, val_ip1)) {
      update += 1;
    }
  }
};

template <class ExecutionSpace, class InputIterator, class OutputIterator,
          class PredicateType>
OutputIterator unique_copy_exespace_impl(
    const std::string& label, const ExecutionSpace& ex, InputIterator first,
    InputIterator last, OutputIterator d_first, PredicateType pred) {
  // checks
  Impl::static_assert_random_access_and_accessible(ex, first, d_first);
  Impl::static_assert_iterators_have_matching_difference_type(first, d_first);
  Impl::expect_valid_range(first, last);

  // branch for trivial vs non trivial case
  const auto num_elements = Kokkos::Experimental::distance(first, last);
  if (num_elements == 0) {
    return d_first;
  } else if (num_elements == 1) {
    return Impl::copy_exespace_impl("Kokkos::copy_from_unique_copy", ex, first,
                                    last, d_first);
  } else {
    // note here that we run scan for num_elements - 1
    // because of the way we implement this, the last element is always needed.
    // We avoid performing checks inside functor that we are within limits
    // and run a "safe" scan and then copy the last element.
    const auto scan_size = num_elements - 1;
    std::size_t count    = 0;
    ::Kokkos::parallel_scan(
        label, RangePolicy<ExecutionSpace>(ex, 0, scan_size),
        // use CTAD
        StdUniqueCopyFunctor(first, last, d_first, pred), count);

    return Impl::copy_exespace_impl("Kokkos::copy_from_unique_copy", ex,
                                    first + scan_size, last, d_first + count);
  }
}

template <class ExecutionSpace, class InputIterator, class OutputIterator>
OutputIterator unique_copy_exespace_impl(const std::string& label,
                                         const ExecutionSpace& ex,
                                         InputIterator first,
                                         InputIterator last,
                                         OutputIterator d_first) {
  // checks
  Impl::static_assert_random_access_and_accessible(ex, first, d_first);
  Impl::static_assert_iterators_have_matching_difference_type(first, d_first);
  Impl::expect_valid_range(first, last);

  // aliases
  using value_type1   = typename InputIterator::value_type;
  using value_type2   = typename OutputIterator::value_type;
  using binary_pred_t = StdAlgoEqualBinaryPredicate<value_type1, value_type2>;

  // run
  return unique_copy_exespace_impl(label, ex, first, last, d_first,
                                   binary_pred_t());
}

//
// team level
//

template <class TeamHandleType, class InputIterator, class OutputIterator,
          class PredicateType>
KOKKOS_FUNCTION OutputIterator unique_copy_team_impl(
    const TeamHandleType& teamHandle, InputIterator first, InputIterator last,
    OutputIterator d_first, PredicateType pred) {
  // checks
  Impl::static_assert_random_access_and_accessible(teamHandle, first, d_first);
  Impl::static_assert_iterators_have_matching_difference_type(first, d_first);
  Impl::expect_valid_range(first, last);

  // branch for trivial vs non trivial case
  const std::size_t num_elements = Kokkos::Experimental::distance(first, last);
  if (num_elements == 0) {
    return d_first;
  } else if (num_elements == 1) {
    d_first[0] = first[0];
    return d_first + 1;
  }

  else {
    if constexpr (stdalgo_must_use_kokkos_single_for_team_scan_v<
                      typename TeamHandleType::execution_space>) {
      std::size_t count = 0;
      Kokkos::single(
          Kokkos::PerTeam(teamHandle),
          [=](std::size_t& lcount) {
            lcount = 0;
            for (std::size_t i = 0; i < num_elements - 1; ++i) {
              const auto& val_i   = first[i];
              const auto& val_ip1 = first[i + 1];
              if (!pred(val_i, val_ip1)) {
                d_first[lcount++] = val_i;
              }
            }
            // we need to copy the last element always
            d_first[lcount++] = first[num_elements - 1];
          },
          count);
      // no barrier needed since single above broadcasts to all members

      return d_first + count;
    } else {
      const auto scan_size = num_elements - 1;
      std::size_t count    = 0;
      ::Kokkos::parallel_scan(TeamThreadRange(teamHandle, 0, scan_size),
                              StdUniqueCopyFunctor(first, last, d_first, pred),
                              count);
      // no barrier needed since reducing into count

      return Impl::copy_team_impl(teamHandle, first + scan_size, last,
                                  d_first + count);
    }
  }
}

template <class TeamHandleType, class InputIterator, class OutputIterator>
KOKKOS_FUNCTION OutputIterator
unique_copy_team_impl(const TeamHandleType& teamHandle, InputIterator first,
                      InputIterator last, OutputIterator d_first) {
  // checks
  Impl::static_assert_random_access_and_accessible(teamHandle, first, d_first);
  Impl::static_assert_iterators_have_matching_difference_type(first, d_first);
  Impl::expect_valid_range(first, last);

  // aliases
  using value_type1 = typename InputIterator::value_type;
  using value_type2 = typename OutputIterator::value_type;

  // default binary predicate uses ==
  using binary_pred_t = StdAlgoEqualBinaryPredicate<value_type1, value_type2>;

  // run
  return unique_copy_team_impl(teamHandle, first, last, d_first,
                               binary_pred_t());
}

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

#endif
