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

#ifndef KOKKOS_STD_ALGORITHMS_COPY_IF_IMPL_HPP
#define KOKKOS_STD_ALGORITHMS_COPY_IF_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 <string>

namespace Kokkos {
namespace Experimental {
namespace Impl {

template <class FirstFrom, class FirstDest, class PredType>
struct StdCopyIfFunctor {
  using index_type = typename FirstFrom::difference_type;

  FirstFrom m_first_from;
  FirstDest m_first_dest;
  PredType m_pred;

  KOKKOS_FUNCTION
  StdCopyIfFunctor(FirstFrom first_from, FirstDest first_dest, PredType pred)
      : m_first_from(std::move(first_from)),
        m_first_dest(std::move(first_dest)),
        m_pred(std::move(pred)) {}

  KOKKOS_FUNCTION
  void operator()(const index_type i, index_type& update,
                  const bool final_pass) const {
    const auto& myval = m_first_from[i];
    if (final_pass) {
      if (m_pred(myval)) {
        m_first_dest[update] = myval;
      }
    }

    if (m_pred(myval)) {
      update += 1;
    }
  }
};

template <class ExecutionSpace, class InputIterator, class OutputIterator,
          class PredicateType>
OutputIterator copy_if_exespace_impl(const std::string& label,
                                     const ExecutionSpace& ex,
                                     InputIterator first, InputIterator last,
                                     OutputIterator d_first,
                                     PredicateType pred) {
  /*
    To explain the impl, suppose that our data is:

    | 1 | 1 | 2 | 2 | 3 | -2 | 4 | 4 | 4 | 5 | 7 | -10 |

    and we want to copy only the even entries,
    We can use an exclusive scan where the "update"
    is incremented only for the elements that satisfy the predicate.
    This way, the update allows us to track where in the destination
    we need to copy the elements:

    In this case, counting only the even entries, the exlusive scan
    during the final pass would yield:

    | 0 | 0 | 0 | 1 | 2 | 2 | 3 | 4 | 5 | 6 | 6 | 6 |
              *   *       *   *   *   *           *

    which provides the indexing in the destination where
    each starred (*) element needs to be copied to since
    the starred elements are those that satisfy the predicate.
   */

  // 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);

  if (first == last) {
    return d_first;
  } else {
    // run
    const auto num_elements = Kokkos::Experimental::distance(first, last);

    typename InputIterator::difference_type count = 0;
    ::Kokkos::parallel_scan(label,
                            RangePolicy<ExecutionSpace>(ex, 0, num_elements),
                            // use CTAD
                            StdCopyIfFunctor(first, d_first, pred), count);

    // fence not needed because of the scan accumulating into count
    return d_first + count;
  }
}

template <class TeamHandleType, class InputIterator, class OutputIterator,
          class PredicateType>
KOKKOS_FUNCTION OutputIterator copy_if_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);

  if (first == last) {
    return d_first;
  }

  const std::size_t num_elements = Kokkos::Experimental::distance(first, last);
  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; ++i) {
            const auto& myval = first[i];
            if (pred(myval)) {
              d_first[lcount++] = myval;
            }
          }
        },
        count);
    // no barrier needed since single above broadcasts to all members
    return d_first + count;

  } else {
    typename InputIterator::difference_type count = 0;
    ::Kokkos::parallel_scan(TeamThreadRange(teamHandle, 0, num_elements),
                            StdCopyIfFunctor(first, d_first, pred), count);
    // no barrier needed because of the scan accumulating into count
    return d_first + count;
  }
}

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

#endif
