/*
 *  Copyright 2008-2021 NVIDIA Corporation
 *
 *  Licensed under the Apache License, Version 2.0 (the "License");
 *  you may not use this file except in compliance with the License.
 *  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 *  Unless required by applicable law or agreed to in writing, software
 *  distributed under the License is distributed on an "AS IS" BASIS,
 *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 *  See the License for the specific language governing permissions and
 *  limitations under the License.
 */

#pragma once

#include <thrust/detail/config.h>

#if defined(_CCCL_IMPLICIT_SYSTEM_HEADER_GCC)
#  pragma GCC system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_CLANG)
#  pragma clang system_header
#elif defined(_CCCL_IMPLICIT_SYSTEM_HEADER_MSVC)
#  pragma system_header
#endif // no system header

#include <thrust/detail/temporary_array.h>
#include <thrust/iterator/iterator_traits.h>
#include <thrust/merge.h>
#include <thrust/system/detail/sequential/execution_policy.h>
#include <thrust/system/detail/sequential/insertion_sort.h>

#include <cuda/std/__algorithm/min.h>

#include <nv/target>

THRUST_NAMESPACE_BEGIN
namespace system::detail::sequential
{
namespace stable_merge_sort_detail
{
template <typename DerivedPolicy, typename RandomAccessIterator, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void inplace_merge(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator first,
  RandomAccessIterator middle,
  RandomAccessIterator last,
  StrictWeakOrdering comp)
{
  using value_type = thrust::detail::it_value_t<RandomAccessIterator>;

  thrust::detail::temporary_array<value_type, DerivedPolicy> a(exec, first, middle);
  thrust::detail::temporary_array<value_type, DerivedPolicy> b(exec, middle, last);

  thrust::merge(exec, a.begin(), a.end(), b.begin(), b.end(), first, comp);
}

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename RandomAccessIterator2,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void inplace_merge_by_key(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 first1,
  RandomAccessIterator1 middle1,
  RandomAccessIterator1 last1,
  RandomAccessIterator2 first2,
  StrictWeakOrdering comp)
{
  using value_type1 = thrust::detail::it_value_t<RandomAccessIterator1>;
  using value_type2 = thrust::detail::it_value_t<RandomAccessIterator2>;

  RandomAccessIterator2 middle2 = first2 + (middle1 - first1);
  RandomAccessIterator2 last2   = first2 + (last1 - first1);

  thrust::detail::temporary_array<value_type1, DerivedPolicy> lhs1(exec, first1, middle1);
  thrust::detail::temporary_array<value_type1, DerivedPolicy> rhs1(exec, middle1, last1);
  thrust::detail::temporary_array<value_type2, DerivedPolicy> lhs2(exec, first2, middle2);
  thrust::detail::temporary_array<value_type2, DerivedPolicy> rhs2(exec, middle2, last2);

  thrust::merge_by_key(
    exec, lhs1.begin(), lhs1.end(), rhs1.begin(), rhs1.end(), lhs2.begin(), rhs2.begin(), first1, first2, comp);
}

template <typename RandomAccessIterator, typename Size, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void
insertion_sort_each(RandomAccessIterator first, RandomAccessIterator last, Size partition_size, StrictWeakOrdering comp)
{
  if (partition_size > 1)
  {
    for (; first < last; first += partition_size)
    {
      RandomAccessIterator partition_last = (::cuda::std::min) (last, first + partition_size);

      thrust::system::detail::sequential::insertion_sort(first, partition_last, comp);
    } // end for
  } // end if
} // end insertion_sort_each()

template <typename RandomAccessIterator1, typename RandomAccessIterator2, typename Size, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void insertion_sort_each_by_key(
  RandomAccessIterator1 keys_first,
  RandomAccessIterator1 keys_last,
  RandomAccessIterator2 values_first,
  Size partition_size,
  StrictWeakOrdering comp)
{
  if (partition_size > 1)
  {
    for (; keys_first < keys_last; keys_first += partition_size, values_first += partition_size)
    {
      RandomAccessIterator1 keys_partition_last = (::cuda::std::min) (keys_last, keys_first + partition_size);

      thrust::system::detail::sequential::insertion_sort_by_key(keys_first, keys_partition_last, values_first, comp);
    } // end for
  } // end if
} // end insertion_sort_each()

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename Size,
          typename RandomAccessIterator2,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void merge_adjacent_partitions(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 first,
  RandomAccessIterator1 last,
  Size partition_size,
  RandomAccessIterator2 result,
  StrictWeakOrdering comp)
{
  for (; first < last; first += 2 * partition_size, result += 2 * partition_size)
  {
    RandomAccessIterator1 interval_middle = (::cuda::std::min) (last, first + partition_size);
    RandomAccessIterator1 interval_last   = (::cuda::std::min) (last, interval_middle + partition_size);

    thrust::merge(exec, first, interval_middle, interval_middle, interval_last, result, comp);
  } // end for
} // end merge_adjacent_partitions()

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename RandomAccessIterator2,
          typename Size,
          typename RandomAccessIterator3,
          typename RandomAccessIterator4,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void merge_adjacent_partitions_by_key(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 keys_first,
  RandomAccessIterator1 keys_last,
  RandomAccessIterator2 values_first,
  Size partition_size,
  RandomAccessIterator3 keys_result,
  RandomAccessIterator4 values_result,
  StrictWeakOrdering comp)
{
  Size stride = 2 * partition_size;

  for (; keys_first < keys_last;
       keys_first += stride, values_first += stride, keys_result += stride, values_result += stride)
  {
    RandomAccessIterator1 keys_interval_middle = (::cuda::std::min) (keys_last, keys_first + partition_size);
    RandomAccessIterator1 keys_interval_last   = (::cuda::std::min) (keys_last, keys_interval_middle + partition_size);

    RandomAccessIterator2 values_first2 = values_first + (keys_interval_middle - keys_first);

    thrust::merge_by_key(
      exec,
      keys_first,
      keys_interval_middle,
      keys_interval_middle,
      keys_interval_last,
      values_first,
      values_first2,
      keys_result,
      values_result,
      comp);
  } // end for
} // end merge_adjacent_partitions()

template <typename DerivedPolicy, typename RandomAccessIterator, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void iterative_stable_merge_sort(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator first,
  RandomAccessIterator last,
  StrictWeakOrdering comp)
{
  using value_type      = thrust::detail::it_value_t<RandomAccessIterator>;
  using difference_type = thrust::detail::it_difference_t<RandomAccessIterator>;

  difference_type n = last - first;

  thrust::detail::temporary_array<value_type, DerivedPolicy> temp(exec, n);

  // insertion sort each 32 element partition
  difference_type partition_size = 32;
  insertion_sort_each(first, last, partition_size, comp);

  // ping indicates whether or not the latest data is in the source range [first, last)
  bool ping = true;

  // merge adjacent partitions until the partition size covers the entire range
  for (; partition_size < n; partition_size *= 2, ping = !ping)
  {
    if (ping)
    {
      merge_adjacent_partitions(exec, first, last, partition_size, temp.begin(), comp);
    } // end if
    else
    {
      merge_adjacent_partitions(exec, temp.begin(), temp.end(), partition_size, first, comp);
    } // end else
  } // end for m

  if (!ping)
  {
    thrust::copy(exec, temp.begin(), temp.end(), first);
  } // end if
} // end iterative_stable_merge_sort()

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename RandomAccessIterator2,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void iterative_stable_merge_sort_by_key(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 keys_first,
  RandomAccessIterator1 keys_last,
  RandomAccessIterator2 values_first,
  StrictWeakOrdering comp)
{
  using value_type1     = thrust::detail::it_value_t<RandomAccessIterator1>;
  using value_type2     = thrust::detail::it_value_t<RandomAccessIterator2>;
  using difference_type = thrust::detail::it_difference_t<RandomAccessIterator1>;

  difference_type n = keys_last - keys_first;

  thrust::detail::temporary_array<value_type1, DerivedPolicy> keys_temp(exec, n);
  thrust::detail::temporary_array<value_type2, DerivedPolicy> values_temp(exec, n);

  // insertion sort each 32 element partition
  difference_type partition_size = 32;
  insertion_sort_each_by_key(keys_first, keys_last, values_first, partition_size, comp);

  // ping indicates whether or not the latest data is in the source range [first, last)
  bool ping = true;

  // merge adjacent partitions until the partition size covers the entire range
  for (; partition_size < n; partition_size *= 2, ping = !ping)
  {
    if (ping)
    {
      merge_adjacent_partitions_by_key(
        exec, keys_first, keys_last, values_first, partition_size, keys_temp.begin(), values_temp.begin(), comp);
    } // end if
    else
    {
      merge_adjacent_partitions_by_key(
        exec, keys_temp.begin(), keys_temp.end(), values_temp.begin(), partition_size, keys_first, values_first, comp);
    } // end else
  } // end for m

  if (!ping)
  {
    thrust::copy(exec, keys_temp.begin(), keys_temp.end(), keys_first);
    thrust::copy(exec, values_temp.begin(), values_temp.end(), values_first);
  } // end if
} // end iterative_stable_merge_sort()

template <typename DerivedPolicy, typename RandomAccessIterator, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void recursive_stable_merge_sort(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator first,
  RandomAccessIterator last,
  StrictWeakOrdering comp)
{
  if (last - first <= 32)
  {
    thrust::system::detail::sequential::insertion_sort(first, last, comp);
  } // end if
  else
  {
    RandomAccessIterator middle = first + (last - first) / 2;

    stable_merge_sort_detail::recursive_stable_merge_sort(exec, first, middle, comp);
    stable_merge_sort_detail::recursive_stable_merge_sort(exec, middle, last, comp);
    stable_merge_sort_detail::inplace_merge(exec, first, middle, last, comp);
  } // end else
} // end recursive_stable_merge_sort()

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename RandomAccessIterator2,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void recursive_stable_merge_sort_by_key(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 first1,
  RandomAccessIterator1 last1,
  RandomAccessIterator2 first2,
  StrictWeakOrdering comp)
{
  if (last1 - first1 <= 32)
  {
    thrust::system::detail::sequential::insertion_sort_by_key(first1, last1, first2, comp);
  } // end if
  else
  {
    RandomAccessIterator1 middle1 = first1 + (last1 - first1) / 2;
    RandomAccessIterator2 middle2 = first2 + (last1 - first1) / 2;

    stable_merge_sort_detail::recursive_stable_merge_sort_by_key(exec, first1, middle1, first2, comp);
    stable_merge_sort_detail::recursive_stable_merge_sort_by_key(exec, middle1, last1, middle2, comp);
    stable_merge_sort_detail::inplace_merge_by_key(exec, first1, middle1, last1, first2, comp);
  } // end else
} // end recursive_stable_merge_sort_by_key()
} // end namespace stable_merge_sort_detail

template <typename DerivedPolicy, typename RandomAccessIterator, typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void stable_merge_sort(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator first,
  RandomAccessIterator last,
  StrictWeakOrdering comp)
{
  NV_IF_TARGET(NV_IS_DEVICE,
               (
                 // avoid recursion in CUDA threads
                 stable_merge_sort_detail::iterative_stable_merge_sort(exec, first, last, comp);),
               (stable_merge_sort_detail::recursive_stable_merge_sort(exec, first, last, comp);));
}

template <typename DerivedPolicy,
          typename RandomAccessIterator1,
          typename RandomAccessIterator2,
          typename StrictWeakOrdering>
_CCCL_HOST_DEVICE void stable_merge_sort_by_key(
  sequential::execution_policy<DerivedPolicy>& exec,
  RandomAccessIterator1 first1,
  RandomAccessIterator1 last1,
  RandomAccessIterator2 first2,
  StrictWeakOrdering comp)
{
  NV_IF_TARGET(NV_IS_DEVICE,
               (
                 // avoid recursion in CUDA threads
                 stable_merge_sort_detail::iterative_stable_merge_sort_by_key(exec, first1, last1, first2, comp);),
               (stable_merge_sort_detail::recursive_stable_merge_sort_by_key(exec, first1, last1, first2, comp);));
}
} // namespace system::detail::sequential
THRUST_NAMESPACE_END
