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#pragma once

#include <cub/config.cuh>

#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 <cub/thread/thread_sort.cuh>
#include <cub/util_math.cuh>
#include <cub/util_namespace.cuh>
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>


CUB_NAMESPACE_BEGIN


// Additional details of the Merge-Path Algorithm can be found in:
// S. Odeh, O. Green, Z. Mwassi, O. Shmueli, Y. Birk, " Merge Path - Parallel
// Merging Made Simple", Multithreaded Architectures and Applications (MTAAP)
// Workshop, IEEE 26th International Parallel & Distributed Processing
// Symposium (IPDPS), 2012
template <typename KeyT,
          typename KeyIteratorT,
          typename OffsetT,
          typename BinaryPred>
__device__ __forceinline__ OffsetT MergePath(KeyIteratorT keys1,
                                             KeyIteratorT keys2,
                                             OffsetT keys1_count,
                                             OffsetT keys2_count,
                                             OffsetT diag,
                                             BinaryPred binary_pred)
{
  OffsetT keys1_begin = diag < keys2_count ? 0 : diag - keys2_count;
  OffsetT keys1_end   = (cub::min)(diag, keys1_count);

  while (keys1_begin < keys1_end)
  {
    OffsetT mid = cub::MidPoint<OffsetT>(keys1_begin, keys1_end);
    KeyT key1   = keys1[mid];
    KeyT key2   = keys2[diag - 1 - mid];
    bool pred   = binary_pred(key2, key1);

    if (pred)
    {
      keys1_end = mid;
    }
    else
    {
      keys1_begin = mid + 1;
    }
  }
  return keys1_begin;
}

template <typename KeyT, typename CompareOp, int ITEMS_PER_THREAD>
__device__ __forceinline__ void SerialMerge(KeyT *keys_shared,
                                            int keys1_beg,
                                            int keys2_beg,
                                            int keys1_count,
                                            int keys2_count,
                                            KeyT (&output)[ITEMS_PER_THREAD],
                                            int (&indices)[ITEMS_PER_THREAD],
                                            CompareOp compare_op)
{
  int keys1_end = keys1_beg + keys1_count;
  int keys2_end = keys2_beg + keys2_count;

  KeyT key1 = keys_shared[keys1_beg];
  KeyT key2 = keys_shared[keys2_beg];

#pragma unroll
  for (int item = 0; item < ITEMS_PER_THREAD; ++item)
  {
    bool p = (keys2_beg < keys2_end) &&
             ((keys1_beg >= keys1_end)
              || compare_op(key2, key1));

    output[item]  = p ? key2 : key1;
    indices[item] = p ? keys2_beg++ : keys1_beg++;

    if (p)
    {
      key2 = keys_shared[keys2_beg];
    }
    else
    {
      key1 = keys_shared[keys1_beg];
    }
  }
}

/**
 * @brief Generalized merge sort algorithm
 *
 * This class is used to reduce code duplication. Warp and Block merge sort
 * differ only in how they compute thread index and how they synchronize
 * threads. Since synchronization might require access to custom data
 * (like member mask), CRTP is used.
 *
 * @par
 * The code snippet below illustrates the way this class can be used.
 * @par
 * @code
 * #include <cub/cub.cuh> // or equivalently <cub/block/block_merge_sort.cuh>
 *
 * constexpr int BLOCK_THREADS = 256;
 * constexpr int ITEMS_PER_THREAD = 9;
 *
 * class BlockMergeSort : public BlockMergeSortStrategy<int,
 *                                                      cub::NullType,
 *                                                      BLOCK_THREADS,
 *                                                      ITEMS_PER_THREAD,
 *                                                      BlockMergeSort>
 * {
 *   using BlockMergeSortStrategyT =
 *     BlockMergeSortStrategy<int,
 *                            cub::NullType,
 *                            BLOCK_THREADS,
 *                            ITEMS_PER_THREAD,
 *                            BlockMergeSort>;
 * public:
 *   __device__ __forceinline__ explicit BlockMergeSort(
 *     typename BlockMergeSortStrategyT::TempStorage &temp_storage)
 *       : BlockMergeSortStrategyT(temp_storage, threadIdx.x)
 *   {}
 *
 *   __device__ __forceinline__ void SyncImplementation() const
 *   {
 *     __syncthreads();
 *   }
 * };
 * @endcode
 *
 * @tparam KeyT
 *   KeyT type
 *
 * @tparam ValueT
 *   ValueT type. cub::NullType indicates a keys-only sort
 *
 * @tparam SynchronizationPolicy
 *   Provides a way of synchronizing threads. Should be derived from
 *   `BlockMergeSortStrategy`.
 */
template <typename KeyT,
          typename ValueT,
          int NUM_THREADS,
          int ITEMS_PER_THREAD,
          typename SynchronizationPolicy>
class BlockMergeSortStrategy
{
  static_assert(PowerOfTwo<NUM_THREADS>::VALUE,
                "NUM_THREADS must be a power of two");

private:

  static constexpr int ITEMS_PER_TILE = ITEMS_PER_THREAD * NUM_THREADS;

  // Whether or not there are values to be trucked along with keys
  static constexpr bool KEYS_ONLY = std::is_same<ValueT, NullType>::value;

  /// Shared memory type required by this thread block
  union _TempStorage
  {
    KeyT keys_shared[ITEMS_PER_TILE + 1];
    ValueT items_shared[ITEMS_PER_TILE + 1];
  }; // union TempStorage

  /// Shared storage reference
  _TempStorage &temp_storage;

  /// Internal storage allocator
  __device__ __forceinline__ _TempStorage& PrivateStorage()
  {
    __shared__ _TempStorage private_storage;
    return private_storage;
  }

  const unsigned int linear_tid;

public:
  /// \smemstorage{BlockMergeSort}
  struct TempStorage : Uninitialized<_TempStorage> {};

  BlockMergeSortStrategy() = delete;
  explicit __device__ __forceinline__
  BlockMergeSortStrategy(unsigned int linear_tid)
      : temp_storage(PrivateStorage())
      , linear_tid(linear_tid)
  {}

  __device__ __forceinline__ BlockMergeSortStrategy(TempStorage &temp_storage,
                                                    unsigned int linear_tid)
      : temp_storage(temp_storage.Alias())
      , linear_tid(linear_tid)
  {}

  __device__ __forceinline__ unsigned int get_linear_tid() const
  {
    return linear_tid;
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * Sort is not guaranteed to be stable. That is, suppose that i and j are
   * equivalent: neither one is less than the other. It is not guaranteed
   * that the relative order of these two elements will be preserved by sort.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void Sort(KeyT (&keys)[ITEMS_PER_THREAD],
                                       CompareOp compare_op)
  {
    ValueT items[ITEMS_PER_THREAD];
    Sort<CompareOp, false>(keys, items, compare_op, ITEMS_PER_TILE, keys[0]);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * - Sort is not guaranteed to be stable. That is, suppose that `i` and `j`
   *   are equivalent: neither one is less than the other. It is not guaranteed
   *   that the relative order of these two elements will be preserved by sort.
   * - The value of `oob_default` is assigned to all elements that are out of
   *   `valid_items` boundaries. It's expected that `oob_default` is ordered
   *   after any value in the `valid_items` boundaries. The algorithm always
   *   sorts a fixed amount of elements, which is equal to
   *   `ITEMS_PER_THREAD * BLOCK_THREADS`. If there is a value that is ordered
   *   after `oob_default`, it won't be placed within `valid_items` boundaries.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * @param[in] valid_items
   *   Number of valid items to sort
   *
   * @param[in] oob_default
   *   Default value to assign out-of-bound items
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void Sort(KeyT (&keys)[ITEMS_PER_THREAD],
                                       CompareOp compare_op,
                                       int valid_items,
                                       KeyT oob_default)
  {
    ValueT items[ITEMS_PER_THREAD];
    Sort<CompareOp, true>(keys, items, compare_op, valid_items, oob_default);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using a merge sorting method.
   *
   * @par
   * Sort is not guaranteed to be stable. That is, suppose that `i` and `j` are
   * equivalent: neither one is less than the other. It is not guaranteed
   * that the relative order of these two elements will be preserved by sort.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in,out] items
   *   Values to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void Sort(KeyT (&keys)[ITEMS_PER_THREAD],
                                       ValueT (&items)[ITEMS_PER_THREAD],
                                       CompareOp compare_op)
  {
    Sort<CompareOp, false>(keys, items, compare_op, ITEMS_PER_TILE, keys[0]);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * - Sort is not guaranteed to be stable. That is, suppose that `i` and `j`
   *   are equivalent: neither one is less than the other. It is not guaranteed
   *   that the relative order of these two elements will be preserved by sort.
   * - The value of `oob_default` is assigned to all elements that are out of
   *   `valid_items` boundaries. It's expected that `oob_default` is ordered
   *   after any value in the `valid_items` boundaries. The algorithm always
   *   sorts a fixed amount of elements, which is equal to
   *   `ITEMS_PER_THREAD * BLOCK_THREADS`. If there is a value that is ordered
   *   after `oob_default`, it won't be placed within `valid_items` boundaries.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @tparam IS_LAST_TILE
   *   True if `valid_items` isn't equal to the `ITEMS_PER_TILE`
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in,out] items
   *   Values to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * @param[in] valid_items
   *   Number of valid items to sort
   *
   * @param[in] oob_default
   *   Default value to assign out-of-bound items
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp,
            bool IS_LAST_TILE = true>
  __device__ __forceinline__ void Sort(KeyT (&keys)[ITEMS_PER_THREAD],
                                       ValueT (&items)[ITEMS_PER_THREAD],
                                       CompareOp compare_op,
                                       int valid_items,
                                       KeyT oob_default)
  {
    if (IS_LAST_TILE)
    {
      // if last tile, find valid max_key
      // and fill the remaining keys with it
      //
      KeyT max_key = oob_default;

      #pragma unroll
      for (int item = 1; item < ITEMS_PER_THREAD; ++item)
      {
        if (ITEMS_PER_THREAD * linear_tid + item < valid_items)
        {
          max_key = compare_op(max_key, keys[item]) ? keys[item] : max_key;
        }
        else
        {
          keys[item] = max_key;
        }
      }
    }

    // if first element of thread is in input range, stable sort items
    //
    if (!IS_LAST_TILE || ITEMS_PER_THREAD * linear_tid < valid_items)
    {
      StableOddEvenSort(keys, items, compare_op);
    }

    // each thread has sorted keys
    // merge sort keys in shared memory
    //
    for (int target_merged_threads_number = 2;
         target_merged_threads_number <= NUM_THREADS;
         target_merged_threads_number *= 2)
    {
      int merged_threads_number = target_merged_threads_number / 2;
      int mask = target_merged_threads_number - 1;

      Sync();

      // store keys in shmem
      //
      #pragma unroll
      for (int item = 0; item < ITEMS_PER_THREAD; ++item)
      {
        int idx                       = ITEMS_PER_THREAD * linear_tid + item;
        temp_storage.keys_shared[idx] = keys[item];
      }

      Sync();

      int indices[ITEMS_PER_THREAD];

      int first_thread_idx_in_thread_group_being_merged = ~mask & linear_tid;
      int start = ITEMS_PER_THREAD * first_thread_idx_in_thread_group_being_merged;
      int size  = ITEMS_PER_THREAD * merged_threads_number;

      int thread_idx_in_thread_group_being_merged = mask & linear_tid;

      int diag =
        (cub::min)(valid_items,
                   ITEMS_PER_THREAD * thread_idx_in_thread_group_being_merged);

      int keys1_beg = (cub::min)(valid_items, start);
      int keys1_end = (cub::min)(valid_items, keys1_beg + size);
      int keys2_beg = keys1_end;
      int keys2_end = (cub::min)(valid_items, keys2_beg + size);

      int keys1_count = keys1_end - keys1_beg;
      int keys2_count = keys2_end - keys2_beg;

      int partition_diag = MergePath<KeyT>(&temp_storage.keys_shared[keys1_beg],
                                           &temp_storage.keys_shared[keys2_beg],
                                           keys1_count,
                                           keys2_count,
                                           diag,
                                           compare_op);

      int keys1_beg_loc   = keys1_beg + partition_diag;
      int keys1_end_loc   = keys1_end;
      int keys2_beg_loc   = keys2_beg + diag - partition_diag;
      int keys2_end_loc   = keys2_end;
      int keys1_count_loc = keys1_end_loc - keys1_beg_loc;
      int keys2_count_loc = keys2_end_loc - keys2_beg_loc;
      SerialMerge(&temp_storage.keys_shared[0],
                  keys1_beg_loc,
                  keys2_beg_loc,
                  keys1_count_loc,
                  keys2_count_loc,
                  keys,
                  indices,
                  compare_op);

      if (!KEYS_ONLY)
      {
        Sync();

        // store keys in shmem
        //
        #pragma unroll
        for (int item = 0; item < ITEMS_PER_THREAD; ++item)
        {
          int idx = ITEMS_PER_THREAD * linear_tid + item;
          temp_storage.items_shared[idx] = items[item];
        }

        Sync();

        // gather items from shmem
        //
        #pragma unroll
        for (int item = 0; item < ITEMS_PER_THREAD; ++item)
        {
          items[item] = temp_storage.items_shared[indices[item]];
        }
      }
    }
  } // func block_merge_sort

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * StableSort is stable: it preserves the relative ordering of equivalent
   * elements. That is, if `x` and `y` are elements such that `x` precedes `y`,
   * and if the two elements are equivalent (neither `x < y` nor `y < x`) then
   * a postcondition of StableSort is that `x` still precedes `y`.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void StableSort(KeyT (&keys)[ITEMS_PER_THREAD],
                                             CompareOp compare_op)
  {
    Sort(keys, compare_op);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * StableSort is stable: it preserves the relative ordering of equivalent
   * elements. That is, if `x` and `y` are elements such that `x` precedes `y`,
   * and if the two elements are equivalent (neither `x < y` nor `y < x`) then
   * a postcondition of StableSort is that `x` still precedes `y`.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in,out] items
   *   Values to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void StableSort(KeyT (&keys)[ITEMS_PER_THREAD],
                                             ValueT (&items)[ITEMS_PER_THREAD],
                                             CompareOp compare_op)
  {
    Sort(keys, items, compare_op);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * - StableSort is stable: it preserves the relative ordering of equivalent
   *   elements. That is, if `x` and `y` are elements such that `x` precedes
   *   `y`, and if the two elements are equivalent (neither `x < y` nor `y < x`)
   *   then a postcondition of StableSort is that `x` still precedes `y`.
   * - The value of `oob_default` is assigned to all elements that are out of
   *   `valid_items` boundaries. It's expected that `oob_default` is ordered
   *   after any value in the `valid_items` boundaries. The algorithm always
   *   sorts a fixed amount of elements, which is equal to
   *   `ITEMS_PER_THREAD * BLOCK_THREADS`.
   *   If there is a value that is ordered after `oob_default`, it won't be
   *   placed within `valid_items` boundaries.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * @param[in] valid_items
   *   Number of valid items to sort
   *
   * @param[in] oob_default
   *   Default value to assign out-of-bound items
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp>
  __device__ __forceinline__ void StableSort(KeyT (&keys)[ITEMS_PER_THREAD],
                                             CompareOp compare_op,
                                             int valid_items,
                                             KeyT oob_default)
  {
    Sort(keys, compare_op, valid_items, oob_default);
  }

  /**
   * @brief Sorts items partitioned across a CUDA thread block using
   *        a merge sorting method.
   *
   * @par
   * - StableSort is stable: it preserves the relative ordering of equivalent
   *   elements. That is, if `x` and `y` are elements such that `x` precedes
   *   `y`, and if the two elements are equivalent (neither `x < y` nor `y < x`)
   *   then a postcondition of StableSort is that `x` still precedes `y`.
   * - The value of `oob_default` is assigned to all elements that are out of
   *   `valid_items` boundaries. It's expected that `oob_default` is ordered
   *   after any value in the `valid_items` boundaries. The algorithm always
   *   sorts a fixed amount of elements, which is equal to
   *   `ITEMS_PER_THREAD * BLOCK_THREADS`. If there is a value that is ordered
   *   after `oob_default`, it won't be placed within `valid_items` boundaries.
   *
   * @tparam CompareOp
   *   functor type having member `bool operator()(KeyT lhs, KeyT rhs)`.
   *   `CompareOp` is a model of [Strict Weak Ordering].
   *
   * @tparam IS_LAST_TILE
   *   True if `valid_items` isn't equal to the `ITEMS_PER_TILE`
   *
   * @param[in,out] keys
   *   Keys to sort
   *
   * @param[in,out] items
   *   Values to sort
   *
   * @param[in] compare_op
   *   Comparison function object which returns true if the first argument is
   *   ordered before the second
   *
   * @param[in] valid_items
   *   Number of valid items to sort
   *
   * @param[in] oob_default
   *   Default value to assign out-of-bound items
   *
   * [Strict Weak Ordering]: https://en.cppreference.com/w/cpp/concepts/strict_weak_order
   */
  template <typename CompareOp,
            bool IS_LAST_TILE = true>
  __device__ __forceinline__ void StableSort(KeyT (&keys)[ITEMS_PER_THREAD],
                                             ValueT (&items)[ITEMS_PER_THREAD],
                                             CompareOp compare_op,
                                             int valid_items,
                                             KeyT oob_default)
  {
    Sort<CompareOp, IS_LAST_TILE>(keys,
                                  items,
                                  compare_op,
                                  valid_items,
                                  oob_default);
  }

private:
  __device__ __forceinline__ void Sync() const
  {
    static_cast<const SynchronizationPolicy*>(this)->SyncImplementation();
  }
};


/**
 * @brief The BlockMergeSort class provides methods for sorting items
 *        partitioned across a CUDA thread block using a merge sorting method.
 * @ingroup BlockModule
 *
 * @tparam KeyT
 *   KeyT type
 *
 * @tparam BLOCK_DIM_X
 *   The thread block length in threads along the X dimension
 *
 * @tparam ITEMS_PER_THREAD
 *   The number of items per thread
 *
 * @tparam ValueT
 *   **[optional]** ValueT type (default: `cub::NullType`, which indicates
 *   a keys-only sort)
 *
 * @tparam BLOCK_DIM_Y
 *   **[optional]** The thread block length in threads along the Y dimension
 *   (default: 1)
 *
 * @tparam BLOCK_DIM_Z
 *   **[optional]** The thread block length in threads along the Z dimension
 *   (default: 1)
 *
 * @par Overview
 *   BlockMergeSort arranges items into ascending order using a comparison
 *   functor with less-than semantics. Merge sort can handle arbitrary types
 *   and comparison functors, but is slower than BlockRadixSort when sorting
 *   arithmetic types into ascending/descending order.
 *
 * @par A Simple Example
 * @blockcollective{BlockMergeSort}
 * @par
 * The code snippet below illustrates a sort of 512 integer keys that are
 * partitioned across 128 threads * where each thread owns 4 consecutive items.
 * @par
 * @code
 * #include <cub/cub.cuh>  // or equivalently <cub/block/block_merge_sort.cuh>
 *
 * struct CustomLess
 * {
 *   template <typename DataType>
 *   __device__ bool operator()(const DataType &lhs, const DataType &rhs)
 *   {
 *     return lhs < rhs;
 *   }
 * };
 *
 * __global__ void ExampleKernel(...)
 * {
 *     // Specialize BlockMergeSort for a 1D block of 128 threads owning 4 integer items each
 *     typedef cub::BlockMergeSort<int, 128, 4> BlockMergeSort;
 *
 *     // Allocate shared memory for BlockMergeSort
 *     __shared__ typename BlockMergeSort::TempStorage temp_storage_shuffle;
 *
 *     // Obtain a segment of consecutive items that are blocked across threads
 *     int thread_keys[4];
 *     ...
 *
 *     BlockMergeSort(temp_storage_shuffle).Sort(thread_keys, CustomLess());
 *     ...
 * }
 * @endcode
 * @par
 * Suppose the set of input `thread_keys` across the block of threads is
 * `{ [0,511,1,510], [2,509,3,508], [4,507,5,506], ..., [254,257,255,256] }`.
 * The corresponding output `thread_keys` in those threads will be
 * `{ [0,1,2,3], [4,5,6,7], [8,9,10,11], ..., [508,509,510,511] }`.
 *
 * @par Re-using dynamically allocating shared memory
 * The following example under the examples/block folder illustrates usage of
 * dynamically shared memory with BlockReduce and how to re-purpose
 * the same memory region:
 * <a href="../../examples/block/example_block_reduce_dyn_smem.cu">example_block_reduce_dyn_smem.cu</a>
 *
 * This example can be easily adapted to the storage required by BlockMergeSort.
 */
template <typename KeyT,
          int BLOCK_DIM_X,
          int ITEMS_PER_THREAD,
          typename ValueT = NullType,
          int BLOCK_DIM_Y = 1,
          int BLOCK_DIM_Z = 1>
class BlockMergeSort
    : public BlockMergeSortStrategy<KeyT,
                                    ValueT,
                                    BLOCK_DIM_X * BLOCK_DIM_Y * BLOCK_DIM_Z,
                                    ITEMS_PER_THREAD,
                                    BlockMergeSort<KeyT,
                                                   BLOCK_DIM_X,
                                                   ITEMS_PER_THREAD,
                                                   ValueT,
                                                   BLOCK_DIM_Y,
                                                   BLOCK_DIM_Z>>
{
private:
  // The thread block size in threads
  static constexpr int BLOCK_THREADS = BLOCK_DIM_X * BLOCK_DIM_Y * BLOCK_DIM_Z;
  static constexpr int ITEMS_PER_TILE = ITEMS_PER_THREAD * BLOCK_THREADS;

  using BlockMergeSortStrategyT =
    BlockMergeSortStrategy<KeyT,
                           ValueT,
                           BLOCK_THREADS,
                           ITEMS_PER_THREAD,
                           BlockMergeSort>;

public:
  __device__ __forceinline__ BlockMergeSort()
      : BlockMergeSortStrategyT(
          RowMajorTid(BLOCK_DIM_X, BLOCK_DIM_Y, BLOCK_DIM_Z))
  {}

  __device__ __forceinline__ explicit BlockMergeSort(
    typename BlockMergeSortStrategyT::TempStorage &temp_storage)
      : BlockMergeSortStrategyT(
          temp_storage,
          RowMajorTid(BLOCK_DIM_X, BLOCK_DIM_Y, BLOCK_DIM_Z))
  {}

private:
  __device__ __forceinline__ void SyncImplementation() const
  {
    CTA_SYNC();
  }

  friend BlockMergeSortStrategyT;
};


CUB_NAMESPACE_END
