// SPDX-FileCopyrightText: Copyright (c) 2011-2021, NVIDIA CORPORATION. All rights reserved.
// SPDX-License-Identifier: BSD-3

#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/block/block_scan.cuh>
#include <cub/thread/thread_search.cuh>
#include <cub/util_math.cuh>
#include <cub/util_namespace.cuh>
#include <cub/util_ptx.cuh>
#include <cub/util_type.cuh>

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

CUB_NAMESPACE_BEGIN

//! @rst
//! The BlockRunLengthDecode class supports decoding a run-length encoded array of items. That
//! is, given the two arrays ``run_value[N]`` and ``run_lengths[N]``, ``run_value[i]`` is repeated ``run_lengths[i]``
//! many times in the output array. Due to the nature of the run-length decoding algorithm
//! ("decompression"), the output size of the run-length decoded array is runtime-dependent and
//! potentially without any upper bound. To address this, BlockRunLengthDecode allows retrieving a
//! "window" from the run-length decoded array. The window's offset can be specified and
//! BLOCK_THREADS * DecodedItemsPerThread (i.e., referred to as window_size) decoded items from
//! the specified window will be returned.
//!
//! .. note::
//!
//!    Trailing runs of length 0 are supported (i.e., they may only appear at the end of the run_lengths array).
//!    A run of length zero may not be followed by a run length that is not zero.
//!
//! .. code-block:: c++
//!
//!    __global__ void ExampleKernel(...)
//!    {
//!      // Specialising BlockRunLengthDecode to run-length decode items of type uint64_t
//!      using RunItemT = uint64_t;
//!      // Type large enough to index into the run-length decoded array
//!      using RunLengthT = uint32_t;
//!
//!      // Specialising BlockRunLengthDecode for a 1D block of 128 threads
//!      constexpr int BlockDimX = 128;
//!      // Specialising BlockRunLengthDecode to have each thread contribute 2 run-length encoded runs
//!      constexpr int RunsPerThread = 2;
//!      // Specialising BlockRunLengthDecode to have each thread hold 4 run-length decoded items
//!      constexpr int DecodedItemsPerThread = 4;
//!
//!      // Specialize BlockRunLengthDecode for a 1D block of 128 threads owning 4 integer items each
//!      using BlockRunLengthDecodeT =
//!        cub::BlockRunLengthDecode<RunItemT, BlockDimX, RunsPerThread, DecodedItemsPerThread>;
//!
//!      // Allocate shared memory for BlockRunLengthDecode
//!      __shared__ typename BlockRunLengthDecodeT::TempStorage temp_storage;
//!
//!      // The run-length encoded items and how often they shall be repeated in the run-length decoded output
//!      RunItemT run_values[RunsPerThread];
//!      RunLengthT run_lengths[RunsPerThread];
//!      ...
//!
//!      // Initialize the BlockRunLengthDecode with the runs that we want to run-length decode
//!      uint32_t total_decoded_size = 0;
//!      BlockRunLengthDecodeT block_rld(temp_storage, run_values, run_lengths, total_decoded_size);
//!
//!      // Run-length decode ("decompress") the runs into a window buffer of limited size. This is repeated until all
//!      runs
//!      // have been decoded.
//!      uint32_t decoded_window_offset = 0U;
//!      while (decoded_window_offset < total_decoded_size)
//!      {
//!        RunLengthT relative_offsets[DecodedItemsPerThread];
//!        RunItemT decoded_items[DecodedItemsPerThread];
//!
//!        // The number of decoded items that are valid within this window (aka pass) of run-length decoding
//!        uint32_t num_valid_items = total_decoded_size - decoded_window_offset;
//!        block_rld.RunLengthDecode(decoded_items, relative_offsets, decoded_window_offset);
//!
//!        decoded_window_offset += BlockDimX * DecodedItemsPerThread;
//!
//!        ...
//!      }
//!    }
//!
//! Suppose the set of input ``run_values`` across the block of threads is
//! ``{ [0, 1], [2, 3], [4, 5], [6, 7], ..., [254, 255] }`` and
//! ``run_lengths`` is ``{ [1, 2], [3, 4], [5, 1], [2, 3], ..., [5, 1] }``.
//! The corresponding output ``decoded_items`` in those threads will be
//! ``{ [0, 1, 1, 2], [2, 2, 3, 3], [3, 3, 4, 4], [4, 4, 4, 5], ..., [169, 169, 170, 171] }``
//! and ``relative_offsets`` will be
//! ``{ [0, 0, 1, 0], [1, 2, 0, 1], [2, 3, 0, 1], [2, 3, 4, 0], ..., [3, 4, 0, 0] }`` during the
//! first iteration of the while loop.
//!
//! @endrst
//!
//! @tparam ItemT
//!   The data type of the items being run-length decoded
//!
//! @tparam BlockDimX
//!   The thread block length in threads along the X dimension
//!
//! @tparam RunsPerThread
//!   The number of consecutive runs that each thread contributes
//!
//! @tparam DecodedItemsPerThread
//!   The maximum number of decoded items that each thread holds
//!
//! @tparam DecodedOffsetT
//!   Type used to index into the block's decoded items (large enough to hold the sum over all the
//!   runs' lengths)
//!
//! @tparam BlockDimY
//!   The thread block length in threads along the Y dimension
//!
//! @tparam BlockDimZ
//!   The thread block length in threads along the Z dimension
template <typename ItemT,
          int BlockDimX,
          int RunsPerThread,
          int DecodedItemsPerThread,
          typename DecodedOffsetT = uint32_t,
          int BlockDimY           = 1,
          int BlockDimZ           = 1>
class BlockRunLengthDecode
{
  //---------------------------------------------------------------------
  // CONFIGS & TYPE ALIASES
  //---------------------------------------------------------------------

private:
  /// The thread block size in threads
  static constexpr int BLOCK_THREADS = BlockDimX * BlockDimY * BlockDimZ;

  /// The number of runs that the block decodes (out-of-bounds items may be padded with run lengths of '0')
  static constexpr int BLOCK_RUNS = BLOCK_THREADS * RunsPerThread;

  /// BlockScan used to determine the beginning of each run (i.e., prefix sum over the runs' length)
  using RunOffsetScanT = BlockScan<DecodedOffsetT, BlockDimX, BLOCK_SCAN_RAKING_MEMOIZE, BlockDimY, BlockDimZ>;

  /// Type used to index into the block's runs
  using RunOffsetT = uint32_t;

#ifndef _CCCL_DOXYGEN_INVOKED // Do not document
  /// Shared memory type required by this thread block
  union _TempStorage
  {
    typename RunOffsetScanT::TempStorage offset_scan;
    struct
    {
      ItemT run_values[BLOCK_RUNS];
      DecodedOffsetT run_offsets[BLOCK_RUNS];
    } runs;
  }; // union TempStorage
#endif // _CCCL_DOXYGEN_INVOKED

  /// Internal storage allocator (used when the user does not provide pre-allocated shared memory)
  _CCCL_DEVICE _CCCL_FORCEINLINE _TempStorage& PrivateStorage()
  {
    __shared__ _TempStorage private_storage;
    return private_storage;
  }

  /// Shared storage reference
  _TempStorage& temp_storage;

  /// Linear thread-id
  uint32_t linear_tid;

public:
  struct TempStorage : Uninitialized<_TempStorage>
  {};

  //---------------------------------------------------------------------
  // CONSTRUCTOR
  //---------------------------------------------------------------------

  //! @brief Constructor specialised for user-provided temporary storage, initializing using the runs' lengths.
  //!        The algorithm's temporary storage may not be repurposed between the constructor call and subsequent
  //!        `RunLengthDecode` calls.
  template <typename RunLengthT, typename TotalDecodedSizeT>
  _CCCL_DEVICE _CCCL_FORCEINLINE BlockRunLengthDecode(
    TempStorage& temp_storage,
    ItemT (&run_values)[RunsPerThread],
    RunLengthT (&run_lengths)[RunsPerThread],
    TotalDecodedSizeT& total_decoded_size)
      : temp_storage(temp_storage.Alias())
      , linear_tid(RowMajorTid(BlockDimX, BlockDimY, BlockDimZ))
  {
    InitWithRunLengths(run_values, run_lengths, total_decoded_size);
  }

  //! @brief Constructor specialised for user-provided temporary storage, initializing using the runs' offsets.
  //!        The algorithm's temporary storage may not be repurposed between the constructor call and subsequent
  //!        `RunLengthDecode` calls.
  template <typename UserRunOffsetT>
  _CCCL_DEVICE _CCCL_FORCEINLINE BlockRunLengthDecode(
    TempStorage& temp_storage, ItemT (&run_values)[RunsPerThread], UserRunOffsetT (&run_offsets)[RunsPerThread])
      : temp_storage(temp_storage.Alias())
      , linear_tid(RowMajorTid(BlockDimX, BlockDimY, BlockDimZ))
  {
    InitWithRunOffsets(run_values, run_offsets);
  }

  /**
   * \brief Constructor specialised for static temporary storage, initializing using the runs' lengths.
   */
  template <typename RunLengthT, typename TotalDecodedSizeT>
  _CCCL_DEVICE _CCCL_FORCEINLINE BlockRunLengthDecode(
    ItemT (&run_values)[RunsPerThread], RunLengthT (&run_lengths)[RunsPerThread], TotalDecodedSizeT& total_decoded_size)
      : temp_storage(PrivateStorage())
      , linear_tid(RowMajorTid(BlockDimX, BlockDimY, BlockDimZ))
  {
    InitWithRunLengths(run_values, run_lengths, total_decoded_size);
  }

  /**
   * \brief Constructor specialised for static temporary storage, initializing using the runs' offsets.
   */
  template <typename UserRunOffsetT>
  _CCCL_DEVICE _CCCL_FORCEINLINE
  BlockRunLengthDecode(ItemT (&run_values)[RunsPerThread], UserRunOffsetT (&run_offsets)[RunsPerThread])
      : temp_storage(PrivateStorage())
      , linear_tid(RowMajorTid(BlockDimX, BlockDimY, BlockDimZ))
  {
    InitWithRunOffsets(run_values, run_offsets);
  }

private:
  /**
   * @brief Returns the offset of the first value within @p input which compares greater than
   * @p val. This version takes @p MAX_NUM_ITEMS, an upper bound of the array size, which will
   * be used to determine the number of binary search iterations at compile time.
   *
   * @param[in] input
   *   Input sequence
   *
   * @param[in] num_items
   *   Input sequence length
   *
   * @param[in] val
   *   Search key
   */
  template <int MAX_NUM_ITEMS, typename InputIteratorT, typename OffsetT, typename T>
  _CCCL_DEVICE _CCCL_FORCEINLINE OffsetT StaticUpperBound(InputIteratorT input, OffsetT num_items, T val)
  {
    OffsetT lower_bound = 0;
    OffsetT upper_bound = num_items;

    _CCCL_PRAGMA_UNROLL_FULL()
    for (int i = 0; i <= Log2<MAX_NUM_ITEMS>::VALUE; i++)
    {
      OffsetT mid = cub::MidPoint<OffsetT>(lower_bound, upper_bound);
      mid         = (::cuda::std::min) (mid, num_items - 1);

      if (val < input[mid])
      {
        upper_bound = mid;
      }
      else
      {
        lower_bound = mid + 1;
      }
    }

    return lower_bound;
  }

  template <typename RunOffsetT>
  _CCCL_DEVICE _CCCL_FORCEINLINE void
  InitWithRunOffsets(ItemT (&run_values)[RunsPerThread], RunOffsetT (&run_offsets)[RunsPerThread])
  {
    // Keep the runs' items and the offsets of each run's beginning in the temporary storage
    RunOffsetT thread_dst_offset = static_cast<RunOffsetT>(linear_tid) * static_cast<RunOffsetT>(RunsPerThread);

    _CCCL_PRAGMA_UNROLL_FULL()
    for (int i = 0; i < RunsPerThread; i++)
    {
      temp_storage.runs.run_values[thread_dst_offset]  = run_values[i];
      temp_storage.runs.run_offsets[thread_dst_offset] = run_offsets[i];
      thread_dst_offset++;
    }

    // Ensure run offsets and run values have been written to shared memory
    __syncthreads();
  }

  template <typename RunLengthT, typename TotalDecodedSizeT>
  _CCCL_DEVICE _CCCL_FORCEINLINE void InitWithRunLengths(
    ItemT (&run_values)[RunsPerThread], RunLengthT (&run_lengths)[RunsPerThread], TotalDecodedSizeT& total_decoded_size)
  {
    // Compute the offset for the beginning of each run
    DecodedOffsetT run_offsets[RunsPerThread];

    _CCCL_PRAGMA_UNROLL_FULL()
    for (int i = 0; i < RunsPerThread; i++)
    {
      run_offsets[i] = static_cast<DecodedOffsetT>(run_lengths[i]);
    }
    DecodedOffsetT decoded_size_aggregate;
    RunOffsetScanT(this->temp_storage.offset_scan).ExclusiveSum(run_offsets, run_offsets, decoded_size_aggregate);
    total_decoded_size = static_cast<TotalDecodedSizeT>(decoded_size_aggregate);

    // Ensure the prefix scan's temporary storage can be reused (may be superfluous, but depends on scan implementation)
    __syncthreads();

    InitWithRunOffsets(run_values, run_offsets);
  }

public:
  /**
   * \brief Run-length decodes the runs previously passed via a call to Init(...) and returns the run-length decoded
   * items in a blocked arrangement to \p decoded_items. If the number of run-length decoded items exceeds the
   * run-length decode buffer (i.e., `DecodedItemsPerThread * BLOCK_THREADS`), only the items that fit within
   * the buffer are returned. Subsequent calls to `RunLengthDecode` adjusting \p from_decoded_offset can be
   * used to retrieve the remaining run-length decoded items. Calling __syncthreads() between any two calls to
   * `RunLengthDecode` is not required.
   * \p item_offsets can be used to retrieve each run-length decoded item's relative index within its run. E.g., the
   * run-length encoded array of `3, 1, 4` with the respective run lengths of `2, 1, 3` would yield the run-length
   * decoded array of `3, 3, 1, 4, 4, 4` with the relative offsets of `0, 1, 0, 0, 1, 2`.
   * \smemreuse
   *
   * \param[out] decoded_items The run-length decoded items to be returned in a blocked arrangement
   * \param[out] item_offsets The run-length decoded items' relative offset within the run they belong to
   * \param[in] from_decoded_offset If invoked with from_decoded_offset that is larger than total_decoded_size results
   * in undefined behavior.
   */
  template <typename RelativeOffsetT>
  _CCCL_DEVICE _CCCL_FORCEINLINE void RunLengthDecode(
    ItemT (&decoded_items)[DecodedItemsPerThread],
    RelativeOffsetT (&item_offsets)[DecodedItemsPerThread],
    DecodedOffsetT from_decoded_offset = 0)
  {
    // The (global) offset of the first item decoded by this thread
    DecodedOffsetT thread_decoded_offset = from_decoded_offset + linear_tid * DecodedItemsPerThread;

    // The run that the first decoded item of this thread belongs to
    // If this thread's <thread_decoded_offset> is already beyond the total decoded size, it will be assigned to the
    // last run
    RunOffsetT assigned_run =
      StaticUpperBound<BLOCK_RUNS>(temp_storage.runs.run_offsets, BLOCK_RUNS, thread_decoded_offset)
      - static_cast<RunOffsetT>(1U);

    DecodedOffsetT assigned_run_begin = temp_storage.runs.run_offsets[assigned_run];

    // If this thread is getting assigned the last run, we make sure it will not fetch any other run after this
    DecodedOffsetT assigned_run_end =
      (assigned_run == BLOCK_RUNS - 1)
        ? thread_decoded_offset + DecodedItemsPerThread
        : temp_storage.runs.run_offsets[assigned_run + 1];

    ItemT val = temp_storage.runs.run_values[assigned_run];

    _CCCL_PRAGMA_UNROLL_FULL()
    for (DecodedOffsetT i = 0; i < DecodedItemsPerThread; i++)
    {
      decoded_items[i] = val;
      item_offsets[i]  = thread_decoded_offset - assigned_run_begin;

      // A thread only needs to fetch the next run if this was not the last loop iteration
      const bool is_final_loop_iteration = (i + 1 >= DecodedItemsPerThread);
      if (!is_final_loop_iteration && (thread_decoded_offset == assigned_run_end - 1))
      {
        // We make sure that a thread is not re-entering this conditional when being assigned to the last run already by
        // extending the last run's length to all the thread's item
        assigned_run++;
        assigned_run_begin = temp_storage.runs.run_offsets[assigned_run];

        // If this thread is getting assigned the last run, we make sure it will not fetch any other run after this
        assigned_run_end = (assigned_run == BLOCK_RUNS - 1)
                           ? thread_decoded_offset + DecodedItemsPerThread
                           : temp_storage.runs.run_offsets[assigned_run + 1];
        val              = temp_storage.runs.run_values[assigned_run];
      }
      thread_decoded_offset++;
    }
  }

  /**
   * \brief Run-length decodes the runs previously passed via a call to Init(...) and returns the run-length decoded
   * items in a blocked arrangement to `decoded_items`. If the number of run-length decoded items exceeds the
   * run-length decode buffer (i.e., `DecodedItemsPerThread * BLOCK_THREADS`), only the items that fit within
   * the buffer are returned. Subsequent calls to `RunLengthDecode` adjusting `from_decoded_offset` can be
   * used to retrieve the remaining run-length decoded items. Calling __syncthreads() between any two calls to
   * `RunLengthDecode` is not required.
   *
   * \param[out] decoded_items The run-length decoded items to be returned in a blocked arrangement
   * \param[in] from_decoded_offset If invoked with from_decoded_offset that is larger than total_decoded_size results
   * in undefined behavior.
   */
  _CCCL_DEVICE _CCCL_FORCEINLINE void
  RunLengthDecode(ItemT (&decoded_items)[DecodedItemsPerThread], DecodedOffsetT from_decoded_offset = 0)
  {
    DecodedOffsetT item_offsets[DecodedItemsPerThread];
    RunLengthDecode(decoded_items, item_offsets, from_decoded_offset);
  }
};

CUB_NAMESPACE_END
