// SPDX-FileCopyrightText: Copyright (c) 2018, NVIDIA Corporation. All rights reserved.
// SPDX-License-Identifier: Apache-2.0

/*! \file
 *  \brief A caching and pooling memory resource adaptor which uses a single
 *  upstream resource for memory allocation, and embeds bookkeeping information
 *  in allocated blocks.
 */

#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/host_vector.h>
#include <thrust/mr/allocator.h>
#include <thrust/mr/memory_resource.h>
#include <thrust/mr/pool_options.h>

#include <cuda/__cmath/ilog.h>
#include <cuda/__cmath/pow2.h>
#include <cuda/std/__host_stdlib/algorithm>
#include <cuda/std/cassert>
#include <cuda/std/cstdint>

THRUST_NAMESPACE_BEGIN
namespace mr
{
/** \addtogroup memory_resources Memory Resources
 *  \ingroup memory_management
 *  \{
 */

/*! A memory resource adaptor allowing for pooling and caching allocations from \p Upstream, using memory allocated
 *      from it for both blocks then allocated to the user and for internal bookkeeping of the cached memory.
 *
 *  On a typical memory resource, calls to \p allocate and \p deallocate actually allocate and deallocate memory.
 * Pooling memory resources only allocate and deallocate memory from an external resource (the upstream memory resource)
 * when there's no suitable memory currently cached; otherwise, they use memory they have acquired beforehand, to make
 *      memory allocation faster and more efficient.
 *
 *  The non-disjoint version of the pool resource uses a single upstream memory resource. Every allocation is larger
 * than strictly necessary to fulfill the end-user's request, because it needs to account for the memory overhead of
 * tracking the memory blocks and chunks inside those same memory regions. Nevertheless, this version should be more
 * memory-efficient than the \p disjoint_unsynchronized_pool_resource, because it doesn't need to allocate additional
 * blocks of memory from a separate resource, which in turn would necessitate the bookkeeping overhead in the upstream
 * resource.
 *
 *  This version requires that memory allocated from Upstream is accessible from device. It supports smart references,
 *      meaning that the non-managed CUDA resource, returning a device-tagged pointer, will work, but will be much less
 *      efficient than the disjoint version, which wouldn't need to touch device memory at all, and therefore wouldn't
 * need to transfer it back and forth between the host and the device whenever an allocation or a deallocation happens.
 *
 *  \tparam Upstream the type of memory resources that will be used for allocating memory blocks
 */
template <typename Upstream>
class unsynchronized_pool_resource final
    : public memory_resource<typename Upstream::pointer>
    , private validator<Upstream>
{
public:
  /*! Get the default options for a pool. These are meant to be a sensible set of values for many use cases,
   *      and as such, may be tuned in the future. This function is exposed so that creating a set of options that are
   *      just a slight departure from the defaults is easy.
   */
  static pool_options get_default_options()
  {
    pool_options ret;

    ret.min_blocks_per_chunk = 16;
    ret.min_bytes_per_chunk  = 1024;
    ret.max_blocks_per_chunk = static_cast<std::size_t>(1) << 20;
    ret.max_bytes_per_chunk  = static_cast<std::size_t>(1) << 30;

    ret.smallest_block_size = THRUST_MR_DEFAULT_ALIGNMENT;
    ret.largest_block_size  = static_cast<std::size_t>(1) << 20;

    ret.alignment = THRUST_MR_DEFAULT_ALIGNMENT;

    ret.cache_oversized = true;

    ret.cached_size_cutoff_factor      = 16;
    ret.cached_alignment_cutoff_factor = 16;

    return ret;
  }

  /*! Constructor.
   *
   *  \param upstream the upstream memory resource for allocations
   *  \param options pool options to use
   */
  unsynchronized_pool_resource(Upstream* upstream, pool_options options = get_default_options())
      : m_upstream(upstream)
      , m_options(options)
      , m_smallest_block_log2(::cuda::ceil_ilog2(m_options.smallest_block_size))
      , m_pools(upstream)
      , m_allocated()
      , m_oversized()
      , m_cached_oversized()
  {
    assert(m_options.validate());

    pool p = {block_descriptor_ptr(), 0};
    m_pools.resize(::cuda::ceil_ilog2(m_options.largest_block_size) - m_smallest_block_log2 + 1, p);
  }

  // TODO: C++11: use delegating constructors

  /*! Constructor. The upstream resource is obtained by calling \p get_global_resource<Upstream>.
   *
   *  \param options pool options to use
   */
  unsynchronized_pool_resource(pool_options options = get_default_options())
      : m_upstream(get_global_resource<Upstream>())
      , m_options(options)
      , m_smallest_block_log2(::cuda::ceil_ilog2(m_options.smallest_block_size))
      , m_pools(get_global_resource<Upstream>())
      , m_allocated()
      , m_oversized()
      , m_cached_oversized()
  {
    assert(m_options.validate());

    pool p = {block_descriptor_ptr(), 0};
    m_pools.resize(::cuda::ceil_ilog2(m_options.largest_block_size) - m_smallest_block_log2 + 1, p);
  }

  /*! Destructor. Releases all held memory to upstream.
   */
  ~unsynchronized_pool_resource()
  {
    release();
  }

private:
  using void_ptr        = typename Upstream::pointer;
  using void_ptr_traits = thrust::detail::pointer_traits<void_ptr>;
  using char_ptr        = typename void_ptr_traits::template rebind<char>::other;

  struct block_descriptor;
  struct chunk_descriptor;
  struct oversized_block_descriptor;

  using block_descriptor_ptr           = typename void_ptr_traits::template rebind<block_descriptor>::other;
  using chunk_descriptor_ptr           = typename void_ptr_traits::template rebind<chunk_descriptor>::other;
  using oversized_block_descriptor_ptr = typename void_ptr_traits::template rebind<oversized_block_descriptor>::other;
  using oversized_block_ptr_traits     = thrust::detail::pointer_traits<oversized_block_descriptor_ptr>;

  struct block_descriptor
  {
    block_descriptor_ptr next;
  };

  struct chunk_descriptor
  {
    std::size_t size;
    chunk_descriptor_ptr next;
  };

  // this was originally a forward list, but I made it a doubly linked list
  // because that way deallocation when not caching is faster and doesn't require
  // traversal of a linked list (it's still a forward list for the cached list,
  // because allocation from that list already traverses)
  //
  // TODO: investigate whether it's better to have this be a doubly-linked list
  // with fast do_deallocate when !m_options.cache_oversized, or to have this be
  // a forward list and require traversal in do_deallocate
  //
  // I assume that it is better this way, but the additional pointer could
  // potentially hurt? these are supposed to be oversized and/or overaligned,
  // so they are kinda memory intensive already
  struct oversized_block_descriptor
  {
    std::size_t size;
    std::size_t alignment;
    oversized_block_descriptor_ptr prev;
    oversized_block_descriptor_ptr next;
    oversized_block_descriptor_ptr next_cached;
    std::size_t current_size;
  };

  struct pool
  {
    block_descriptor_ptr free_list;
    std::size_t previous_allocated_count;
  };

  using pool_vector = thrust::host_vector<pool, allocator<pool, Upstream>>;

  Upstream* m_upstream;

  pool_options m_options;
  std::size_t m_smallest_block_log2;

  pool_vector m_pools;
  chunk_descriptor_ptr m_allocated;
  oversized_block_descriptor_ptr m_oversized;
  oversized_block_descriptor_ptr m_cached_oversized;

public:
  /*! Releases all held memory to upstream.
   */
  void release()
  {
    // reset the buckets
    for (std::size_t i = 0; i < m_pools.size(); ++i)
    {
      thrust::raw_reference_cast(m_pools[i]).free_list                = block_descriptor_ptr();
      thrust::raw_reference_cast(m_pools[i]).previous_allocated_count = 0;
    }

    // deallocate memory allocated for the buckets
    while (detail::pointer_traits<chunk_descriptor_ptr>::get(m_allocated))
    {
      chunk_descriptor_ptr alloc = m_allocated;
      m_allocated                = thrust::raw_reference_cast(*m_allocated).next;

      void_ptr p = static_cast<void_ptr>(
        static_cast<char_ptr>(static_cast<void_ptr>(alloc)) - thrust::raw_reference_cast(*alloc).size);
      m_upstream->do_deallocate(
        p, thrust::raw_reference_cast(*alloc).size + sizeof(chunk_descriptor), m_options.alignment);
    }

    // deallocate cached oversized/overaligned memory
    while (oversized_block_ptr_traits::get(m_oversized))
    {
      oversized_block_descriptor_ptr alloc = m_oversized;
      m_oversized                          = thrust::raw_reference_cast(*m_oversized).next;

      oversized_block_descriptor desc = thrust::raw_reference_cast(*alloc);

      void_ptr p = static_cast<void_ptr>(static_cast<char_ptr>(static_cast<void_ptr>(alloc)) - desc.current_size);
      m_upstream->do_deallocate(p, desc.size + sizeof(oversized_block_descriptor), desc.alignment);
    }

    m_cached_oversized = oversized_block_descriptor_ptr();
  }

  [[nodiscard]] virtual void_ptr
  do_allocate(std::size_t bytes, std::size_t alignment = THRUST_MR_DEFAULT_ALIGNMENT) override
  {
    bytes = (std::max) (bytes, m_options.smallest_block_size);
    assert(::cuda::is_power_of_two(alignment));

    // an oversized and/or overaligned allocation requested; needs to be allocated separately
    if (bytes > m_options.largest_block_size || alignment > m_options.alignment)
    {
      if (m_options.cache_oversized)
      {
        oversized_block_descriptor_ptr ptr       = m_cached_oversized;
        oversized_block_descriptor_ptr* previous = &m_cached_oversized;
        while (oversized_block_ptr_traits::get(ptr))
        {
          oversized_block_descriptor desc = *ptr;
          bool is_good                    = desc.size >= bytes && desc.alignment >= alignment;

          // if the size is bigger than the requested size by a factor
          // bigger than or equal to the specified cutoff for size,
          // allocate a new block
          if (is_good)
          {
            std::size_t size_factor = desc.size / bytes;
            if (size_factor >= m_options.cached_size_cutoff_factor)
            {
              is_good = false;
            }
          }

          // if the alignment is bigger than the requested one by a factor
          // bigger than or equal to the specified cutoff for alignment,
          // allocate a new block
          if (is_good)
          {
            std::size_t alignment_factor = desc.alignment / alignment;
            if (alignment_factor >= m_options.cached_alignment_cutoff_factor)
            {
              is_good = false;
            }
          }

          if (is_good)
          {
            if (previous != &m_cached_oversized)
            {
              *previous = desc.next_cached;
            }
            else
            {
              m_cached_oversized = desc.next_cached;
            }

            desc.next_cached = oversized_block_descriptor_ptr();

            auto ret = static_cast<char_ptr>(static_cast<void_ptr>(ptr)) - desc.size;

            if (bytes != desc.size)
            {
              desc.current_size = bytes;

              ptr = static_cast<oversized_block_descriptor_ptr>(static_cast<void_ptr>(ret + bytes));

              if (oversized_block_ptr_traits::get(desc.prev))
              {
                thrust::raw_reference_cast(*desc.prev).next = ptr;
              }
              else
              {
                m_oversized = ptr;
              }

              if (oversized_block_ptr_traits::get(desc.next))
              {
                thrust::raw_reference_cast(*desc.next).prev = ptr;
              }
            }

            *ptr = desc;

            return static_cast<void_ptr>(ret);
          }

          previous = &thrust::raw_reference_cast(*ptr).next_cached;
          ptr      = *previous;
        }
      }

      // no fitting cached block found; allocate a new one that's just up to the specs
      void_ptr allocated = m_upstream->do_allocate(bytes + sizeof(oversized_block_descriptor), alignment);
      oversized_block_descriptor_ptr block =
        static_cast<oversized_block_descriptor_ptr>(static_cast<void_ptr>(static_cast<char_ptr>(allocated) + bytes));

      oversized_block_descriptor desc;
      desc.size         = bytes;
      desc.alignment    = alignment;
      desc.prev         = oversized_block_descriptor_ptr();
      desc.next         = m_oversized;
      desc.next_cached  = oversized_block_descriptor_ptr();
      desc.current_size = bytes;
      *block            = desc;
      m_oversized       = block;

      if (oversized_block_ptr_traits::get(desc.next))
      {
        oversized_block_descriptor next = *desc.next;
        next.prev                       = block;
        *desc.next                      = next;
      }

      return allocated;
    }

    // the request is NOT for oversized and/or overaligned memory
    // allocate a block from an appropriate bucket
    std::size_t bytes_log2 = ::cuda::ceil_ilog2(bytes);
    std::size_t bucket_idx = bytes_log2 - m_smallest_block_log2;
    pool& bucket           = thrust::raw_reference_cast(m_pools[bucket_idx]);

    bytes = static_cast<std::size_t>(1) << bytes_log2;

    // if the free list of the bucket has no elements, allocate a new chunk
    // and split it into blocks pushed to the free list
    if (!detail::pointer_traits<block_descriptor_ptr>::get(bucket.free_list))
    {
      std::size_t n = bucket.previous_allocated_count;
      if (n == 0)
      {
        n = m_options.min_blocks_per_chunk;
        if (n < (m_options.min_bytes_per_chunk >> bytes_log2))
        {
          n = m_options.min_bytes_per_chunk >> bytes_log2;
        }
      }
      else
      {
        n = n * 3 / 2;
        if (n > (m_options.max_bytes_per_chunk >> bytes_log2))
        {
          n = m_options.max_bytes_per_chunk >> bytes_log2;
        }
        if (n > m_options.max_blocks_per_chunk)
        {
          n = m_options.max_blocks_per_chunk;
        }
      }

      std::size_t descriptor_size = (std::max) (sizeof(block_descriptor), m_options.alignment);
      std::size_t block_size      = bytes + descriptor_size;
      block_size += m_options.alignment - block_size % m_options.alignment;
      std::size_t chunk_size = block_size * n;

      void_ptr allocated = m_upstream->do_allocate(chunk_size + sizeof(chunk_descriptor), m_options.alignment);
      chunk_descriptor_ptr chunk =
        static_cast<chunk_descriptor_ptr>(static_cast<void_ptr>(static_cast<char_ptr>(allocated) + chunk_size));

      chunk_descriptor chunk_desc;
      chunk_desc.size = chunk_size;
      chunk_desc.next = m_allocated;
      *chunk          = chunk_desc;
      m_allocated     = chunk;

      for (std::size_t i = 0; i < n; ++i)
      {
        block_descriptor_ptr block = static_cast<block_descriptor_ptr>(
          static_cast<void_ptr>(static_cast<char_ptr>(allocated) + block_size * i + bytes));

        block_descriptor block_desc;
        block_desc.next  = bucket.free_list;
        *block           = block_desc;
        bucket.free_list = block;
      }
    }

    // allocate a block from the front of the bucket's free list
    block_descriptor_ptr block = bucket.free_list;
    bucket.free_list           = thrust::raw_reference_cast(*block).next;
    return static_cast<void_ptr>(static_cast<char_ptr>(static_cast<void_ptr>(block)) - bytes);
  }

  virtual void do_deallocate(void_ptr p, std::size_t n, std::size_t alignment = THRUST_MR_DEFAULT_ALIGNMENT) override
  {
    n = (std::max) (n, m_options.smallest_block_size);
    assert(::cuda::is_power_of_two(alignment));

    // verify that the pointer is at least as aligned as claimed
    assert(reinterpret_cast<::cuda::std::intmax_t>(void_ptr_traits::get(p)) % alignment == 0);

    // the deallocated block is oversized and/or overaligned
    if (n > m_options.largest_block_size || alignment > m_options.alignment)
    {
      oversized_block_descriptor_ptr block =
        static_cast<oversized_block_descriptor_ptr>(static_cast<void_ptr>(static_cast<char_ptr>(p) + n));

      oversized_block_descriptor desc = *block;
      assert(desc.current_size == n);
      assert(desc.alignment == alignment);

      if (m_options.cache_oversized)
      {
        desc.next_cached = m_cached_oversized;

        if (desc.size != n)
        {
          desc.current_size = desc.size;
          block =
            static_cast<oversized_block_descriptor_ptr>(static_cast<void_ptr>(static_cast<char_ptr>(p) + desc.size));
          if (oversized_block_ptr_traits::get(desc.prev))
          {
            thrust::raw_reference_cast(*desc.prev).next = block;
          }
          else
          {
            m_oversized = block;
          }

          if (oversized_block_ptr_traits::get(desc.next))
          {
            thrust::raw_reference_cast(*desc.next).prev = block;
          }
        }

        m_cached_oversized = block;
        *block             = desc;

        return;
      }

      if (oversized_block_ptr_traits::get(desc.prev))
      {
        thrust::raw_reference_cast(*desc.prev).next = desc.next;
      }
      else
      {
        m_oversized = desc.next;
      }

      if (oversized_block_ptr_traits::get(desc.next))
      {
        thrust::raw_reference_cast(*desc.next).prev = desc.prev;
      }

      m_upstream->do_deallocate(p, desc.size + sizeof(oversized_block_descriptor), desc.alignment);

      return;
    }

    // push the block to the front of the appropriate bucket's free list
    std::size_t n_log2     = ::cuda::ceil_ilog2(n);
    std::size_t bucket_idx = n_log2 - m_smallest_block_log2;
    pool& bucket           = thrust::raw_reference_cast(m_pools[bucket_idx]);

    n = static_cast<std::size_t>(1) << n_log2;

    block_descriptor_ptr block = static_cast<block_descriptor_ptr>(static_cast<void_ptr>(static_cast<char_ptr>(p) + n));

    block_descriptor desc;
    desc.next        = bucket.free_list;
    *block           = desc;
    bucket.free_list = block;
  }
};

/*! \} // memory_resources
 */
} // namespace mr
THRUST_NAMESPACE_END
