/*
 * Copyright (c) 2020-2021, 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 <rmm/cuda_stream_view.hpp>
#include <rmm/detail/error.hpp>
#include <rmm/detail/exec_check_disable.hpp>
#include <rmm/device_buffer.hpp>
#include <rmm/mr/device/device_memory_resource.hpp>
#include <rmm/mr/device/per_device_resource.hpp>

#include <cstddef>
#include <vector>

namespace rmm {

/**
 * @brief An *uninitialized* vector of elements in device memory.
 *
 * Similar to a `thrust::device_vector`, `device_uvector` is a random access container of elements
 * stored contiguously in device memory. However, unlike `thrust::device_vector`, `device_uvector`
 * does *not* default initialize the vector elements.
 *
 * If initialization is desired, this must be done explicitly by the caller, e.g., with
 * `thrust::uninitialized_fill`.
 *
 * Example:
 * @code
 * rmm::mr::device_memory_resource * mr = new my_custom_resource();
 * rmm::cuda_stream_view s{};
 *
 * // Allocates *uninitialized* device memory on stream `s` sufficient for 100 ints using the
 * // supplied resource `mr`
 * rmm::device_uvector<int> uv(100, s, mr);
 *
 * // Initializes all elements to 0 on stream `s`
 * thrust::uninitialized_fill(thrust::cuda::par.on(s), uv.begin(), uv.end(), 0);
 * @endcode
 *
 * Avoiding default initialization improves performance by eliminating the kernel launch required to
 * default initialize the elements. This initialization is often unnecessary, e.g., when the vector
 * is created to hold some output from some operation.
 *
 * However, this restricts the element type `T` to only trivially copyable types. In short,
 * trivially copyable types can be safely copied with `memcpy`. For more information, see
 * https://en.cppreference.com/w/cpp/types/is_trivially_copyable.
 *
 * Another key difference over `thrust::device_vector` is that all operations that invoke
 * allocation, kernels, or memcpys take a CUDA stream parameter to indicate on which stream the
 * operation will be performed.
 *
 * @tparam T Trivially copyable element type
 */
template <typename T>
class device_uvector {
  static_assert(std::is_trivially_copyable<T>::value,
                "device_uvector only supports types that are trivially copyable.");

 public:
  using value_type      = T;
  using size_type       = std::size_t;
  using reference       = value_type&;
  using const_reference = value_type const&;
  using pointer         = value_type*;
  using const_pointer   = value_type const*;
  using iterator        = pointer;
  using const_iterator  = const_pointer;

  RMM_EXEC_CHECK_DISABLE
  ~device_uvector() = default;

  RMM_EXEC_CHECK_DISABLE
  device_uvector(device_uvector&&) noexcept = default;

  device_uvector& operator=(device_uvector&&) noexcept = default;

  /**
   * @brief Copy ctor is deleted as it doesn't allow a stream argument
   */
  device_uvector(device_uvector const&) = delete;

  /**
   * @brief Copy assignment is deleted as it doesn't allow a stream argument
   */
  device_uvector& operator=(device_uvector const&) = delete;

  /**
   * @brief Default constructor is deleted as it doesn't allow a stream argument
   */
  device_uvector() = delete;

  /**
   * @brief Construct a new `device_uvector` with sufficient uninitialized storage for `size`
   * elements.
   *
   * Elements are uninitialized. Reading an element before it is initialized results in undefined
   * behavior.
   *
   * @param size The number of elements to allocate storage for
   * @param stream The stream on which to perform the allocation
   * @param mr The resource used to allocate the device storage
   */
  explicit device_uvector(
    std::size_t size,
    cuda_stream_view stream,
    rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource())
    : _storage{elements_to_bytes(size), stream, mr}
  {
  }

  /**
   * @brief Construct a new device_uvector by deep copying the contents of another `device_uvector`.
   *
   * Elements are copied as if by `memcpy`, i.e., `T`'s copy constructor is not invoked.
   *
   * @param other The vector to copy from
   * @param stream The stream on which to perform the copy
   * @param mr The resource used to allocate device memory for the new vector
   */
  explicit device_uvector(
    device_uvector const& other,
    cuda_stream_view stream,
    rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource())
    : _storage{other._storage, stream, mr}
  {
  }

  /**
   * @brief Returns pointer to the specified element
   *
   * Behavior is undefined if `element_index >= size()`.
   *
   * @param element_index Index of the specified element.
   * @return T* Pointer to the desired element
   */
  [[nodiscard]] pointer element_ptr(std::size_t element_index) noexcept
  {
    assert(element_index < size());
    return data() + element_index;
  }

  /**
   * @brief Returns pointer to the specified element
   *
   * Behavior is undefined if `element_index >= size()`.
   *
   * @param element_index Index of the specified element.
   * @return T* Pointer to the desired element
   */
  [[nodiscard]] const_pointer element_ptr(std::size_t element_index) const noexcept
  {
    assert(element_index < size());
    return data() + element_index;
  }

  /**
   * @brief Performs an asynchronous copy of `v` to the specified element in device memory.
   *
   * This specialization for fundamental types is optimized to use `cudaMemsetAsync` when
   * `host_value` is zero.
   *
   * This function does not synchronize stream `s` before returning. Therefore, the object
   * referenced by `v` should not be destroyed or modified until `stream` has been synchronized.
   * Otherwise, behavior is undefined.
   *
   * @note This function incurs a host to device memcpy and should be used sparingly.
   *
   * @note Calling this function with a literal or other r-value reference for `v` is disallowed
   *  to prevent the implementation from asynchronously copying from a literal or other implicit
   *  temporary after it is deleted or goes out of scope.
   *
   * Example:
   * \code{cpp}
   * rmm::device_uvector<int32_t> vec(100, stream);
   *
   * int v{42};
   *
   * // Copies 42 to element 0 on `stream`. Does _not_ synchronize
   * vec.set_element_async(0, v, stream);
   * ...
   * cudaStreamSynchronize(stream);
   * // Synchronization is required before `v` can be modified
   * v = 13;
   * \endcode
   *
   * @throws rmm::out_of_range exception if `element_index >= size()`
   *
   * @param element_index Index of the target element
   * @param v The value to copy to the specified element
   * @param s The stream on which to perform the copy
   */
  void set_element_async(std::size_t element_index,
                         value_type const& value,
                         cuda_stream_view stream)
  {
    RMM_EXPECTS(
      element_index < size(), rmm::out_of_range, "Attempt to access out of bounds element.");

    if constexpr (std::is_same<value_type, bool>::value) {
      RMM_CUDA_TRY(
        cudaMemsetAsync(element_ptr(element_index), value, sizeof(value), stream.value()));
      return;
    }

    if constexpr (std::is_fundamental<value_type>::value) {
      if (value == value_type{0}) {
        set_element_to_zero_async(element_index, stream);
        return;
      }
    }

    RMM_CUDA_TRY(cudaMemcpyAsync(
      element_ptr(element_index), &value, sizeof(value), cudaMemcpyDefault, stream.value()));
  }

  // We delete the r-value reference overload to prevent asynchronously copying from a literal or
  // implicit temporary value after it is deleted or goes out of scope.
  void set_element_async(std::size_t, value_type const&&, cuda_stream_view) = delete;

  /**
   * @brief Asynchronously sets the specified element to zero in device memory.
   *
   * This function does not synchronize stream `s` before returning
   *
   * @note This function incurs a device memset and should be used sparingly.
   *
   * Example:
   * \code{cpp}
   * rmm::device_uvector<int32_t> vec(100, stream);
   *
   * int v{42};
   *
   * // Sets element at index 42 to 0 on `stream`. Does _not_ synchronize
   * vec.set_element_to_zero_async(42, stream);
   * \endcode
   *
   * @throws rmm::out_of_range exception if `element_index >= size()`
   *
   * @param element_index Index of the target element
   * @param s The stream on which to perform the copy
   */
  void set_element_to_zero_async(std::size_t element_index, cuda_stream_view stream)
  {
    RMM_EXPECTS(
      element_index < size(), rmm::out_of_range, "Attempt to access out of bounds element.");
    RMM_CUDA_TRY(
      cudaMemsetAsync(element_ptr(element_index), 0, sizeof(value_type), stream.value()));
  }

  /**
   * @brief Performs a synchronous copy of `v` to the specified element in device memory.
   *
   * Because this function synchronizes the stream `s`, it is safe to destroy or modify the object
   * referenced by `v` after this function has returned.
   *
   * @note This function incurs a host to device memcpy and should be used sparingly.
   * @note This function synchronizes `stream`.
   *
   * Example:
   * \code{cpp}
   * rmm::device_uvector<int32_t> vec(100, stream);
   *
   * int v{42};
   *
   * // Copies 42 to element 0 on `stream` and synchronizes the stream
   * vec.set_element(0, v, stream);
   *
   * // It is safe to destroy or modify `v`
   * v = 13;
   * \endcode
   *
   *
   * @throws rmm::out_of_range exception if `element_index >= size()`
   *
   * @param element_index Index of the target element
   * @param value The value to copy to the specified element
   * @param stream The stream on which to perform the copy
   */
  void set_element(std::size_t element_index, T const& value, cuda_stream_view stream)
  {
    set_element_async(element_index, value, stream);
    stream.synchronize_no_throw();
  }

  /**
   * @brief Returns the specified element from device memory
   *
   * @note This function incurs a device to host memcpy and should be used sparingly.
   * @note This function synchronizes `stream`.
   *
   * @throws rmm::out_of_range exception if `element_index >= size()`
   *
   * @param element_index Index of the desired element
   * @param stream The stream on which to perform the copy
   * @return The value of the specified element
   */
  [[nodiscard]] value_type element(std::size_t element_index, cuda_stream_view stream) const
  {
    RMM_EXPECTS(
      element_index < size(), rmm::out_of_range, "Attempt to access out of bounds element.");
    value_type value;
    RMM_CUDA_TRY(cudaMemcpyAsync(
      &value, element_ptr(element_index), sizeof(value), cudaMemcpyDefault, stream.value()));
    stream.synchronize();
    return value;
  }

  /**
   * @brief Returns the first element.
   *
   * @note This function incurs a device-to-host memcpy and should be used sparingly.
   * @note This function synchronizes `stream`.
   *
   * @throws rmm::out_of_range exception if the vector is empty.
   *
   * @param stream The stream on which to perform the copy
   * @return The value of the first element
   */
  [[nodiscard]] value_type front_element(cuda_stream_view stream) const
  {
    return element(0, stream);
  }

  /**
   * @brief Returns the last element.
   *
   * @note This function incurs a device-to-host memcpy and should be used sparingly.
   * @note This function synchronizes `stream`.
   *
   * @throws rmm::out_of_range exception if the vector is empty.
   *
   * @param stream The stream on which to perform the copy
   * @return The value of the last element
   */
  [[nodiscard]] value_type back_element(cuda_stream_view stream) const
  {
    return element(size() - 1, stream);
  }

  /**
   * @brief Resizes the vector to contain `new_size` elements.
   *
   * If `new_size > size()`, the additional elements are uninitialized.
   *
   * If `new_size < capacity()`, no action is taken other than updating the value of `size()`. No
   * memory is allocated nor copied. `shrink_to_fit()` may be used to force deallocation of unused
   * memory.
   *
   * If `new_size > capacity()`, elements are copied as if by mempcy to a new allocation.
   *
   * The invariant `size() <= capacity()` holds.
   *
   * @param new_size The desired number of elements
   * @param stream The stream on which to perform the allocation/copy (if any)
   */
  void resize(std::size_t new_size, cuda_stream_view stream)
  {
    _storage.resize(elements_to_bytes(new_size), stream);
  }

  /**
   * @brief Forces deallocation of unused device memory.
   *
   * If `capacity() > size()`, reallocates and copies vector contents to eliminate unused memory.
   *
   * @param stream Stream on which to perform allocation and copy
   */
  void shrink_to_fit(cuda_stream_view stream) { _storage.shrink_to_fit(stream); }

  /**
   * @brief Release ownership of device memory storage.
   *
   * @return The `device_buffer` used to store the vector elements
   */
  device_buffer release() noexcept { return std::move(_storage); }

  /**
   * @brief Returns the number of elements that can be held in currently allocated storage.
   *
   * @return std::size_t The number of elements that can be stored without requiring a new
   * allocation.
   */
  [[nodiscard]] std::size_t capacity() const noexcept
  {
    return bytes_to_elements(_storage.capacity());
  }

  /**
   * @brief Returns pointer to underlying device storage.
   *
   * @note If `size() == 0` it is undefined behavior to deference the returned pointer. Furthermore,
   * the returned pointer may or may not be equal to `nullptr`.
   *
   * @return Raw pointer to element storage in device memory.
   */
  [[nodiscard]] pointer data() noexcept { return static_cast<pointer>(_storage.data()); }

  /**
   * @brief Returns const pointer to underlying device storage.
   *
   * @note If `size() == 0` it is undefined behavior to deference the returned pointer. Furthermore,
   * the returned pointer may or may not be equal to `nullptr`.
   *
   * @return const_pointer Raw const pointer to element storage in device memory.
   */
  [[nodiscard]] const_pointer data() const noexcept
  {
    return static_cast<const_pointer>(_storage.data());
  }

  /**
   * @brief Returns an iterator to the first element.
   *
   * If the vector is empty, then `begin() == end()`.
   *
   * @return Iterator to the first element.
   */
  [[nodiscard]] iterator begin() noexcept { return data(); }

  /**
   * @brief Returns a const_iterator to the first element.
   *
   * If the vector is empty, then `cbegin() == cend()`.
   *
   * @return Immutable iterator to the first element.
   */
  [[nodiscard]] const_iterator cbegin() const noexcept { return data(); }

  /**
   * @brief Returns a const_iterator to the first element.
   *
   * If the vector is empty, then `begin() == end()`.
   *
   * @return Immutable iterator to the first element.
   */
  [[nodiscard]] const_iterator begin() const noexcept { return cbegin(); }

  /**
   * @brief Returns an iterator to the element following the last element of the vector.
   *
   * The element referenced by `end()` is a placeholder and dereferencing it results in undefined
   * behavior.
   *
   * @return Iterator to one past the last element.
   */
  [[nodiscard]] iterator end() noexcept { return data() + size(); }

  /**
   * @brief Returns a const_iterator to the element following the last element of the vector.
   *
   * The element referenced by `end()` is a placeholder and dereferencing it results in undefined
   * behavior.
   *
   * @return Immutable iterator to one past the last element.
   */
  [[nodiscard]] const_iterator cend() const noexcept { return data() + size(); }

  /**
   * @brief Returns an iterator to the element following the last element of the vector.
   *
   * The element referenced by `end()` is a placeholder and dereferencing it results in undefined
   * behavior.
   *
   * @return Immutable iterator to one past the last element.
   */
  [[nodiscard]] const_iterator end() const noexcept { return cend(); }

  /**
   * @brief Returns the number of elements in the vector.
   *
   * @return The number of elements.
   */
  [[nodiscard]] std::size_t size() const noexcept { return bytes_to_elements(_storage.size()); }

  /**
   * @brief Returns true if the vector contains no elements, i.e., `size() == 0`.
   *
   * @return true The vector is empty
   * @return false The vector is not empty
   */
  [[nodiscard]] bool is_empty() const noexcept { return size() == 0; }

  /**
   * @brief Returns pointer to the resource used to allocate and deallocate the device storage.
   *
   * @return Pointer to underlying resource
   */
  [[nodiscard]] mr::device_memory_resource* memory_resource() const noexcept
  {
    return _storage.memory_resource();
  }

 private:
  device_buffer _storage{};  ///< Device memory storage for vector elements

  [[nodiscard]] std::size_t constexpr elements_to_bytes(std::size_t num_elements) const noexcept
  {
    return num_elements * sizeof(value_type);
  }

  [[nodiscard]] std::size_t constexpr bytes_to_elements(std::size_t num_bytes) const noexcept
  {
    return num_bytes / sizeof(value_type);
  }
};
}  // namespace rmm
