#pragma once

#include <torch/csrc/distributed/c10d/Backend.hpp>
#include <torch/csrc/distributed/c10d/Work.hpp>
#include <memory>
#include <unordered_map>
#include <utility>
#include <vector>

#include <ATen/ATen.h>
#include <ATen/core/dispatch/Dispatcher.h>
#include <c10/macros/Macros.h>

// *************************************************************************
// PROCESS GROUP collective communication API IS BEING CHANGED BETWEEN
// versions 1.7 and 1.8.
// PLEASE DO NOT ADD ANY DEPENDENCIES.
// SEE RFC: https://github.com/pytorch/pytorch/issues/39662
// *************************************************************************

constexpr auto kProcessGroupDefaultTimeout =
    std::chrono::milliseconds(30 * 60 * 1000);

namespace c10d {

// We only call `register_work()` in two cases:
// 1. If the work object is created from a functional collective call.
// 2. If the work object is created from a non-functional collective call within
//    the `with allow_inflight_collective_as_graph_input_ctx()` context manager.
C10_EXPORT void register_work(
    const at::Tensor& tensor,
    const c10::intrusive_ptr<c10d::Work>& work);

C10_EXPORT at::Tensor wait_tensor(const at::Tensor& tensor);

// We only call `unregister_work()` in one case:
// 1. If the work object is created from a non-functional collective call within
//    the `with allow_inflight_collective_as_graph_input_ctx()` context manager.
//
// Q: What about the functional collective case?
// A: The unregistration of work object for functional collective is done in
//    the required user-side explicit call to `wait_tensor()`.
C10_EXPORT void unregister_work(const c10::intrusive_ptr<c10d::Work>& work);

C10_EXPORT size_t get_work_registry_size();

C10_EXPORT void set_allow_inflight_collective_as_graph_input(bool value);

C10_EXPORT bool allow_inflight_collective_as_graph_input();

// ProcessGroup is a base class that captures collective and point to
// point communication in a fixed set of processes.
//
// The functions specified in the class below describe the API alone;
// implementations are provided in subclasses.
//
// Every function that performs I/O is executed asynchronously by a
// thread pool owned by the ProcessGroup (by default). They return an
// object that can be used to wait for completion or error.
//
// The ProcessGroup can instantiate subgroups with fewer or an equal
// number of members. Implementations must take care that multiple
// process groups can be used in parallel and synchronize accordingly.
//
// The ProcessGroup assumes a fixed set of processes. If the set
// changes, existing instances must be destructed and instantiation
// and initialization must start from scratch. For members of the
// process group to find each other (referred to as rendezvous from
// hereon)
//
class TORCH_API ProcessGroup : public torch::CustomClassHolder {
 public:
  struct TORCH_API MergeOptions : torch::CustomClassHolder {
    explicit MergeOptions(
        const std::chrono::milliseconds timeout = kProcessGroupDefaultTimeout,
        const std::optional<std::string> group_name = std::nullopt,
        const std::optional<std::string> group_desc = std::nullopt)
        : timeout(timeout), group_name(group_name), group_desc(group_desc) {}
    ~MergeOptions() override = default;
    MergeOptions(const MergeOptions&) = delete;
    MergeOptions& operator=(const MergeOptions&) = delete;

    std::chrono::milliseconds timeout;
    std::optional<std::string> group_name;
    std::optional<std::string> group_desc;
  };

  enum BackendType : uint8_t {
    UNDEFINED = 0,
    GLOO = 1,
    NCCL = 2,
    UCC = 3,
    MPI = 4,
    XCCL = 5,
    CUSTOM = 6,
  };

  static std::string backendTypeToString(const BackendType& type) {
    switch (type) {
      case BackendType::GLOO:
        return "gloo";
      case BackendType::NCCL:
        return "nccl";
      case BackendType::XCCL:
        return "xccl";
      case BackendType::UCC:
        return "ucc";
      case BackendType::MPI:
        return "mpi";
      case BackendType::UNDEFINED:
        return "undefined";
      case BackendType::CUSTOM:
        return "custom";
      default:
        TORCH_CHECK(false, "THis should never happen!");
    }
  }

  static BackendType strToBackendType(const std::string& backend) {
    if (backend == "undefined") {
      return BackendType::UNDEFINED;
    } else if (backend == "gloo") {
      return BackendType::GLOO;
    } else if (backend == "nccl") {
      return BackendType::NCCL;
    } else if (backend == "xccl") {
      return BackendType::XCCL;
    } else if (backend == "ucc") {
      return BackendType::UCC;
    } else if (backend == "mpi") {
      return BackendType::MPI;
    } else {
      return BackendType::CUSTOM;
    }
  }

  // Not used, set for backwards compatibility and only used for TypeDef in
  // Ops.cpp
  explicit ProcessGroup(int rank, int size);

  explicit ProcessGroup(
      c10::intrusive_ptr<::c10d::Store> store,
      int rank,
      int size);
  ~ProcessGroup() override;

  virtual int getRank() const {
    return rank_;
  }

  virtual int getSize() const {
    return size_;
  }

  // Returns an unique opaque ID of this process group object.
  int64_t getID() const {
    return reinterpret_cast<std::intptr_t>(this);
  }

  // Returns an unique opaque ID of a backend for the specific backend type
  // that can correlate with this process group's collectives.
  int64_t getBackendID(BackendType backend_type) const {
    return reinterpret_cast<std::intptr_t>(getBackend(backend_type).get());
  }

  virtual const std::string getBackendName() const {
    return backendTypeToString(backendType_);
  }

  BackendType getBackendType() const {
    return backendType_;
  }

  inline bool backendSupportsSequenceNumbers(BackendType backendType) {
    if (backendType == BackendType::GLOO || backendType == BackendType::NCCL ||
        backendType == BackendType::XCCL || backendType == BackendType::UCC)
      return true;
    return false;
  }

  virtual void setTimeout(std::chrono::milliseconds timeout) {
    for (auto& backend : backendTypeToBackend_) {
      backend.second->setTimeout(timeout);
    }
  }

  int64_t incrementSplitCount() {
    return splitCounter_++;
  }

  virtual void startCoalescing(c10::DeviceType deviceType) {
    // only nccl has implemented startCoalescing so only execute for nccl
    // backends
    auto backend = getBackend(deviceType);
    backend->startCoalescing();
  }

  virtual c10::intrusive_ptr<Work> endCoalescing(c10::DeviceType deviceType) {
    // only nccl has implemented endCoalescing so only execute for nccl
    // backends
    auto backend = getBackend(deviceType);
    auto work = backend->endCoalescing();
    return work;
  }

  virtual c10::intrusive_ptr<Work> broadcast(
      std::vector<at::Tensor>& tensors,
      const BroadcastOptions& opts = BroadcastOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::broadcast_", "")
            .typed<
                std::tuple<std::vector<at::Tensor>, c10::intrusive_ptr<Work>>(
                    at::TensorList,
                    const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                    int64_t,
                    int64_t,
                    bool,
                    int64_t)>();
    // It's awakward to unbox the opts here and box them again in the custom C++
    // op. But it's also complicated to make opts as a CustomClassHolder. Leave
    // it as it is now.
    auto work = std::get<1>(op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.rootRank,
        opts.rootTensor,
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> allreduce(
      std::vector<at::Tensor>& tensors,
      const AllreduceOptions& opts = AllreduceOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::allreduce_", "")
            .typed<
                std::tuple<std::vector<at::Tensor>, c10::intrusive_ptr<Work>>(
                    at::TensorList,
                    const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                    const c10::intrusive_ptr<::c10d::ReduceOp>&,
                    const std::optional<at::Tensor>& sparse_indices,
                    bool,
                    int64_t)>();

    auto work = std::get<1>(op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<ReduceOp>(opts.reduceOp),
        opts.sparseIndices,
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> allreduce_coalesced(
      std::vector<at::Tensor>& tensors,
      const AllreduceCoalescedOptions& opts = AllreduceCoalescedOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::allreduce_coalesced_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             const c10::intrusive_ptr<::c10d::ReduceOp>&,
                             bool,
                             int64_t)>();

    auto work = op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<ReduceOp>(opts.reduceOp),
        opts.asyncOp,
        opts.timeout.count());

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> reduce(
      std::vector<at::Tensor>& tensors,
      const ReduceOptions& opts = ReduceOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::reduce_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             const c10::intrusive_ptr<::c10d::ReduceOp>&,
                             int64_t,
                             int64_t,
                             bool,
                             int64_t)>();
    auto work = op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<ReduceOp>(opts.reduceOp),
        opts.rootRank,
        opts.rootTensor,
        opts.asyncOp,
        opts.timeout.count());

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> allgather(
      std::vector<std::vector<at::Tensor>>& outputTensors,
      std::vector<at::Tensor>& inputTensors,
      const AllgatherOptions& opts = AllgatherOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::allgather_", "")
                         .typed<std::tuple<
                             std::vector<std::vector<at::Tensor>>,
                             c10::intrusive_ptr<Work>>(
                             const std::vector<std::vector<at::Tensor>>&,
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             bool,
                             int64_t)>();

    auto work = std::get<1>(op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor_list : outputTensors) {
        for (const auto& tensor : tensor_list) {
          c10d::register_work(tensor, work);
        }
      }
    }
    return work;
  }

  // Gathers a single tensor inputBuffer into a single buffer outputBuffer that
  // is interpreted as a contiguous collection of size inputBuffer * WORLD_SIZE.
  // For implementers of ProcessGroup API and advanced users only.
  // Note: this function will be deprecated in near future.
  virtual c10::intrusive_ptr<Work> _allgather_base(
      at::Tensor& outputBuffer,
      at::Tensor& inputBuffer,
      const AllgatherOptions& opts = AllgatherOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::_allgather_base_", "")
            .typed<std::tuple<at::Tensor, c10::intrusive_ptr<Work>>(
                at::Tensor&,
                at::Tensor&,
                const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                bool,
                int64_t)>();

    auto work = std::get<1>(op.call(
        outputBuffer,
        inputBuffer,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      c10d::register_work(outputBuffer, work);
    }
    return work;
  }

  // This function is deprecated and will be moved out of ProcessGroup to comms:
  // * do not add dependencies on this function,
  // * do not implement it in your ProcessGroup, implement _allgather_base
  //   instead.
  virtual c10::intrusive_ptr<Work> allgather_coalesced(
      std::vector<std::vector<at::Tensor>>& outputTensorLists,
      std::vector<at::Tensor>& inputTensors,
      const AllgatherOptions& opts = AllgatherOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::allgather_coalesced_", "")
                         .typed<c10::intrusive_ptr<Work>(
                             const std::vector<std::vector<at::Tensor>>&,
                             const at::TensorList&,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             bool)>();

    auto work = op.call(
        outputTensorLists,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.asyncOp);

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor_list : outputTensorLists) {
        for (const auto& tensor : tensor_list) {
          c10d::register_work(tensor, work);
        }
      }
    }
    return work;
  }

  // This function is a coalesced version of `allgather_into_tensor` (currently
  // still named as `_allgather_base`). Each tensor in the vector corresponds to
  // an input/output of one `allgather_into_tensor` operation.
  virtual c10::intrusive_ptr<Work> allgather_into_tensor_coalesced(
      std::vector<at::Tensor>& outputTensors,
      std::vector<at::Tensor>& inputTensors,
      const AllgatherOptions& opts = AllgatherOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::allgather_into_tensor_coalesced_", "")
            .typed<c10::intrusive_ptr<Work>(
                const at::TensorList,
                const at::TensorList,
                const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                bool)>();

    auto work = op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.asyncOp);

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : outputTensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> gather(
      std::vector<std::vector<at::Tensor>>& outputTensors,
      std::vector<at::Tensor>& inputTensors,
      const GatherOptions& opts = GatherOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::gather_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             const std::vector<std::vector<at::Tensor>>&,
                             const at::TensorList&,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             int64_t,
                             bool,
                             int64_t)>();
    auto work = op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.rootRank,
        opts.asyncOp,
        opts.timeout.count());

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor_list : outputTensors) {
        for (const auto& tensor : tensor_list) {
          c10d::register_work(tensor, work);
        }
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> scatter(
      std::vector<at::Tensor>& outputTensors,
      std::vector<std::vector<at::Tensor>>& inputTensors,
      const ScatterOptions& opts = ScatterOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::scatter_", "")
            .typed<
                std::tuple<std::vector<at::Tensor>, c10::intrusive_ptr<Work>>(
                    const at::TensorList&,
                    const std::vector<std::vector<at::Tensor>>&,
                    const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                    int64_t,
                    bool,
                    int64_t)>();
    auto work = std::get<1>(op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.rootRank,
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : outputTensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> reduce_scatter(
      std::vector<at::Tensor>& outputTensors,
      std::vector<std::vector<at::Tensor>>& inputTensors,
      const ReduceScatterOptions& opts = ReduceScatterOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::reduce_scatter_", "")
            .typed<
                std::tuple<std::vector<at::Tensor>, c10::intrusive_ptr<Work>>(
                    const at::TensorList&,
                    const std::vector<std::vector<at::Tensor>>&,
                    const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                    const c10::intrusive_ptr<::c10d::ReduceOp>&,
                    bool,
                    int64_t)>();
    auto work = std::get<1>(op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<::c10d::ReduceOp>(opts.reduceOp),
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : outputTensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> _reduce_scatter_base(
      at::Tensor& outputBuffer,
      at::Tensor& inputBuffer,
      const ReduceScatterOptions& opts = ReduceScatterOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::_reduce_scatter_base_", "")
            .typed<std::tuple<at::Tensor, c10::intrusive_ptr<Work>>(
                at::Tensor&,
                at::Tensor&,
                const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                const c10::intrusive_ptr<::c10d::ReduceOp>&,
                bool,
                int64_t)>();
    auto work = std::get<1>(op.call(
        outputBuffer,
        inputBuffer,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<::c10d::ReduceOp>(opts.reduceOp),
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      c10d::register_work(outputBuffer, work);
    }
    return work;
  }

  // This function is a coalesced version of `reduce_scatter_tensor` (currently
  // still named as `_reduce_scatter_base`). Each tensor in the vector
  // corresponds to an input/output of one `reduce_scatter_tensor` operation.
  virtual c10::intrusive_ptr<Work> reduce_scatter_tensor_coalesced(
      std::vector<at::Tensor>& outputTensors,
      std::vector<at::Tensor>& inputTensors,
      const ReduceScatterOptions& opts = ReduceScatterOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::reduce_scatter_tensor_coalesced_", "")
            .typed<c10::intrusive_ptr<Work>(
                const at::TensorList,
                const at::TensorList,
                const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                const c10::intrusive_ptr<::c10d::ReduceOp>&,
                bool,
                int64_t)>();

    auto work = op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        c10::make_intrusive<::c10d::ReduceOp>(opts.reduceOp),
        opts.asyncOp,
        opts.timeout.count());

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : outputTensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> alltoall_base(
      at::Tensor& outputBuffer,
      at::Tensor& inputBuffer,
      std::vector<int64_t>& outputSplitSizes,
      std::vector<int64_t>& inputSplitSizes,
      const AllToAllOptions& opts = AllToAllOptions()) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::alltoall_base_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::Tensor&,
                             at::Tensor&,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             std::vector<int64_t>,
                             std::vector<int64_t>,
                             bool,
                             int64_t)>();
    auto work = op.call(
        outputBuffer,
        inputBuffer,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        outputSplitSizes,
        inputSplitSizes,
        opts.asyncOp,
        opts.timeout.count());

    if (c10d::allow_inflight_collective_as_graph_input()) {
      c10d::register_work(outputBuffer, work);
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> alltoall(
      std::vector<at::Tensor>& outputTensors,
      std::vector<at::Tensor>& inputTensors,
      const AllToAllOptions& opts = AllToAllOptions()) {
    static auto op =
        c10::Dispatcher::singleton()
            .findSchemaOrThrow("c10d::alltoall_", "")
            .typed<
                std::tuple<std::vector<at::Tensor>, c10::intrusive_ptr<Work>>(
                    const at::TensorList&,
                    const at::TensorList&,
                    const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                    bool,
                    int64_t)>();
    auto work = std::get<1>(op.call(
        outputTensors,
        inputTensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.asyncOp,
        opts.timeout.count()));

    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : outputTensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual void monitoredBarrier(
      const BarrierOptions& opts,
      bool wait_all_ranks = false) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::monitored_barrier_", "")
                         .typed<void(
                             at::Tensor,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             const std::vector<int64_t>&,
                             int64_t,
                             bool)>();
    // Default to using cpu implementation, monitored barrier is only for GLOO
    at::Tensor tensor = at::empty({0}, at::TensorOptions().device(at::kCPU));
    op.call(
        tensor,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.device_ids,
        opts.timeout.count(),
        wait_all_ranks);
  }

  // Agrees on an initial sequence number for the whole group by having rank 0
  // create it and broadcast it to other ranks using the store. Only implemented
  // for GLOO and NCCL backends currently.
  virtual void setSequenceNumberForGroup() {
    auto backendType = getBackendType();
    // TODO: HACK for backend name to get sequence number for that backend.
    if (backendSupportsSequenceNumbers(backendType)) {
      getDefaultBackend()->setSequenceNumberForGroup();
    } else {
      TORCH_CHECK(
          false,
          c10::str(
              "ProcessGroup ",
              getBackendName(),
              " does not yet support sequence numbers."));
    }
  }

  // Retrieves the current sequence number for the whole group, which should be
  // in sync. If the returned number is not consistent across the group, it
  // may indicate that there is some sort of collective desynchronization.
  virtual uint64_t getSequenceNumberForGroup() {
    auto backendType = getBackendType();

    // TODO: HACK for backend name to get sequence number for that backend.
    if (backendSupportsSequenceNumbers(backendType)) {
      return getDefaultBackend()->getSequenceNumberForGroup();
    } else {
      TORCH_CHECK(
          false,
          c10::str(
              "ProcessGroup ",
              getBackendName(),
              " does not yet support sequence numbers."));
    }
  }

  virtual c10::intrusive_ptr<Work> send(
      std::vector<at::Tensor>& tensors,
      int dstRank,
      int tag) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::send", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             int64_t,
                             int64_t)>();
    auto work = op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        dstRank,
        tag);
    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> recv(
      std::vector<at::Tensor>& tensors,
      int srcRank,
      int tag) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::recv_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             int64_t,
                             int64_t)>();
    auto work = op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        srcRank,
        tag);
    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> recvAnysource(
      std::vector<at::Tensor>& tensors,
      int tag) {
    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::recv_any_source_", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::TensorList,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             int64_t)>();
    auto work = op.call(
        tensors,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        tag);
    if (c10d::allow_inflight_collective_as_graph_input()) {
      for (const auto& tensor : tensors) {
        c10d::register_work(tensor, work);
      }
    }
    return work;
  }

  virtual c10::intrusive_ptr<Work> barrier(
      const BarrierOptions& opts = BarrierOptions()) {
    static at::Tensor tensor;
    // TODO: if nccl was specified then use it
    auto device = opts.device;
    if (device.has_value()) {
      // set device tensor from argument
      tensor = at::empty(
          {1}, at::TensorOptions().device(device.value()).dtype(at::kByte));
    } else if (backendType_ == c10d::ProcessGroup::BackendType::NCCL) {
      // set cuda tensor
      tensor = at::empty(
          {1},
          at::TensorOptions().device(at::DeviceType::CUDA).dtype(at::kByte));
    } else if (backendType_ == c10d::ProcessGroup::BackendType::XCCL) {
      // set xpu tensor for override cpu dispatch
      tensor = at::empty(
          {1},
          at::TensorOptions().device(at::DeviceType::XPU).dtype(at::kByte));
    } else {
      // Default to using cpu implementation
      tensor = at::empty(
          {1},
          at::TensorOptions().device(at::DeviceType::CPU).dtype(at::kByte));
    }

    static auto op = c10::Dispatcher::singleton()
                         .findSchemaOrThrow("c10d::barrier", "")
                         .typed<c10::intrusive_ptr<::c10d::Work>(
                             at::Tensor,
                             const c10::intrusive_ptr<::c10d::ProcessGroup>&,
                             const std::vector<int64_t>&,
                             bool,
                             int64_t)>();

    auto work = op.call(
        tensor,
        c10::intrusive_ptr<ProcessGroup>::unsafe_reclaim_from_nonowning(this),
        opts.device_ids,
        opts.asyncOp,
        opts.timeout.count());
    if (c10d::allow_inflight_collective_as_graph_input()) {
      c10d::register_work(tensor, work);
    }
    return work;
  }

  bool hasBackends() {
    return !deviceTypeToBackendType_.empty();
  }

  void setBackend(
      c10::DeviceType deviceType,
      BackendType backendType,
      const std::optional<c10::intrusive_ptr<Backend>>& backend) {
    // TODO: should we add these entries after the backend setting succeeds?
    deviceTypeToBackendType_[deviceType] = backendType;
    deviceTypes_.insert(deviceType);
    // if the backendType is already set then reuse it for this device
    if (backendTypeToBackend_.find(backendType) !=
        backendTypeToBackend_.end()) {
      auto existingBackend = backendTypeToBackend_.at(backendType);
      deviceTypeToBackend_[deviceType] = existingBackend;
      TORCH_CHECK(
          existingBackend->getBoundDeviceId() ==
          (*backend)->getBoundDeviceId());
    } else {
      // check if backend has value
      if (backend.has_value()) {
        deviceTypeToBackend_[deviceType] = backend.value();
        backendTypeToBackend_[backendType] = backend.value();
        (*backend)->setBoundDeviceId(bound_device_id_);
      }
    }
  }

  c10::intrusive_ptr<Backend> getDefaultBackend() const {
    auto backend_iter = backendTypeToBackend_.find(backendType_);
    TORCH_CHECK(
        backend_iter != backendTypeToBackend_.end(),
        "Could not find the default backend type ",
        uint16_t(backendType_),
        " for Process Group with name ",
        getBackendName(),
        ".");
    return backend_iter->second;
  }

  void setDefaultBackend(const BackendType& backendType) {
    backendType_ = backendType;
  }

  void setDefaultBackend(const std::string& backend) {
    backendType_ = strToBackendType(backend);
  }

  c10::intrusive_ptr<Backend> getBackend(c10::DeviceType deviceType);

  c10::intrusive_ptr<Backend> getBackend(BackendType backendType) const {
    TORCH_CHECK(
        backendTypeToBackend_.find(backendType) != backendTypeToBackend_.end(),
        "Could not find backend type ",
        uint16_t(backendType),
        " for Process Group with name ",
        backendTypeToString(backendType),
        ".");
    return backendTypeToBackend_.at(backendType);
  }

  // Return device types supported by this ProcessGroup.
  // Note: the return type is `Device` rather than `DeviceType` for the purpose
  // of easy comparison at Python level. The `Device` will have default index
  // (-1).
  std::vector<c10::Device> getDeviceTypes() const {
    std::vector<c10::Device> devices;
    devices.reserve(deviceTypes_.size());
    for (auto& dt : deviceTypes_) {
      devices.emplace_back(dt);
    }
    return devices;
  }

  void registerOnCompletionHook(
      std::function<void(std::shared_ptr<WorkInfo>)>&& hook) {
    getDefaultBackend()->registerOnCompletionHook(std::move(hook));
  }

  void waitForPendingWorks() {
    getDefaultBackend()->waitForPendingWorks();
  }

  virtual void shutdown() {
    for (auto& backend : backendTypeToBackend_) {
      backend.second->shutdown();
    }
  }

  virtual void abort() {
    for (auto& backend : backendTypeToBackend_) {
      backend.second->abort();
    }
  }

  bool hasHooks() const {
    auto backend_iter = backendTypeToBackend_.find(backendType_);
    if (backend_iter == backendTypeToBackend_.end()) {
      TORCH_WARN(
          "No backend of type ",
          uint16_t(backendType_),
          " found for Process Group with name ",
          getBackendName(),
          ". Assuming no hooks are registered.");
      return false;
    }

    return backend_iter->second->hasHooks();
  }

  virtual const std::string& getGroupName() const;
  virtual void setGroupName(const std::string& name);
  virtual const std::string& getGroupDesc() const;
  virtual void setGroupDesc(const std::string& name);
  void enableCollectivesTiming();

  void release_resources() override;

  // ProcessGroups optionally can be "bound" to a specific device.
  // Currently this is only for nccl and allows for some opt-in
  // optimizations such as automatic use of ncclCommSplit.  The device
  // is specified in `init_process_group` and eventually makes it
  // here and then down into the actual backend instances.
  std::optional<at::Device> getBoundDeviceId() const {
    return bound_device_id_;
  }

  c10::intrusive_ptr<c10d::Store> getStore() const {
    return store_;
  }

  void setBoundDeviceId(std::optional<at::Device> device) {
    if (device) {
      TORCH_CHECK(device->has_index(), "setBoundDeviceId must have an index");
    }
    bound_device_id_ = device;
  }

  // This creates a new subgroup using the specified ranks.
  // The current rank must be included in the list of new_ranks.
  virtual c10::intrusive_ptr<ProcessGroup> splitGroup(
      const std::vector<int>& ranks,
      const std::optional<std::chrono::milliseconds>& timeout,
      const std::optional<c10::intrusive_ptr<Backend::Options>>& opts,
      const std::optional<std::string>& name,
      const std::optional<std::string>& groupDesc);

  // This creates a new subgroup using the specified ranks.
  // The current rank must be included in the list of new_ranks.
  virtual c10::intrusive_ptr<ProcessGroup> mergeRemoteGroup(
      const c10::intrusive_ptr<Store>& store,
      const MergeOptions& opts,
      const int& size);

 protected:
  // Implementations of this interface need to call this to setup
  // appropriate logging etc.
  void init();

  c10::intrusive_ptr<c10d::Store> store_;
  // NOLINTNEXTLINE(cppcoreguidelines-avoid-const-or-ref-data-members)
  const int rank_;
  // NOLINTNEXTLINE(cppcoreguidelines-avoid-const-or-ref-data-members)
  const int size_;
  // NOLINTNEXTLINE(cppcoreguidelines-avoid-const-or-ref-data-members)
  BackendType backendType_;
  std::string pg_desc_;
  int64_t splitCounter_;

  // Debug level setting. It is parsed once when ProcessGroup is constructed and
  // remains the same across use of this process group.
  DebugLevel dist_debug_level_{DebugLevel::Off};

  // Backend classes for this ProcessGroup
  std::unordered_set<c10::DeviceType> deviceTypes_;
  // This mapping is ordered, as splitGroup must call split on the underlying
  // backends in a consistent order.
  std::map<c10::DeviceType, BackendType> deviceTypeToBackendType_;
  std::unordered_map<c10::DeviceType, c10::intrusive_ptr<Backend>>
      deviceTypeToBackend_;
  std::unordered_map<BackendType, c10::intrusive_ptr<Backend>>
      backendTypeToBackend_;

  std::optional<at::Device> bound_device_id_;
};

// Thread local functions for managing the currently active process group.
TORCH_API c10::intrusive_ptr<ProcessGroup>& currentProcessGroup();
TORCH_API void setProcessGroup(c10::intrusive_ptr<ProcessGroup> processGroup);

} // namespace c10d