// Copyright 2019 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef PLATFORM_IMPL_SOCKET_HANDLE_WAITER_H_
#define PLATFORM_IMPL_SOCKET_HANDLE_WAITER_H_

#include <condition_variable>
#include <functional>
#include <memory>
#include <mutex>
#include <unordered_map>
#include <vector>

#include "platform/api/time.h"
#include "platform/base/error.h"
#include "platform/impl/socket_handle.h"

namespace openscreen {

// The class responsible for calling platform-level method to watch UDP sockets
// for available read data. Reading from these sockets is handled at a higher
// layer.
class SocketHandleWaiter {
 public:
  using SocketHandleRef = std::reference_wrapper<const SocketHandle>;

  // Used to manage what types of events subscribers are subscribed to.
  enum Flags {
    kReadable = 1 << 0,
    kWritable = 1 << 1,
  };

  // Common flag configurations.
  static inline constexpr uint32_t kReadWriteFlags =
      Flags::kReadable | Flags::kWritable;

  class Subscriber {
   public:
    virtual ~Subscriber();

    // Provides a socket handle to the subscriber which has data waiting to be
    // processed.
    virtual void ProcessReadyHandle(SocketHandleRef handle, uint32_t flags) = 0;

    // Method used to optimize event notifications. Generally speaking,
    // sockets are ready for writing very often, causing the network event
    // loop to be really busy -- a select() call may complete as frequently as
    // every few nanoseconds -- so we really only want to be notified that a
    // socket is ready for writing when we actually have something to write.
    //
    // NOTE: this is only used if the subscriber is subscribed to write events.
    virtual bool HasPendingWrite(SocketHandleRef handle) = 0;
  };

  explicit SocketHandleWaiter(ClockNowFunctionPtr now_function);
  SocketHandleWaiter(const SocketHandleWaiter&) = delete;
  SocketHandleWaiter(SocketHandleWaiter&&) noexcept = delete;
  SocketHandleWaiter& operator=(const SocketHandleWaiter&) = delete;
  SocketHandleWaiter& operator=(SocketHandleWaiter&&) = delete;
  virtual ~SocketHandleWaiter();

  // Start notifying `subscriber` whenever `handle` has an event. May be called
  // multiple times, to be notified for multiple handles, but should not be
  // called multiple times for the same handle.
  void Subscribe(Subscriber* subscriber,
                 SocketHandleRef handle,
                 uint32_t flags);

  // Stop receiving notifications for one of the handles currently subscribed
  // to.
  void Unsubscribe(Subscriber* subscriber, SocketHandleRef handle);

  // Stop receiving notifications for all handles currently subscribed to, or
  // no-op if there are no subscriptions.
  void UnsubscribeAll(Subscriber* subscriber);

  // Called when a handle will be deleted to ensure that deletion can proceed
  // safely.
  void OnHandleDeletion(Subscriber* subscriber,
                        SocketHandleRef handle,
                        bool disable_locking_for_testing = false);

  // Gets all socket handles to process, checks them for readable data, and
  // handles any changes that have occurred.
  Error ProcessHandles(Clock::duration timeout);

 protected:
  struct HandleWithFlags {
    SocketHandleRef handle;
    uint32_t flags;
  };

  // Waits until data is available in one of the provided sockets or the
  // provided timeout has passed - whichever is first. If any sockets have data
  // available, they are returned.
  //
  // NOTE: The handle `flags` are checked against the subscriber's
  // HasPendingWrite() method to ensure that the kWritable flag is only passed
  // if there is a pending write before this method is called. The subscriber
  // may be deleted while this method is being invoked, however the handle
  // itself is guaranteed to not be deleted until the invocation of this method
  // has been completed.
  virtual ErrorOr<std::vector<HandleWithFlags>> AwaitSocketsReady(
      const std::vector<HandleWithFlags>& sockets,
      const Clock::duration& timeout) = 0;

 private:
  struct SocketSubscription {
    Subscriber* subscriber = nullptr;
    // Subscribers are only informed of flags that they are interested in.
    uint32_t flags = 0;
    Clock::time_point last_updated = Clock::time_point::min();
  };

  struct HandleWithSubscription {
    HandleWithFlags ready_handle;
    // Reference to the original subscription in the unordered map, so
    // we can keep track of when we updated this socket handle.
    SocketSubscription* subscription;
  };

  // Call the subscriber associated with each changed handle.  Handles are only
  // processed until `timeout` is exceeded.  Must be called with `mutex_` held.
  void ProcessReadyHandles(std::vector<HandleWithSubscription>* handles,
                           Clock::duration timeout);

  // Guards against concurrent access to all other class data members.
  std::mutex mutex_;

  // Blocks deletion of handles until they are no longer being watched.
  std::condition_variable handle_deletion_block_;

  // Set of handles currently being deleted, for ensuring handle_deletion_block_
  // does not exit prematurely.
  std::vector<SocketHandleRef> handles_being_deleted_;

  // Set of all socket handles currently being watched, mapped to the subscriber
  // that is watching them.
  std::unordered_map<SocketHandleRef, SocketSubscription, SocketHandleHash>
      handle_mappings_;

  const ClockNowFunctionPtr now_function_;
};

}  // namespace openscreen

#endif  // PLATFORM_IMPL_SOCKET_HANDLE_WAITER_H_