/*
 * Copyright (c) 2023, Alibaba Group Holding Limited;
 *
 * 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.
 */

#ifndef ASYNC_SIMPLE_CORO_LATCH_H
#define ASYNC_SIMPLE_CORO_LATCH_H

#include <chrono>
#include <cstddef>

#include "async_simple/coro/ConditionVariable.h"
#include "async_simple/coro/Lazy.h"
#include "async_simple/coro/Mutex.h"
#include "async_simple/coro/SpinLock.h"

namespace async_simple::coro {

// The latch class is a downward counter of type std::size_t which can be
// used to synchronize coroutines. The value of the counter is initialized on
// creation. Coroutines may block on the latch until the counter is decremented
// to zero. It will suspend the current coroutine and switch to other coroutines
// to run.
// There is no possibility to increase or reset the counter, which
// makes the latch a single-use barrier.
class Latch {
 public:
  explicit Latch(std::size_t count) : count_(count) {}
  ~Latch() = default;
  Latch(const Latch&) = delete;
  Latch& operator=(const Latch&) = delete;

  // decrements the counter in a non-blocking manner
  Lazy<void> count_down(std::size_t update = 1) {
    auto lk = co_await mutex_.coScopedLock();
    assert(count_ >= update);
    count_ -= update;
    if (!count_) {
      cv_.notify();
    }
  }

  // tests if the internal counter equals zero
  Lazy<bool> try_wait() const noexcept {
    auto lk = co_await mutex_.coScopedLock();
    co_return !count_;
  }

  // blocks until the counter reaches zero
  // If the counter is not 0, the current coroutine will be suspended
  Lazy<void> wait() const noexcept {
    auto lk = co_await mutex_.coScopedLock();
    co_await cv_.wait(mutex_, [&] {
      return count_ == 0;
    });
  }

  // decrements the counter and blocks until it reaches zero
  Lazy<void> arrive_and_wait(std::size_t update = 1) noexcept {
    co_await count_down(update);
    co_await wait();
  }

 private:
  using MutexType = SpinLock;

  mutable MutexType mutex_;
  mutable ConditionVariable<MutexType> cv_;
  std::size_t count_;
};

}  // namespace async_simple::coro

#endif
