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// Copyright 2017 The Abseil Authors.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// -----------------------------------------------------------------------------
// blocking_counter.h
// -----------------------------------------------------------------------------
#include "absl/base/thread_annotations.h"
#include "absl/synchronization/mutex.h"
namespace absl {
inline namespace lts_2018_12_18 {
// BlockingCounter
// This class allows a thread to block for a pre-specified number of actions.
// `BlockingCounter` maintains a single non-negative abstract integer "count"
// with an initial value `initial_count`. A thread can then call `Wait()` on
// this blocking counter to block until the specified number of events occur;
// worker threads then call 'DecrementCount()` on the counter upon completion of
// their work. Once the counter's internal "count" reaches zero, the blocked
// thread unblocks.
// A `BlockingCounter` requires the following:
// - its `initial_count` is non-negative.
// - the number of calls to `DecrementCount()` on it is at most
// `initial_count`.
// - `Wait()` is called at most once on it.
// Given the above requirements, a `BlockingCounter` provides the following
// guarantees:
// - Once its internal "count" reaches zero, no legal action on the object
// can further change the value of "count".
// - When `Wait()` returns, it is legal to destroy the `BlockingCounter`.
// - When `Wait()` returns, the number of calls to `DecrementCount()` on
// this blocking counter exactly equals `initial_count`.
// Example:
// BlockingCounter bcount(N); // there are N items of work
// ... Allow worker threads to start.
// ... On completing each work item, workers do:
// ... bcount.DecrementCount(); // an item of work has been completed
// bcount.Wait(); // wait for all work to be complete
class BlockingCounter {
explicit BlockingCounter(int initial_count)
: count_(initial_count), num_waiting_(0) {}
BlockingCounter(const BlockingCounter&) = delete;
BlockingCounter& operator=(const BlockingCounter&) = delete;
// BlockingCounter::DecrementCount()
// Decrements the counter's "count" by one, and return "count == 0". This
// function requires that "count != 0" when it is called.
// Memory ordering: For any threads X and Y, any action taken by X
// before it calls `DecrementCount()` is visible to thread Y after
// Y's call to `DecrementCount()`, provided Y's call returns `true`.
bool DecrementCount();
// BlockingCounter::Wait()
// Blocks until the counter reaches zero. This function may be called at most
// once. On return, `DecrementCount()` will have been called "initial_count"
// times and the blocking counter may be destroyed.
// Memory ordering: For any threads X and Y, any action taken by X
// before X calls `DecrementCount()` is visible to Y after Y returns
// from `Wait()`.
void Wait();
Mutex lock_;
int count_ GUARDED_BY(lock_);
int num_waiting_ GUARDED_BY(lock_);
} // inline namespace lts_2018_12_18
} // namespace absl