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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
+//
+//      https://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.
+//
+// -----------------------------------------------------------------------------
+// blocking_counter.h
+// -----------------------------------------------------------------------------
+
+#ifndef ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
+#define ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_
+
+#include "absl/base/thread_annotations.h"
+#include "absl/synchronization/mutex.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+
+// 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 {
+ public:
+  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();
+
+ private:
+  Mutex lock_;
+  int count_ ABSL_GUARDED_BY(lock_);
+  int num_waiting_ ABSL_GUARDED_BY(lock_);
+};
+
+ABSL_NAMESPACE_END
+}  // namespace absl
+
+#endif  // ABSL_SYNCHRONIZATION_BLOCKING_COUNTER_H_