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+/*
+ * Copyright 2019 The libgav1 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
+ *
+ *      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 LIBGAV1_SRC_UTILS_BLOCKING_COUNTER_H_
+#define LIBGAV1_SRC_UTILS_BLOCKING_COUNTER_H_
+
+#include <cassert>
+#include <condition_variable>  // NOLINT (unapproved c++11 header)
+#include <mutex>               // NOLINT (unapproved c++11 header)
+
+#include "src/utils/compiler_attributes.h"
+
+namespace libgav1 {
+
+// Implementation of a Blocking Counter that is used for the "fork-join"
+// use case. Typical usage would be as follows:
+//   BlockingCounter counter(num_jobs);
+//     - spawn the jobs.
+//     - call counter.Wait() on the master thread.
+//     - worker threads will call counter.Decrement().
+//     - master thread will return from counter.Wait() when all workers are
+//     complete.
+template <bool has_failure_status>
+class BlockingCounterImpl {
+ public:
+  explicit BlockingCounterImpl(int initial_count)
+      : count_(initial_count), job_failed_(false) {}
+
+  // Increment the counter by |count|. This must be called before Wait() is
+  // called. This must be called from the same thread that will call Wait().
+  void IncrementBy(int count) {
+    assert(count >= 0);
+    std::unique_lock<std::mutex> lock(mutex_);
+    count_ += count;
+  }
+
+  // Decrement the counter by 1. This function can be called only when
+  // |has_failure_status| is false (i.e.) when this class is being used with the
+  // |BlockingCounter| alias.
+  void Decrement() {
+    static_assert(!has_failure_status, "");
+    std::unique_lock<std::mutex> lock(mutex_);
+    if (--count_ == 0) {
+      condition_.notify_one();
+    }
+  }
+
+  // Decrement the counter by 1. This function can be called only when
+  // |has_failure_status| is true (i.e.) when this class is being used with the
+  // |BlockingCounterWithStatus| alias. |job_succeeded| is used to update the
+  // state of |job_failed_|.
+  void Decrement(bool job_succeeded) {
+    static_assert(has_failure_status, "");
+    std::unique_lock<std::mutex> lock(mutex_);
+    job_failed_ |= !job_succeeded;
+    if (--count_ == 0) {
+      condition_.notify_one();
+    }
+  }
+
+  // Block until the counter becomes 0. This function can be called only once
+  // per object. If |has_failure_status| is true, true is returned if all the
+  // jobs succeeded and false is returned if any of the jobs failed. If
+  // |has_failure_status| is false, this function always returns true.
+  bool Wait() {
+    std::unique_lock<std::mutex> lock(mutex_);
+    condition_.wait(lock, [this]() { return count_ == 0; });
+    // If |has_failure_status| is false, we simply return true.
+    return has_failure_status ? !job_failed_ : true;
+  }
+
+ private:
+  std::mutex mutex_;
+  std::condition_variable condition_;
+  int count_ LIBGAV1_GUARDED_BY(mutex_);
+  bool job_failed_ LIBGAV1_GUARDED_BY(mutex_);
+};
+
+using BlockingCounterWithStatus = BlockingCounterImpl<true>;
+using BlockingCounter = BlockingCounterImpl<false>;
+
+}  // namespace libgav1
+
+#endif  // LIBGAV1_SRC_UTILS_BLOCKING_COUNTER_H_