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+/*
+ * Copyright 2020 Google LLC
+ *
+ * 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 FCP_BASE_SIMULATED_CLOCK_H_
+#define FCP_BASE_SIMULATED_CLOCK_H_
+
+#include "fcp/base/clock.h"
+
+namespace fcp {
+
+/*
+ * A simulated clock is a concrete Clock implementation that does not "tick"
+ * on its own.  Time is advanced by explicit calls to the AdvanceTime() or
+ * SetTime() functions.
+ */
+class SimulatedClock : public Clock {
+ public:
+  // Construct SimulatedClock with a specific initial time.
+  explicit SimulatedClock(absl::Time t) : now_(t) {}
+
+  // Construct SimulatedClock with default initial time (1970-01-01 00:00:00)
+  SimulatedClock() : SimulatedClock(absl::UnixEpoch()) {}
+
+  // Returns the simulated time.
+  absl::Time Now() override;
+
+  // Sleeps until the specified duration has elapsed according to this clock.
+  void Sleep(absl::Duration d) override;
+
+  // Sets the simulated time. Wakes up any waiters whose deadlines have now
+  // expired.
+  void SetTime(absl::Time t);
+
+  // Advances the simulated time. Wakes up any waiters whose deadlines have now
+  // expired.
+  void AdvanceTime(absl::Duration d);
+
+ private:
+  // Returns the simulated time (called internally from the base class).
+  absl::Time NowLocked() override;
+
+  // No specific scheduling is needed for SimulatedClock.
+  void ScheduleWakeup(absl::Time wakeup_time) override {}
+
+  absl::Time now_ ABSL_GUARDED_BY(mutex());
+};
+
+}  // namespace fcp
+
+#endif  // FCP_BASE_SIMULATED_CLOCK_H_