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+// Copyright (C) 2015-2023 Internet Systems Consortium, Inc. ("ISC")
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this
+// file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+#ifndef STOPWATCH_IMPL_H
+#define STOPWATCH_IMPL_H
+
+#include <boost/date_time/posix_time/posix_time.hpp>
+#include <boost/scoped_ptr.hpp>
+
+namespace isc {
+namespace util {
+
+/// @brief @c Stopwatch class implementation.
+///
+/// The @c Stopwatch class uses the plimpl idiom to make it easier to unit
+/// test behavior of the @c Stopwatch class without a need to rely on the system
+/// clock. The @c StopwatchImpl API allows for overriding the @c getCurrentTime
+/// method to return the arbitrary time value as current time to various
+/// methods. By setting the current time to arbitrary values the test can expect
+/// arbitrary values being returned by the class methods.
+///
+/// Also, by using the pimpl idiom the @c Stopwatch class hides its implementation
+/// details and leaves the header with only the pointer to the @c StopwatchImpl
+/// class.
+class StopwatchImpl {
+public:
+
+    /// @brief Constructor.
+    ///
+    /// Initializes the internally used timestamps. It also sets the state of
+    /// the stopwatch to "stopped".
+    StopwatchImpl();
+
+    /// @brief Virtual destructor.
+    ///
+    /// This destructor is virtual because the @c StopwatchImpl::getCurrentTime
+    /// is virtual.
+    virtual ~StopwatchImpl();
+
+    /// @brief Starts the stopwatch.
+    ///
+    /// Sets the stopwatch to the "started" state. It also records the time when
+    /// the stopwatch is started. This method is no-op if the stopwatch is
+    /// already in the "started" state.
+    ///
+    /// Also see the @c Stopwatch::start for details.
+    void start();
+
+    /// @brief Stop the stopwatch.
+    ///
+    /// Sets the stopwatch to the "stopped" state. The stop time is recorded and
+    /// the cumulative time is updated to include the duration between the most
+    /// recent start and stop. This method is no-op if the stopwatch is already
+    /// in the "stopped" state.
+    ///
+    /// Also see the @c Stopwatch::stop for details.
+    void stop();
+
+    /// @brief Reset the stopwatch.
+    ///
+    /// Also see the @c Stopwatch::reset for details.
+    void reset();
+
+    /// @brief Retrieves the measured duration.
+    ///
+    /// Also see the @c Stopwatch::getLastDuration for details.
+    boost::posix_time::time_duration getLastDuration() const;
+
+    /// @brief Retrieves the total measured duration.
+    ///
+    /// Also see the @c Stopwatch::getTotalDuration for details.
+    boost::posix_time::time_duration getTotalDuration() const;
+
+    /// @brief Returns the duration in the textual format which can be
+    /// directly used in log messages.
+    ///
+    /// @param duration Duration to be converted to the textual format.
+    ///
+    /// @return Converted value which can be used to log duration.
+    static std::string
+    logFormat(const boost::posix_time::time_duration& duration);
+
+protected:
+
+    /// @brief Returns the current time.
+    ///
+    /// This method is used internally by the @c StopwatchImpl class and
+    /// its derivations. This class simply returns the value of
+    /// @c boost::posix_time::microsec_clock::univeral_time(), which is
+    /// a current timestamp. The derivations may replace it with the
+    /// custom implementations. The typical use case is for the unit tests
+    /// to customize the behavior of this function to return well known
+    /// (deterministic) values. As a result, it is possible to influence
+    /// the "measured" values returned by accessors of this class, which
+    /// can be compared against some exact values.
+    virtual boost::posix_time::ptime getCurrentTime() const;
+
+private:
+
+    /// @brief Holds the state of the stopwatch.
+    bool started_;
+
+    /// @brief Holds the timestamp when the stopwatch has been last started.
+    boost::posix_time::ptime last_start_;
+
+    /// @brief Holds the timestamp when the stopwatch has been last stopped.
+    boost::posix_time::ptime last_stop_;
+
+    /// @brief Holds the total measured time since the stopwatch has been
+    /// first started after creation or reset.
+    boost::posix_time::time_duration cumulative_time_;
+
+};
+
+}
+}
+
+#endif // STOPWATCH_H
+