/*
 * Copyright 2007-2019 Content Management AG
 * All rights reserved.
 *
 * author: Max Kellermann <mk@cm4all.com>
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the
 * distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
 * FOR A PARTICULAR PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE
 * FOUNDATION OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
 * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
 * (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
 * STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED
 * OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#pragma once

#include <chrono>

/**
 * Cache the now() method of a clock.
 */
template<typename Clock>
class ClockCache {
	using value_type = typename Clock::time_point;
	mutable value_type value;

public:
	ClockCache() = default;
	ClockCache(const ClockCache &) = delete;
	ClockCache &operator=(const ClockCache &) = delete;

	[[gnu::pure]]
	const auto &now() const noexcept {
		if (value <= value_type())
			value = Clock::now();
		return value;
	}

	void flush() noexcept {
		value = {};
	}

	/**
	 * Inject a fake value.  This can be helpful for unit tests.
	 */
	void Mock(value_type _value) noexcept {
		value = _value;
	}
};