/**
 *    Copyright (C) 2018-present MongoDB, Inc.
 *
 *    This program is free software: you can redistribute it and/or modify
 *    it under the terms of the Server Side Public License, version 1,
 *    as published by MongoDB, Inc.
 *
 *    This program is distributed in the hope that it will be useful,
 *    but WITHOUT ANY WARRANTY; without even the implied warranty of
 *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *    Server Side Public License for more details.
 *
 *    You should have received a copy of the Server Side Public License
 *    along with this program. If not, see
 *    <http://www.mongodb.com/licensing/server-side-public-license>.
 *
 *    As a special exception, the copyright holders give permission to link the
 *    code of portions of this program with the OpenSSL library under certain
 *    conditions as described in each individual source file and distribute
 *    linked combinations including the program with the OpenSSL library. You
 *    must comply with the Server Side Public License in all respects for
 *    all of the code used other than as permitted herein. If you modify file(s)
 *    with this exception, you may extend this exception to your version of the
 *    file(s), but you are not obligated to do so. If you do not wish to do so,
 *    delete this exception statement from your version. If you delete this
 *    exception statement from all source files in the program, then also delete
 *    it in the license file.
 */

#ifndef DB_STATS_FINE_CLOCK_HEADER
#define DB_STATS_FINE_CLOCK_HEADER

#include <ctime>  // struct timespec

namespace mongo {

/**
 * This is a nano-second precision clock. We're skipping the
 * harware TSC in favor of clock_gettime() which in some systems
 * does not involve a trip to the OS (VDSO).
 *
 * We're exporting a type WallTime that is and should remain
 * opaque. The business of getting accurate time is still ongoing
 * and we may change the internal representation of this class.
 * (http://lwn.net/Articles/388188/)
 *
 * Really, you shouldn't be using this class in hot code paths for
 * platforms you're not sure whether the overhead is low.
 */
class FineClock {
public:
    typedef timespec WallTime;

    static WallTime now() {
        struct timespec ts;
        clock_gettime(CLOCK_MONOTONIC, &ts);
        return ts;
    }

    static uint64_t diffInNanos(WallTime end, WallTime start) {
        uint64_t diff;
        if (end.tv_nsec < start.tv_nsec) {
            diff = 1000000000 * (end.tv_sec - start.tv_sec - 1);
            diff += 1000000000 + end.tv_nsec - start.tv_nsec;
        } else {
            diff = 1000000000 * (end.tv_sec - start.tv_sec);
            diff += end.tv_nsec - start.tv_nsec;
        }
        return diff;
    }
};
}  // namespace mongo

#endif  // DB_STATS_FINE_CLOCK_HEADER