// Copyright Yahoo. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.concurrent;

import java.time.Clock;
import java.time.Instant;
import java.util.concurrent.TimeUnit;

/**
 * This interface wraps access to some timer that can be used to measure elapsed time, in milliseconds. This
 * abstraction allows for unit testing the behavior of time-based constructs.
 *
 * @author Simon Thoresen Hult
 */
public interface Timer {

    /**
     * Returns the current value of some arbitrary timer, in milliseconds. This method can only be used to measure
     * elapsed time and is not related to any other notion of system or wall-clock time.
     *
     * @return The current value of the timer, in milliseconds.
     */
    long milliTime();
    Timer monotonic = () -> TimeUnit.NANOSECONDS.toMillis(System.nanoTime());
    static Timer wrap(Clock original) {
        return new Timer() {
            private final Clock clock = original;

            @Override
            public long milliTime() {
                return clock.millis();
            }
        }; }

    default Instant instant() { return Instant.ofEpochMilli(milliTime()); }
}