TemporalFuse.java

import java.time.Duration;

/**
 * Tracks if a given activity has happened more times than the requested threshold
 * within the requested time window.
 *
 * <p>
 *     <b>NOTE</b> this class is <b>not</b> thread-safe. This allows for optimal performance when
 *     concurrent access is not a factor.
 * </p>
 *
 * <p>
 *     For example, to determine if an error has occurred more than 10 times within a
 *     5 second window:
 *     <pre>
 *         TemporalFuse fuse = new TemporalFuse(10, Duration.ofSeconds(5));
 *         .
 *         .
 *         .
 *         if (fuse.tick()) {
 *             log.error("That thing happened to many times.")
 *         }
 *     </pre>
 * </p>
 */
public class TemporalFuse {
    private final long[] eventTimes;
    private final long fuseDuration;

    int pos = 0;

    public TemporalFuse(int fuseCount, Duration fuseDuration) {
        eventTimes = new long[fuseCount];
        this.fuseDuration = fuseDuration.toMillis();
    }

    /**
     * A user of this fuse should call this when the measured activity occurs.
     * @return true if the fuse has gone over the threshold within the fuse duration
     */
    public boolean tick() {
        final long currentTime = System.currentTimeMillis();
        eventTimes[pos++] = currentTime;
        if (pos >= eventTimes.length) {
            pos = 0;
        }

        final long fuseWindowStart = currentTime - fuseDuration;
        for (final long eventTime : eventTimes) {
            if (eventTime < fuseWindowStart) {
                return false;
            }
        }
        return true;
    }
}