HitCounter.java

package com.tpoi.monitoring.service;

import java.time.Instant;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.stream.IntStream;
import java.util.stream.Stream;

/**
 * HitCounter count the number of request in a specific range of time.
 */
public class HitCounter {
  // hits, array where each row is the number of hits for this second,
  // we use AtomicInteger because it is thread safe.
  private final AtomicInteger[] hits;
  // times, array where each row is a timestamp, it is here to keep track
  // of which timestamp has already been in the counter in the last X seconds.
  private final long[] times;
  // rangeInSecond, the number of seconds we need to keep.
  private final int rangeInSecond;

  /**
   * Constructor for a HitCounter
   *
   * @param rangeInSecond, number of seconds to keep
   */
  public HitCounter(final int rangeInSecond) {
    this.rangeInSecond = rangeInSecond;
    this.times = new long[rangeInSecond];

    // Init the array with an AtomicInteger of 0.
    this.hits = Stream.generate(() -> new AtomicInteger(0))
        .limit(rangeInSecond)
        .toArray(AtomicInteger[]::new);
  }

  /**
   * hit, is called for each new request and increment the counter
   * for the second of the request.
   *
   * @param timestamp, the timestamp of the request
   */
  public void hit(final long timestamp) {
    final int idx = Math.toIntExact(timestamp % rangeInSecond);
    if (times[idx] != timestamp) {
      // we should re-initialize the number of request
      hits[idx].set(1);
      times[idx] = timestamp;
      return;
    }
    hits[idx].addAndGet(1);
  }

  /**
   * getHits, compute the number of request in the last X seconds.
   *
   * @return number of request in the last X seconds.
   */
  public long getHits() {
    final var timestamp = Instant.now().getEpochSecond();
    // We filter to get only the hits from the last X seconds.
    return IntStream.range(0, times.length)
        .filter(i -> timestamp - times[i] < this.rangeInSecond)
        .map(i -> hits[i].get())
        .reduce(0, Integer::sum);

  }
}