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   * Licensed to the Apache Software Foundation (ASF) under one or more
   * contributor license agreements. See the NOTICE file distributed with
   * this work for additional information regarding copyright ownership.
   * The ASF licenses this file to You under the Apache License, Version 2.0
   * (the "License"); you may not use this file except in compliance with
   * the License. You may obtain a copy of the License at
   *
   *    http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
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  package org.apache.kafka.common.metrics.stats;
  
  import org.apache.kafka.common.metrics.MetricConfig;
  import org.apache.kafka.common.metrics.internals.MetricsUtils;
  import org.apache.kafka.common.utils.MockTime;
  import org.apache.kafka.common.utils.Time;
  
  import org.junit.jupiter.api.BeforeEach;
  import org.junit.jupiter.api.Test;
  import org.junit.jupiter.params.ParameterizedTest;
  import org.junit.jupiter.params.provider.CsvSource;
  
  import java.util.Arrays;
  import java.util.List;
  import java.util.concurrent.TimeUnit;
  
  import static java.util.concurrent.TimeUnit.SECONDS;
  import static org.junit.jupiter.api.Assertions.assertEquals;
  import static org.junit.jupiter.api.Assertions.assertFalse;
  import static org.junit.jupiter.api.Assertions.assertTrue;
  
  public class RateTest {
      private static final double EPS = 0.000001;
      private Rate rate;
      private Time time;
  
      @BeforeEach
      public void setup() {
          rate = new Rate();
          time = new MockTime();
      }
  
      // Tests the scenario where the recording and measurement is done before the window for first sample finishes
      // with no prior samples retained.
      @ParameterizedTest
      @CsvSource({"1,1", "1,11", "11,1", "11,11"})
      public void testRateWithNoPriorAvailableSamples(int numSample, int sampleWindowSizeSec) {
          final MetricConfig config = new MetricConfig().samples(numSample).timeWindow(sampleWindowSizeSec, TimeUnit.SECONDS);
          final double sampleValue = 50.0;
          // record at beginning of the window
          rate.record(config, sampleValue, time.milliseconds());
          // forward time till almost the end of window
          final long measurementTime = TimeUnit.SECONDS.toMillis(sampleWindowSizeSec) - 1;
          time.sleep(measurementTime);
          // calculate rate at almost the end of window
          final double observedRate = rate.measure(config, time.milliseconds());
          assertFalse(Double.isNaN(observedRate));
  
          // In a scenario where sufficient number of samples is not available yet, the rate calculation algorithm assumes
          // presence of N-1 (where N = numSample) prior samples with sample values of 0. Hence, the window size for rate
          // calculation accounts for N-1 prior samples
          final int dummyPriorSamplesAssumedByAlgorithm = numSample - 1;
          final double windowSize = MetricsUtils.convert(measurementTime, TimeUnit.SECONDS) + (dummyPriorSamplesAssumedByAlgorithm * sampleWindowSizeSec);
          double expectedRatePerSec = sampleValue / windowSize;
          assertEquals(expectedRatePerSec, observedRate, EPS);
      }
  
      // Record an event every 100 ms on average, moving some 1 ms back or forth for fine-grained 
      // window control. The expected rate, hence, is 10-11 events/sec depending on the moment of 
      // measurement. Start assertions from the second window. This test covers the case where a
      // sample window partially overlaps with the monitored window.
      @Test
      public void testRateIsConsistentAfterTheFirstWindow() {
          MetricConfig config = new MetricConfig().timeWindow(1, SECONDS).samples(2);
          List<Integer> steps = Arrays.asList(0, 99, 100, 100, 100, 100, 100, 100, 100, 100, 100);
  
          // start the first window and record events at 0,99,199,...,999 ms 
          for (int stepMs : steps) {
              time.sleep(stepMs);
              rate.record(config, 1, time.milliseconds());
          }
  
          // making a gap of 100 ms between windows
          time.sleep(101);
  
          // start the second window and record events at 0,99,199,...,999 ms
          for (int stepMs : steps) {
              time.sleep(stepMs);
              rate.record(config, 1, time.milliseconds());
              double observedRate = rate.measure(config, time.milliseconds());
              assertTrue(10 <= observedRate && observedRate <= 11);
              // make sure measurements are repeatable with the same timestamp
              double measuredAgain = rate.measure(config, time.milliseconds());
              assertEquals(observedRate, measuredAgain);
          }
      }
  }
  