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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.
   */
  
  package org.apache.bifromq.basekv.store.stats;
  
  import static org.mockito.ArgumentMatchers.anyMap;
  import static org.mockito.Mockito.doAnswer;
  import static org.testng.Assert.assertEquals;
  
  import org.apache.bifromq.basekv.MockableTest;
  import com.google.common.util.concurrent.AtomicDouble;
  import com.google.common.util.concurrent.MoreExecutors;
  import io.reactivex.rxjava3.observers.TestObserver;
  import java.time.Duration;
  import java.util.Map;
  import lombok.SneakyThrows;
  import org.mockito.Mockito;
  import org.testng.annotations.Test;
  
  public class StatsCollectorTest extends MockableTest {
      @Test
      public void initAndTick() {
          StatsCollector collector = Mockito.mock(StatsCollector.class, Mockito.withSettings()
              .useConstructor(Duration.ofSeconds(1), MoreExecutors.directExecutor())
              .defaultAnswer(Mockito.CALLS_REAL_METHODS));
  
          doAnswer(invocation -> {
              Map<String, Double> map = invocation.getArgument(0);
              map.put("stat1", 0.0);
              return null;
          }).when(collector).scrap(anyMap());
          TestObserver<Map<String, Double>> statsObserver = TestObserver.create();
          collector.collect().subscribe(statsObserver);
          collector.tick();
          statsObserver.awaitCount(1);
          assertEquals(statsObserver.values().get(0).get("stat1"), 0.0);
      }
  
      @SneakyThrows
      @Test
      public void tickInterval() {
          StatsCollector collector = Mockito.mock(StatsCollector.class, Mockito.withSettings()
              .useConstructor(Duration.ofMillis(500), MoreExecutors.directExecutor())
              .defaultAnswer(Mockito.CALLS_REAL_METHODS));
          AtomicDouble statValue = new AtomicDouble();
          doAnswer(invocation -> {
              Map<String, Double> map = invocation.getArgument(0);
              map.put("stat1", statValue.getAndAdd(1.0));
              return null;
          }).when(collector).scrap(anyMap());
          TestObserver<Map<String, Double>> statsObserver = TestObserver.create();
          collector.collect().subscribe(statsObserver);
          collector.tick();
          collector.tick();
          Thread.sleep(550);
          collector.tick();
          collector.stop().toCompletableFuture().join();
          statsObserver.await();
          assertEquals(statsObserver.values().size(), 2);
      }
  
      @SneakyThrows
      @Test
      public void distinctUntilChange() {
          StatsCollector collector = Mockito.mock(StatsCollector.class, Mockito.withSettings()
              .useConstructor(Duration.ofMillis(10), MoreExecutors.directExecutor())
              .defaultAnswer(Mockito.CALLS_REAL_METHODS));
          doAnswer(invocation -> {
              Map<String, Double> map = invocation.getArgument(0);
              map.put("stat1", 0.0);
              return null;
          }).when(collector).scrap(anyMap());
          TestObserver<Map<String, Double>> statsObserver = TestObserver.create();
          collector.collect().subscribe(statsObserver);
          collector.tick();
          Thread.sleep(10);
          collector.tick();
          Thread.sleep(10);
          collector.tick();
          collector.stop().toCompletableFuture().join();
          statsObserver.await();
          assertEquals(statsObserver.values().size(), 1);
      }
  }
  