code: /*
   * 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.skywalking.generator;
  
  import static com.google.common.base.Preconditions.checkArgument;
  import java.util.UUID;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.concurrent.atomic.AtomicReference;
  import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
  import lombok.Data;
  
  @JsonDeserialize(builder = UUIDGenerator.Builder.class)
  public final class UUIDGenerator implements Generator<String> {
      private final int changingFrequency;
      private final AtomicInteger counter;
      private final AtomicReference<String> last =
          new AtomicReference<>(UUID.randomUUID().toString());
  
      public UUIDGenerator(Builder builder) {
          checkArgument(builder.changingFrequency > 0, "changingFrequency must be greater than 0");
          changingFrequency = builder.changingFrequency;
          counter = new AtomicInteger();
          reset();
      }
  
      @Override
      public String next() {
          if (counter.incrementAndGet() < changingFrequency) {
              return last.get();
          }
          reset();
          return last.getAndSet(UUID.randomUUID().toString());
      }
  
      @Override
      public String toString() {
          return String.valueOf(next());
      }
  
      @Override
      public void reset() {
          counter.set(0);
      }
  
      @Data
      public static class Builder {
          private int changingFrequency = 1;
  
          public UUIDGenerator build() {
              return new UUIDGenerator(this);
          }
      }
  }
  