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 java.util.HashSet;
  import java.util.Random;
  import java.util.Set;
  import java.util.concurrent.ThreadLocalRandom;
  import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
  import com.google.common.base.Preconditions;
  import lombok.Data;
  import lombok.experimental.Accessors;
  
  @JsonDeserialize(builder = SequenceGenerator.Builder.class)
  public final class SequenceGenerator implements Generator<Long> {
      private final boolean limitedDomain;
      private final long min;
      private final long max;
      private final long step;
      private final Integer fluctuation;
      private final Integer domainSize;
      private final Random random = ThreadLocalRandom.current();
      private final Set<Long> domain = new HashSet<>();
      private volatile Long last;
  
      public SequenceGenerator(Builder builder) {
          min = builder.min;
          max = builder.max;
          step = builder.step;
          fluctuation = builder.fluctuation;
          domainSize = builder.domainSize;
          limitedDomain = builder.domainSize != null && builder.domainSize > 0;
  
          reset();
      }
  
      @Override
      public Long next() {
          if (!limitedDomain) {
              return next0();
          }
          return domain
              .stream()
              .skip(random.nextInt(domain.size()))
              .findFirst()
              .orElseThrow(() -> new RuntimeException("Should never happen"));
      }
  
      private synchronized long next0() {
          long next = last == null ? min : last + step;
  
          if (fluctuation != null) {
              int j = random.nextInt(fluctuation);
              next += random.nextBoolean() ? j : -j;
          }
  
          if (next > max) {
              return max;
          }
          if (next < min) {
              return min;
          }
  
          return last = next;
      }
  
      @Override
      public void reset() {
          last = null;
          if (limitedDomain) {
              domain.clear();
              while (domain.size() < domainSize) {
                  domain.add(next0());
              }
          }
      }
  
      @Override
      public String toString() {
          return String.valueOf(next());
      }
  
      @Data
      @Accessors(chain = true)
      public static class Builder {
          private long min = Long.MIN_VALUE;
          private long max = Long.MAX_VALUE;
          private long step = 1;
          private Integer domainSize;
          private Integer fluctuation;
  
          public SequenceGenerator build() {
              if (domainSize != null) {
                  Preconditions.checkArgument(domainSize > 0, "domainSize must be > 0");
                  Preconditions.checkArgument(domainSize + min <= max,
                      "domain size must be <= max - min");
              }
              return new SequenceGenerator(this);
          }
      }
  }
  