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.Strings;
  import org.apache.commons.lang3.RandomStringUtils;
  import lombok.Data;
  
  @JsonDeserialize(builder = StringGenerator.Builder.class)
  public final class StringGenerator implements Generator<String> {
      private final int length;
      private final String prefix;
      private final boolean letters;
      private final boolean numbers;
      private final boolean limitedDomain;
      private final Random random = ThreadLocalRandom.current();
      private final Set<String> domain = new HashSet<>();
  
      public StringGenerator(Builder builder) {
          length = builder.length;
          prefix = builder.prefix;
          letters = builder.letters;
          numbers = builder.numbers;
          limitedDomain = builder.domainSize > 0;
  
          if (limitedDomain) {
              while (domain.size() < builder.domainSize) {
                  final String r = RandomStringUtils.random(length, letters, numbers);
                  if (!Strings.isNullOrEmpty(builder.prefix)) {
                      domain.add(builder.prefix + r);
                  } else {
                      domain.add(r);
                  }
              }
          }
      }
  
      @Override
      public String next() {
          if (!limitedDomain) {
              return Strings.nullToEmpty(prefix)
                  + RandomStringUtils.random(length, letters, numbers);
          }
          return domain
              .stream()
              .skip(random.nextInt(domain.size()))
              .findFirst()
              .orElseThrow(() -> new RuntimeException("Should never happen"));
      }
  
      @Override
      public String toString() {
          return String.valueOf(next());
      }
  
      @Data
      public static class Builder {
          private int length;
          private String prefix;
          private boolean letters;
          private boolean numbers;
          private int domainSize;
  
          public StringGenerator build() {
              return new StringGenerator(this);
          }
      }
  }
  