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   /*
    * Copyright 2018 The Netty Project
    *
    * The Netty Project 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:
    *
    *   https://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 io.netty.resolver.dns;
  
  import io.netty.channel.EventLoop;
  import io.netty.util.AsciiString;
  
  import java.util.List;
  
  import static io.netty.util.internal.ObjectUtil.*;
  
  /**
   * Default implementation of a {@link DnsCnameCache}.
   */
  public final class DefaultDnsCnameCache implements DnsCnameCache {
      private final int minTtl;
      private final int maxTtl;
  
      private final Cache<String> cache = new Cache<String>() {
          @Override
          protected boolean shouldReplaceAll(String entry) {
              // Only one 1:1 mapping is supported as specified in the RFC.
              return true;
          }
  
          @Override
          protected boolean equals(String entry, String otherEntry) {
              return AsciiString.contentEqualsIgnoreCase(entry, otherEntry);
          }
      };
  
      /**
       * Create a cache that respects the TTL returned by the DNS server.
       */
      public DefaultDnsCnameCache() {
          this(0, Cache.MAX_SUPPORTED_TTL_SECS);
      }
  
      /**
       * Create a cache.
       *
       * @param minTtl the minimum TTL
       * @param maxTtl the maximum TTL
       */
      public DefaultDnsCnameCache(int minTtl, int maxTtl) {
          this.minTtl = Math.min(Cache.MAX_SUPPORTED_TTL_SECS, checkPositiveOrZero(minTtl, "minTtl"));
          this.maxTtl = Math.min(Cache.MAX_SUPPORTED_TTL_SECS, checkPositive(maxTtl, "maxTtl"));
          if (minTtl > maxTtl) {
              throw new IllegalArgumentException(
                      "minTtl: " + minTtl + ", maxTtl: " + maxTtl + " (expected: 0 <= minTtl <= maxTtl)");
          }
      }
  
      @SuppressWarnings("unchecked")
      @Override
      public String get(String hostname) {
          List<? extends String> cached =  cache.get(checkNotNull(hostname, "hostname"));
          if (cached == null || cached.isEmpty()) {
              return null;
          }
          // We can never have more then one record.
          return cached.get(0);
      }
  
      @Override
      public void cache(String hostname, String cname, long originalTtl, EventLoop loop) {
          checkNotNull(hostname, "hostname");
          checkNotNull(cname, "cname");
          checkNotNull(loop, "loop");
          cache.cache(hostname, cname, Math.max(minTtl, (int) Math.min(maxTtl, originalTtl)), loop);
      }
  
      @Override
      public void clear() {
          cache.clear();
      }
  
      @Override
      public boolean clear(String hostname) {
          return cache.clear(checkNotNull(hostname, "hostname"));
      }
  
      // Package visibility for testing purposes
      int minTtl() {
          return minTtl;
      }
 
     // Package visibility for testing purposes
     int maxTtl() {
         return maxTtl;
     }
 }