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   /*
    * Copyright 2015 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;
  
  import io.netty.util.CharsetUtil;
  import io.netty.util.internal.ObjectUtil;
  import io.netty.util.internal.PlatformDependent;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.net.InetAddress;
  import java.nio.charset.Charset;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Locale;
  import java.util.Map;
  import java.util.concurrent.atomic.AtomicLong;
  
  /**
   * Default {@link HostsFileEntriesResolver} that resolves hosts file entries only once.
   */
  public final class DefaultHostsFileEntriesResolver implements HostsFileEntriesResolver {
  
      private static final InternalLogger logger =
              InternalLoggerFactory.getInstance(DefaultHostsFileEntriesResolver.class);
      private static final long DEFAULT_REFRESH_INTERVAL;
  
      private final long refreshInterval;
      private final AtomicLong lastRefresh = new AtomicLong(System.nanoTime());
      private final HostsFileEntriesProvider.Parser hostsFileParser;
      private volatile Map<String, List<InetAddress>> inet4Entries;
      private volatile Map<String, List<InetAddress>> inet6Entries;
  
      static {
          DEFAULT_REFRESH_INTERVAL = SystemPropertyUtil.getLong(
                  "io.netty.hostsFileRefreshInterval", /*nanos*/0);
  
          if (logger.isDebugEnabled()) {
              logger.debug("-Dio.netty.hostsFileRefreshInterval: {}", DEFAULT_REFRESH_INTERVAL);
          }
      }
  
      public DefaultHostsFileEntriesResolver() {
          this(HostsFileEntriesProvider.parser(), DEFAULT_REFRESH_INTERVAL);
      }
  
      // for testing purpose only
      DefaultHostsFileEntriesResolver(HostsFileEntriesProvider.Parser hostsFileParser, long refreshInterval) {
          this.hostsFileParser = hostsFileParser;
          this.refreshInterval = ObjectUtil.checkPositiveOrZero(refreshInterval, "refreshInterval");
          HostsFileEntriesProvider entries = parseEntries(hostsFileParser);
          inet4Entries = entries.ipv4Entries();
          inet6Entries = entries.ipv6Entries();
      }
  
      @Override
      public InetAddress address(String inetHost, ResolvedAddressTypes resolvedAddressTypes) {
          return firstAddress(addresses(inetHost, resolvedAddressTypes));
      }
  
      /**
       * Resolves all addresses of a hostname against the entries in a hosts file, depending on the specified
       * {@link ResolvedAddressTypes}.
       *
       * @param inetHost the hostname to resolve
       * @param resolvedAddressTypes the address types to resolve
       * @return all matching addresses or {@code null} in case the hostname cannot be resolved
       */
      public List<InetAddress> addresses(String inetHost, ResolvedAddressTypes resolvedAddressTypes) {
          String normalized = normalize(inetHost);
          ensureHostsFileEntriesAreFresh();
  
          switch (resolvedAddressTypes) {
              case IPV4_ONLY:
                  return inet4Entries.get(normalized);
              case IPV6_ONLY:
                  return inet6Entries.get(normalized);
              case IPV4_PREFERRED:
                  List<InetAddress> allInet4Addresses = inet4Entries.get(normalized);
                  return allInet4Addresses != null ? allAddresses(allInet4Addresses, inet6Entries.get(normalized)) :
                          inet6Entries.get(normalized);
              case IPV6_PREFERRED:
                  List<InetAddress> allInet6Addresses = inet6Entries.get(normalized);
                  return allInet6Addresses != null ? allAddresses(allInet6Addresses, inet4Entries.get(normalized)) :
                          inet4Entries.get(normalized);
             default:
                 throw new IllegalArgumentException("Unknown ResolvedAddressTypes " + resolvedAddressTypes);
         }
     }
 
     private void ensureHostsFileEntriesAreFresh() {
         long interval = refreshInterval;
         if (interval == 0) {
             return;
         }
         long last = lastRefresh.get();
         long currentTime = System.nanoTime();
         if (currentTime - last > interval) {
             if (lastRefresh.compareAndSet(last, currentTime)) {
                 HostsFileEntriesProvider entries = parseEntries(hostsFileParser);
                 inet4Entries = entries.ipv4Entries();
                 inet6Entries = entries.ipv6Entries();
             }
         }
     }
 
     // package-private for testing purposes
     String normalize(String inetHost) {
         return inetHost.toLowerCase(Locale.ENGLISH);
     }
 
     private static List<InetAddress> allAddresses(List<InetAddress> a, List<InetAddress> b) {
         List<InetAddress> result = new ArrayList<InetAddress>(a.size() + (b == null ? 0 : b.size()));
         result.addAll(a);
         if (b != null) {
             result.addAll(b);
         }
         return result;
     }
 
     private static InetAddress firstAddress(List<InetAddress> addresses) {
         return addresses != null && !addresses.isEmpty() ? addresses.get(0) : null;
     }
 
     private static HostsFileEntriesProvider parseEntries(HostsFileEntriesProvider.Parser parser) {
         if (PlatformDependent.isWindows()) {
             // Ony windows there seems to be no standard for the encoding used for the hosts file, so let us
             // try multiple until we either were able to parse it or there is none left and so we return an
             // empty instance.
             return parser.parseSilently(Charset.defaultCharset(), CharsetUtil.UTF_16, CharsetUtil.UTF_8);
         }
         return parser.parseSilently();
     }
 }