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   /*
    * Copyright 2016 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.resolver.NameResolver;
  import io.netty.util.concurrent.EventExecutor;
  import io.netty.util.concurrent.Future;
  import io.netty.util.concurrent.FutureListener;
  import io.netty.util.concurrent.Promise;
  import io.netty.util.internal.StringUtil;
  
  import java.util.List;
  import java.util.concurrent.ConcurrentMap;
  
  import static io.netty.util.internal.ObjectUtil.checkNotNull;
  
  // FIXME(trustin): Find a better name and move it to the 'resolver' module.
  final class InflightNameResolver<T> implements NameResolver<T> {
  
      private final EventExecutor executor;
      private final NameResolver<T> delegate;
      private final ConcurrentMap<String, Promise<T>> resolvesInProgress;
      private final ConcurrentMap<String, Promise<List<T>>> resolveAllsInProgress;
  
      InflightNameResolver(EventExecutor executor, NameResolver<T> delegate,
                           ConcurrentMap<String, Promise<T>> resolvesInProgress,
                           ConcurrentMap<String, Promise<List<T>>> resolveAllsInProgress) {
  
          this.executor = checkNotNull(executor, "executor");
          this.delegate = checkNotNull(delegate, "delegate");
          this.resolvesInProgress = checkNotNull(resolvesInProgress, "resolvesInProgress");
          this.resolveAllsInProgress = checkNotNull(resolveAllsInProgress, "resolveAllsInProgress");
      }
  
      @Override
      public Future<T> resolve(String inetHost) {
          return resolve(inetHost, executor.<T>newPromise());
      }
  
      @Override
      public Future<List<T>> resolveAll(String inetHost) {
          return resolveAll(inetHost, executor.<List<T>>newPromise());
      }
  
      @Override
      public void close() {
          delegate.close();
      }
  
      @Override
      public Promise<T> resolve(String inetHost, Promise<T> promise) {
          return resolve(resolvesInProgress, inetHost, promise, false);
      }
  
      @Override
      public Promise<List<T>> resolveAll(String inetHost, Promise<List<T>> promise) {
          return resolve(resolveAllsInProgress, inetHost, promise, true);
      }
  
      private <U> Promise<U> resolve(
              final ConcurrentMap<String, Promise<U>> resolveMap,
              final String inetHost, final Promise<U> promise, boolean resolveAll) {
  
          final Promise<U> earlyPromise = resolveMap.putIfAbsent(inetHost, promise);
          if (earlyPromise != null) {
              // Name resolution for the specified inetHost is in progress already.
              if (earlyPromise.isDone()) {
                  transferResult(earlyPromise, promise);
              } else {
                  earlyPromise.addListener(new FutureListener<U>() {
                      @Override
                      public void operationComplete(Future<U> f) throws Exception {
                          transferResult(f, promise);
                      }
                  });
              }
          } else {
              try {
                  if (resolveAll) {
                      @SuppressWarnings("unchecked")
                      final Promise<List<T>> castPromise = (Promise<List<T>>) promise; // U is List<T>
                      delegate.resolveAll(inetHost, castPromise);
                  } else {
                      @SuppressWarnings("unchecked")
                      final Promise<T> castPromise = (Promise<T>) promise; // U is T
                     delegate.resolve(inetHost, castPromise);
                 }
             } finally {
                 if (promise.isDone()) {
                     resolveMap.remove(inetHost);
                 } else {
                     promise.addListener(new FutureListener<U>() {
                         @Override
                         public void operationComplete(Future<U> f) throws Exception {
                             resolveMap.remove(inetHost);
                         }
                     });
                 }
             }
         }
 
         return promise;
     }
 
     private static <T> void transferResult(Future<T> src, Promise<T> dst) {
         if (src.isSuccess()) {
             dst.trySuccess(src.getNow());
         } else {
             dst.tryFailure(src.cause());
         }
     }
 
     @Override
     public String toString() {
         return StringUtil.simpleClassName(this) + '(' + delegate + ')';
     }
 }