/*
    *  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.mina.util;
  
  import java.util.Collection;
  import java.util.Map;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  
  import org.apache.mina.core.buffer.IoBuffer;
  
  /**
   * This map is specially useful when reads are much more frequent than writes and 
   * if the cost of instantiating the values is high like allocating an 
   * {@link IoBuffer} for example.
   *  
   * Based on the final implementation of Memoizer written by Brian Goetz and Tim
   * Peierls. This implementation will return an
   * {@link UnsupportedOperationException} on each method that is not intended to
   * be called by user code for performance reasons.
   * 
   * @param <K> The key type
   * @param <V> The value type
   * 
   * @author <a href="http://mina.apache.org">Apache MINA Project</a>
   * @since MINA 2.0.0-M2
   */
  public class LazyInitializedCacheMap<K, V> implements Map<K, V> {
      private ConcurrentMap<K, LazyInitializer<V>> cache;
  
      /**
       * This class provides a noop {@link LazyInitializer} meaning it 
       * will return the same object it received when instantiated.
       */
      public class NoopInitializer extends LazyInitializer<V> {
          private V value;
  
          /**
           * Create a new NoopInitializer instance
           * 
           * @param value The value stored in this initializer
           */
          public NoopInitializer(V value) {
              this.value = value;
          }
  
          /**
           * {@inheritDoc}
           */
          @Override
          public V init() {
              return value;
          }
      }
  
      /**
       * Default constructor. Uses the default parameters to initialize its internal
       * {@link ConcurrentHashMap}.
       */
      public LazyInitializedCacheMap() {
          this.cache = new ConcurrentHashMap<>();
      }
  
      /**
       * This constructor allows to provide a fine tuned {@link ConcurrentHashMap}
       * to stick with each special case the user needs.
       * 
       * @param map The map to use as a cache
       */
      public LazyInitializedCacheMap(ConcurrentHashMap<K, LazyInitializer<V>> map) {
          this.cache = map;
      }
  
      /**
       * {@inheritDoc}
       */
      @Override
      public V get(Object key) {
          LazyInitializer<V> c = cache.get(key);
  
          if (c != null) {
             return c.get();
         }
 
         return null;
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public V remove(Object key) {
         LazyInitializer<V> c = cache.remove(key);
         
         if (c != null) {
             return c.get();
         }
 
         return null;
     }
 
     /**
      * If the specified key is not already associated
      * with a value, associate it with the given value.
      * This is equivalent to
      * <pre>
      *   if (!map.containsKey(key))
      *       return map.put(key, value);
      *   else
      *       return map.get(key);</pre>
      * except that the action is performed atomically.
      *
      * @param key key with which the specified value is to be associated
      * @param value a lazy initialized value object.
      * 
      * @return the previous value associated with the specified key,
      *         or <tt>null</tt> if there was no mapping for the key
      */
     public V putIfAbsent(K key, LazyInitializer<V> value) {
         LazyInitializer<V> v = cache.get(key);
         
         if (v == null) {
             v = cache.putIfAbsent(key, value);
             
             if (v == null) {
                 return value.get();
             }
         }
 
         return v.get();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public V put(K key, V value) {
         LazyInitializer<V> c = cache.put(key, new NoopInitializer(value));
 
         if (c != null) {
             return c.get();
         }
 
         return null;
     }
 
     /**
      * Throws {@link UnsupportedOperationException} as this method would imply
      *         performance drops.
      */
     @Override
     public boolean containsValue(Object value) {
         throw new UnsupportedOperationException();
     }
 
     /**
      * Throws {@link UnsupportedOperationException} as this method would imply
      *         performance drops.
      */
     @Override
     public Collection<V> values() {
         throw new UnsupportedOperationException();
     }
 
     /**
      * Throws {@link UnsupportedOperationException} as this method would imply
      *         performance drops.
      */
     @Override
     public Set<java.util.Map.Entry<K, V>> entrySet() {
         throw new UnsupportedOperationException();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void putAll(Map<? extends K, ? extends V> m) {
         for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) {
             cache.put(e.getKey(), new NoopInitializer(e.getValue()));
         }
     }
 
     /**
      * @return return the values from the cache
      */
     public Collection<LazyInitializer<V>> getValues() {
         return cache.values();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public void clear() {
         cache.clear();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean containsKey(Object key) {
         return cache.containsKey(key);
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public boolean isEmpty() {
         return cache.isEmpty();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public Set<K> keySet() {
         return cache.keySet();
     }
 
     /**
      * {@inheritDoc}
      */
     @Override
     public int size() {
         return cache.size();
     }
 }