/*
    * 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.commons.collections.map;
  
  import java.io.IOException;
  import java.io.ObjectInputStream;
  import java.io.ObjectOutputStream;
  import java.io.Serializable;
  import java.util.Map;
  
  import org.apache.commons.collections.BoundedMap;
  
  /**
   * A <code>Map</code> implementation with a fixed maximum size which removes
   * the least recently used entry if an entry is added when full.
   * <p>
   * The least recently used algorithm works on the get and put operations only.
   * Iteration of any kind, including setting the value by iteration, does not
   * change the order. Queries such as containsKey and containsValue or access
   * via views also do not change the order.
   * <p>
   * A somewhat subtle ramification of the least recently used
   * algorithm is that calls to {@link #get(Object)} stand a very good chance
   * of modifying the map's iteration order and thus invalidating any
   * iterators currently in use.  It is therefore suggested that iterations
   * over an {@link LRUMap} instance access entry values only through a
   * {@link org.apache.commons.collections.MapIterator MapIterator} or {@link #entrySet()} iterator.
   * <p>
   * The map implements <code>OrderedMap</code> and entries may be queried using
   * the bidirectional <code>OrderedMapIterator</code>. The order returned is
   * least recently used to most recently used. Iterators from map views can
   * also be cast to <code>OrderedIterator</code> if required.
   * <p>
   * All the available iterators can be reset back to the start by casting to
   * <code>ResettableIterator</code> and calling <code>reset()</code>.
   * <p>
   * <strong>Note that LRUMap is not synchronized and is not thread-safe.</strong>
   * If you wish to use this map from multiple threads concurrently, you must use
   * appropriate synchronization. The simplest approach is to wrap this map
   * using {@link java.util.Collections#synchronizedMap(Map)}. This class may throw
   * <code>NullPointerException</code>'s when accessed by concurrent threads.
   *
   * @since 3.0 (previously in main package v1.0)
   * @version $Id: LRUMap.java 1436463 2013-01-21 16:35:01Z tn $
   */
  public class LRUMap<K, V>
          extends AbstractLinkedMap<K, V> implements BoundedMap<K, V>, Serializable, Cloneable {
  
      /** Serialisation version */
      private static final long serialVersionUID = -612114643488955218L;
      /** Default maximum size */
      protected static final int DEFAULT_MAX_SIZE = 100;
  
      /** Maximum size */
      private transient int maxSize;
      /** Scan behaviour */
      private boolean scanUntilRemovable;
  
      /**
       * Constructs a new empty map with a maximum size of 100.
       */
      public LRUMap() {
          this(DEFAULT_MAX_SIZE, DEFAULT_LOAD_FACTOR, false);
      }
  
      /**
       * Constructs a new, empty map with the specified maximum size.
       *
       * @param maxSize  the maximum size of the map
       * @throws IllegalArgumentException if the maximum size is less than one
       */
      public LRUMap(final int maxSize) {
          this(maxSize, DEFAULT_LOAD_FACTOR);
      }
  
      /**
       * Constructs a new, empty map with the specified maximum size.
       *
       * @param maxSize  the maximum size of the map
       * @param scanUntilRemovable  scan until a removeable entry is found, default false
       * @throws IllegalArgumentException if the maximum size is less than one
       * @since 3.1
       */
      public LRUMap(final int maxSize, final boolean scanUntilRemovable) {
          this(maxSize, DEFAULT_LOAD_FACTOR, scanUntilRemovable);
     }
 
     /**
      * Constructs a new, empty map with the specified initial capacity and
      * load factor.
      *
      * @param maxSize  the maximum size of the map
      * @param loadFactor  the load factor
      * @throws IllegalArgumentException if the maximum size is less than one
      * @throws IllegalArgumentException if the load factor is less than zero
      */
     public LRUMap(final int maxSize, final float loadFactor) {
         this(maxSize, loadFactor, false);
     }
 
     /**
      * Constructs a new, empty map with the specified initial capacity and
      * load factor.
      *
      * @param maxSize  the maximum size of the ma
      * @param loadFactor  the load factor
      * @param scanUntilRemovable  scan until a removeable entry is found, default false
      * @throws IllegalArgumentException if the maximum size is less than one
      * @throws IllegalArgumentException if the load factor is less than zero
      * @since 3.1
      */
     public LRUMap(final int maxSize, final float loadFactor, final boolean scanUntilRemovable) {
         super(maxSize < 1 ? DEFAULT_CAPACITY : maxSize, loadFactor);
         if (maxSize < 1) {
             throw new IllegalArgumentException("LRUMap max size must be greater than 0");
         }
         this.maxSize = maxSize;
         this.scanUntilRemovable = scanUntilRemovable;
     }
 
     /**
      * Constructor copying elements from another map.
      * <p>
      * The maximum size is set from the map's size.
      *
      * @param map  the map to copy
      * @throws NullPointerException if the map is null
      * @throws IllegalArgumentException if the map is empty
      */
     public LRUMap(final Map<K, V> map) {
         this(map, false);
     }
 
     /**
      * Constructor copying elements from another map.
      * <p/>
      * The maximum size is set from the map's size.
      *
      * @param map  the map to copy
      * @param scanUntilRemovable  scan until a removeable entry is found, default false
      * @throws NullPointerException if the map is null
      * @throws IllegalArgumentException if the map is empty
      * @since 3.1
      */
     public LRUMap(final Map<K, V> map, final boolean scanUntilRemovable) {
         this(map.size(), DEFAULT_LOAD_FACTOR, scanUntilRemovable);
         putAll(map);
     }
 
     //-----------------------------------------------------------------------
     /**
      * Gets the value mapped to the key specified.
      * <p>
      * This operation changes the position of the key in the map to the
      * most recently used position (first).
      *
      * @param key  the key
      * @return the mapped value, null if no match
      */
     @Override
     public V get(final Object key) {
         final LinkEntry<K, V> entry = getEntry(key);
         if (entry == null) {
             return null;
         }
         moveToMRU(entry);
         return entry.getValue();
     }
 
     //-----------------------------------------------------------------------
     /**
      * Moves an entry to the MRU position at the end of the list.
      * <p>
      * This implementation moves the updated entry to the end of the list.
      *
      * @param entry  the entry to update
      */
     protected void moveToMRU(final LinkEntry<K, V> entry) {
         if (entry.after != header) {
             modCount++;
             // remove
             if(entry.before == null) {
                 throw new IllegalStateException("Entry.before is null." + 
                     " Please check that your keys are immutable, and that you have used synchronization properly." +
                     " If so, then please report this to dev@commons.apache.org as a bug.");
             }
             entry.before.after = entry.after;
             entry.after.before = entry.before;
             // add first
             entry.after = header;
             entry.before = header.before;
             header.before.after = entry;
             header.before = entry;
         } else if (entry == header) {
             throw new IllegalStateException("Can't move header to MRU" +
                 " (please report this to dev@commons.apache.org)");
         }
     }
 
     /**
      * Updates an existing key-value mapping.
      * <p>
      * This implementation moves the updated entry to the top of the list
      * using {@link #moveToMRU(AbstractLinkedMap.LinkEntry)}.
      *
      * @param entry  the entry to update
      * @param newValue  the new value to store
      */
     @Override
     protected void updateEntry(final HashEntry<K, V> entry, final V newValue) {
         moveToMRU((LinkEntry<K, V>) entry);  // handles modCount
         entry.setValue(newValue);
     }
 
     /**
      * Adds a new key-value mapping into this map.
      * <p>
      * This implementation checks the LRU size and determines whether to
      * discard an entry or not using {@link #removeLRU(AbstractLinkedMap.LinkEntry)}.
      * <p>
      * From Commons Collections 3.1 this method uses {@link #isFull()} rather
      * than accessing <code>size</code> and <code>maxSize</code> directly.
      * It also handles the scanUntilRemovable functionality.
      *
      * @param hashIndex  the index into the data array to store at
      * @param hashCode  the hash code of the key to add
      * @param key  the key to add
      * @param value  the value to add
      */
     @Override
     protected void addMapping(final int hashIndex, final int hashCode, final K key, final V value) {
         if (isFull()) {
             LinkEntry<K, V> reuse = header.after;
             boolean removeLRUEntry = false;
             if (scanUntilRemovable) {
                 while (reuse != header && reuse != null) {
                     if (removeLRU(reuse)) {
                         removeLRUEntry = true;
                         break;
                     }
                     reuse = reuse.after;
                 }
                 if (reuse == null) {
                     throw new IllegalStateException(
                         "Entry.after=null, header.after" + header.after + " header.before" + header.before +
                         " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
                         " Please check that your keys are immutable, and that you have used synchronization properly." +
                         " If so, then please report this to dev@commons.apache.org as a bug.");
                 }
             } else {
                 removeLRUEntry = removeLRU(reuse);
             }
 
             if (removeLRUEntry) {
                 if (reuse == null) {
                     throw new IllegalStateException(
                         "reuse=null, header.after=" + header.after + " header.before" + header.before +
                         " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
                         " Please check that your keys are immutable, and that you have used synchronization properly." +
                         " If so, then please report this to dev@commons.apache.org as a bug.");
                 }
                 reuseMapping(reuse, hashIndex, hashCode, key, value);
             } else {
                 super.addMapping(hashIndex, hashCode, key, value);
             }
         } else {
             super.addMapping(hashIndex, hashCode, key, value);
         }
     }
 
     /**
      * Reuses an entry by removing it and moving it to a new place in the map.
      * <p>
      * This method uses {@link #removeEntry}, {@link #reuseEntry} and {@link #addEntry}.
      *
      * @param entry  the entry to reuse
      * @param hashIndex  the index into the data array to store at
      * @param hashCode  the hash code of the key to add
      * @param key  the key to add
      * @param value  the value to add
      */
     protected void reuseMapping(final LinkEntry<K, V> entry, final int hashIndex, final int hashCode,
                                 final K key, final V value) {
         // find the entry before the entry specified in the hash table
         // remember that the parameters (except the first) refer to the new entry,
         // not the old one
         try {
             final int removeIndex = hashIndex(entry.hashCode, data.length);
             final HashEntry<K, V>[] tmp = data;  // may protect against some sync issues
             HashEntry<K, V> loop = tmp[removeIndex];
             HashEntry<K, V> previous = null;
             while (loop != entry && loop != null) {
                 previous = loop;
                 loop = loop.next;
             }
             if (loop == null) {
                 throw new IllegalStateException(
                     "Entry.next=null, data[removeIndex]=" + data[removeIndex] + " previous=" + previous +
                     " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
                     " Please check that your keys are immutable, and that you have used synchronization properly." +
                     " If so, then please report this to dev@commons.apache.org as a bug.");
             }
 
             // reuse the entry
             modCount++;
             removeEntry(entry, removeIndex, previous);
             reuseEntry(entry, hashIndex, hashCode, key, value);
             addEntry(entry, hashIndex);
         } catch (final NullPointerException ex) {
             throw new IllegalStateException(
                     "NPE, entry=" + entry + " entryIsHeader=" + (entry==header) +
                     " key=" + key + " value=" + value + " size=" + size + " maxSize=" + maxSize +
                     " Please check that your keys are immutable, and that you have used synchronization properly." +
                     " If so, then please report this to dev@commons.apache.org as a bug.");
         }
     }
 
     /**
      * Subclass method to control removal of the least recently used entry from the map.
      * <p>
      * This method exists for subclasses to override. A subclass may wish to
      * provide cleanup of resources when an entry is removed. For example:
      * <pre>
      * protected boolean removeLRU(LinkEntry entry) {
      *   releaseResources(entry.getValue());  // release resources held by entry
      *   return true;  // actually delete entry
      * }
      * </pre>
      * <p>
      * Alternatively, a subclass may choose to not remove the entry or selectively
      * keep certain LRU entries. For example:
      * <pre>
      * protected boolean removeLRU(LinkEntry entry) {
      *   if (entry.getKey().toString().startsWith("System.")) {
      *     return false;  // entry not removed from LRUMap
      *   } else {
      *     return true;  // actually delete entry
      *   }
      * }
      * </pre>
      * The effect of returning false is dependent on the scanUntilRemovable flag.
      * If the flag is true, the next LRU entry will be passed to this method and so on
      * until one returns false and is removed, or every entry in the map has been passed.
      * If the scanUntilRemovable flag is false, the map will exceed the maximum size.
      * <p>
      * NOTE: Commons Collections 3.0 passed the wrong entry to this method.
      * This is fixed in version 3.1 onwards.
      *
      * @param entry  the entry to be removed
      * @return {@code true}
      */
     protected boolean removeLRU(final LinkEntry<K, V> entry) {
         return true;
     }
 
     //-----------------------------------------------------------------------
     /**
      * Returns true if this map is full and no new mappings can be added.
      *
      * @return <code>true</code> if the map is full
      */
     public boolean isFull() {
         return size >= maxSize;
     }
 
     /**
      * Gets the maximum size of the map (the bound).
      *
      * @return the maximum number of elements the map can hold
      */
     public int maxSize() {
         return maxSize;
     }
 
     /**
      * Whether this LRUMap will scan until a removable entry is found when the
      * map is full.
      *
      * @return true if this map scans
      * @since 3.1
      */
     public boolean isScanUntilRemovable() {
         return scanUntilRemovable;
     }
 
     //-----------------------------------------------------------------------
     /**
      * Clones the map without cloning the keys or values.
      *
      * @return a shallow clone
      */
     @Override
     public LRUMap<K, V> clone() {
         return (LRUMap<K, V>) super.clone();
     }
 
     /**
      * Write the map out using a custom routine.
      */
     private void writeObject(final ObjectOutputStream out) throws IOException {
         out.defaultWriteObject();
         doWriteObject(out);
     }
 
     /**
      * Read the map in using a custom routine.
      */
     private void readObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
         in.defaultReadObject();
         doReadObject(in);
     }
 
     /**
      * Writes the data necessary for <code>put()</code> to work in deserialization.
      * 
      * @param out  the output stream
      * @throws IOException if an error occurs while writing to the stream
      */
     @Override
     protected void doWriteObject(final ObjectOutputStream out) throws IOException {
         out.writeInt(maxSize);
         super.doWriteObject(out);
     }
 
     /**
      * Reads the data necessary for <code>put()</code> to work in the superclass.
      *
      * @param in  the input stream
      * @throws IOException if an error occurs while reading from the stream
      * @throws ClassNotFoundException if an object read from the stream can not be loaded
      */
     @Override
     protected void doReadObject(final ObjectInputStream in) throws IOException, ClassNotFoundException {
         maxSize = in.readInt();
         super.doReadObject(in);
     }
 
 }