/*
 *  Copyright 2001-2004 The Apache Software Foundation
 *
 *  Licensed 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>
 * 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>.
 * 
 * @since Commons Collections 3.0 (previously in main package v1.0)
 * @version $Revision: 1.1 $ $Date: 2004/03/27 00:07:13 $
 *
 * @author James Strachan
 * @author Morgan Delagrange
 * @author Stephen Colebourne
 */
public class LRUMap
        extends AbstractLinkedMap implements BoundedMap, Serializable, Cloneable {
    
    /** Serialisation version */
    static final long serialVersionUID = -612114643488955218L;
    /** Default maximum size */
    protected static final int DEFAULT_MAX_SIZE = 100;
    
    /** Maximum size */
    private transient int maxSize;

    /**
     * Constructs a new empty map with a maximum size of 100.
     */
    public LRUMap() {
        this(DEFAULT_MAX_SIZE, DEFAULT_LOAD_FACTOR);
    }

    /**
     * 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(int maxSize) {
        this(maxSize, DEFAULT_LOAD_FACTOR);
    }

    /**
     * Constructs a new, empty map with the specified initial capacity and
     * load factor. 
     *
     * @param maxSize  the maximum size of the map, -1 for no limit,
     * @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(int maxSize, float loadFactor) {
        super((maxSize < 1 ? DEFAULT_CAPACITY : maxSize), loadFactor);
        if (maxSize < 1) {
            throw new IllegalArgumentException("LRUMap max size must be greater than 0");
        }
        this.maxSize = maxSize;
    }

    /**
     * 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(Map map) {
        this(map.size(), DEFAULT_LOAD_FACTOR);
        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
     */
    public Object get(Object key) {
        LinkEntry entry = (LinkEntry) 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.
     * This implementation moves the updated entry to the end of the list.
     * 
     * @param entry  the entry to update
     */
    protected void moveToMRU(LinkEntry entry) {
        if (entry.after != header) {
            modCount++;
            // remove
            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;
        }
    }
    
    /**
     * Updates an existing key-value mapping.
     * This implementation moves the updated entry to the top of the list.
     * 
     * @param entry  the entry to update
     * @param newValue  the new value to store
     */
    protected void updateEntry(HashEntry entry, Object newValue) {
        moveToMRU((LinkEntry) entry);  // handles modCount
        entry.setValue(newValue);
    }
    
    /**
     * Adds a new key-value mapping into this map.
     * This implementation checks the LRU size and determines whether to
     * discard an entry or not.
     * 
     * @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 addMapping(int hashIndex, int hashCode, Object key, Object value) {
        if (size >= maxSize && removeLRU(header.before)) {
            reuseMapping(header.after, 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.
     * 
     * @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(LinkEntry entry, int hashIndex, int hashCode, Object key, Object 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
        int removeIndex = hashIndex(entry.hashCode, data.length);
        HashEntry loop = data[removeIndex];
        HashEntry previous = null;
        while (loop != entry) {
            previous = loop;
            loop = loop.next;
        }
        
        // reuse the entry
        modCount++;
        removeEntry(entry, removeIndex, previous);
        reuseEntry(entry, hashIndex, hashCode, key, value);
        addEntry(entry, hashIndex);
    }
    
    /**
     * 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>
     * Note that the effect of not removing an LRU is for the Map to exceed the maximum size.
     * 
     * @param entry  the entry to be removed
     */
    protected boolean removeLRU(LinkEntry 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;
    }

    //-----------------------------------------------------------------------
    /**
     * Clones the map without cloning the keys or values.
     *
     * @return a shallow clone
     */
    public Object clone() {
        return super.clone();
    }
    
    /**
     * Write the map out using a custom routine.
     */
    private void writeObject(ObjectOutputStream out) throws IOException {
        out.defaultWriteObject();
        doWriteObject(out);
    }

    /**
     * Read the map in using a custom routine.
     */
    private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        in.defaultReadObject();
        doReadObject(in);
    }
    
    /**
     * Writes the data necessary for <code>put()</code> to work in deserialization.
     */
    protected void doWriteObject(ObjectOutputStream out) throws IOException {
        out.writeInt(maxSize);
        super.doWriteObject(out);
    }

    /**
     * Reads the data necessary for <code>put()</code> to work in the superclass.
     */
    protected void doReadObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
        maxSize = in.readInt();
        super.doReadObject(in);
    }
    
}
