/**
 * $Revision: 1456 $
 * $Date: 2005-06-01 22:04:54 -0700 (Wed, 01 Jun 2005) $
 *
 * Copyright 2003-2005 Jive Software.
 *
 * All rights reserved. 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.jivesoftware.smack.util;

import java.util.*;

/**
 * A specialized Map that is size-limited (using an LRU algorithm) and
 * has an optional expiration time for cache items. The Map is thread-safe.<p>
 *
 * The algorithm for cache is as follows: a HashMap is maintained for fast
 * object lookup. Two linked lists are maintained: one keeps objects in the
 * order they are accessed from cache, the other keeps objects in the order
 * they were originally added to cache. When objects are added to cache, they
 * are first wrapped by a CacheObject which maintains the following pieces
 * of information:<ul>
 * <li> A pointer to the node in the linked list that maintains accessed
 * order for the object. Keeping a reference to the node lets us avoid
 * linear scans of the linked list.
 * <li> A pointer to the node in the linked list that maintains the age
 * of the object in cache. Keeping a reference to the node lets us avoid
 * linear scans of the linked list.</ul>
 * <p/>
 * To get an object from cache, a hash lookup is performed to get a reference
 * to the CacheObject that wraps the real object we are looking for.
 * The object is subsequently moved to the front of the accessed linked list
 * and any necessary cache cleanups are performed. Cache deletion and expiration
 * is performed as needed.
 *
 * @author Matt Tucker
 */
public class Cache implements Map {

    /**
     * The map the keys and values are stored in.
     */
    protected Map map;

    /**
     * Linked list to maintain order that cache objects are accessed
     * in, most used to least used.
     */
    protected LinkedList lastAccessedList;

    /**
     * Linked list to maintain time that cache objects were initially added
     * to the cache, most recently added to oldest added.
     */
    protected LinkedList ageList;

    /**
     * Maximum number of items the cache will hold.
     */
    protected int maxCacheSize;

    /**
     * Maximum length of time objects can exist in cache before expiring.
     */
    protected long maxLifetime;

    /**
     * Maintain the number of cache hits and misses. A cache hit occurs every
     * time the get method is called and the cache contains the requested
     * object. A cache miss represents the opposite occurence.<p>
     *
     * Keeping track of cache hits and misses lets one measure how efficient
     * the cache is; the higher the percentage of hits, the more efficient.
     */
    protected long cacheHits, cacheMisses = 0L;

    /**
     * Create a new cache and specify the maximum size of for the cache in
     * bytes, and the maximum lifetime of objects.
     *
     * @param maxSize the maximum number of objects the cache will hold. -1
     *      means the cache has no max size.
     * @param maxLifetime the maximum amount of time (in ms) objects can exist in
     *      cache before being deleted. -1 means objects never expire.
     */
    public Cache(int maxSize, long maxLifetime) {
        if (maxSize == 0) {
            throw new IllegalArgumentException("Max cache size cannot be 0.");
        }
        this.maxCacheSize = maxSize;
        this.maxLifetime = maxLifetime;

        // Our primary data structure is a hash map. The default capacity of 11
        // is too small in almost all cases, so we set it bigger.
        map = new HashMap(103);

        lastAccessedList = new LinkedList();
        ageList = new LinkedList();
    }

    public synchronized Object put(Object key, Object value) {
        Object oldValue = null;
        // Delete an old entry if it exists.
        if (map.containsKey(key)) {
            oldValue = remove(key, true);
        }

        CacheObject cacheObject = new CacheObject(value);
        map.put(key, cacheObject);
        // Make an entry into the cache order list.
        // Store the cache order list entry so that we can get back to it
        // during later lookups.
        cacheObject.lastAccessedListNode = lastAccessedList.addFirst(key);
        // Add the object to the age list
        LinkedListNode ageNode = ageList.addFirst(key);
        ageNode.timestamp = System.currentTimeMillis();
        cacheObject.ageListNode = ageNode;

        // If cache is too full, remove least used cache entries until it is not too full.
        cullCache();

        return oldValue;
    }

    public synchronized Object get(Object key) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        CacheObject cacheObject = (CacheObject) map.get(key);
        if (cacheObject == null) {
            // The object didn't exist in cache, so increment cache misses.
            cacheMisses++;
            return null;
        }
        // Remove the object from it's current place in the cache order list,
        // and re-insert it at the front of the list.
        cacheObject.lastAccessedListNode.remove();
        lastAccessedList.addFirst(cacheObject.lastAccessedListNode);

        // The object exists in cache, so increment cache hits. Also, increment
        // the object's read count.
        cacheHits++;
        cacheObject.readCount++;

        return cacheObject.object;
    }

    public synchronized Object remove(Object key) {
        return remove(key, false);
    }

    /*
     * Remove operation with a flag so we can tell coherence if the remove was
     * caused by cache internal processing such as eviction or loading
     */
    public synchronized Object remove(Object key, boolean internal) {
        CacheObject cacheObject = (CacheObject) map.remove(key);
        // If the object is not in cache, stop trying to remove it.
        if (cacheObject == null) {
            return null;
        }
        // Remove from the cache order list
        cacheObject.lastAccessedListNode.remove();
        cacheObject.ageListNode.remove();
        // Remove references to linked list nodes
        cacheObject.ageListNode = null;
        cacheObject.lastAccessedListNode = null;

        return cacheObject.object;
    }

    public synchronized void clear() {
        Object[] keys = map.keySet().toArray();
        for (int i = 0; i < keys.length; i++) {
            remove(keys[i]);
        }

        // Now, reset all containers.
        map.clear();
        lastAccessedList.clear();
        ageList.clear();

        cacheHits = 0;
        cacheMisses = 0;
    }

    public synchronized int size() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.size();
    }

    public synchronized boolean isEmpty() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.isEmpty();
    }

    public synchronized Collection values() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        Object[] cacheObjects = map.values().toArray();
        Object[] values = new Object[cacheObjects.length];
        for (int i = 0; i < cacheObjects.length; i++) {
            values[i] = ((CacheObject) cacheObjects[i]).object;
        }
        return Collections.unmodifiableList(Arrays.asList(values));
    }

    public synchronized boolean containsKey(Object key) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return map.containsKey(key);
    }

    public void putAll(Map map) {
        for (Iterator i = map.entrySet().iterator(); i.hasNext();) {
            Map.Entry entry = (Map.Entry) i.next();
            Object value = entry.getValue();
            // If the map is another DefaultCache instance than the
            // entry values will be CacheObject instances that need
            // to be converted to the normal object form.
            if (value instanceof CacheObject) {
                value = ((CacheObject) value).object;
            }
            put(entry.getKey(), value);
        }
    }

    public synchronized boolean containsValue(Object value) {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        CacheObject cacheObject = new CacheObject(value);

        return map.containsValue(cacheObject);
    }

    public synchronized Set entrySet() {
        // Warning -- this method returns CacheObject instances and not Objects
        // in the same form they were put into cache.

        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return Collections.unmodifiableSet(map.entrySet());
    }

    public synchronized Set keySet() {
        // First, clear all entries that have been in cache longer than the
        // maximum defined age.
        deleteExpiredEntries();

        return Collections.unmodifiableSet(map.keySet());
    }

    public long getCacheHits() {
        return cacheHits;
    }

    public long getCacheMisses() {
        return cacheMisses;
    }

    public int getMaxCacheSize() {
        return maxCacheSize;
    }

    public synchronized void setMaxCacheSize(int maxCacheSize) {
        this.maxCacheSize = maxCacheSize;
        // It's possible that the new max size is smaller than our current cache
        // size. If so, we need to delete infrequently used items.
        cullCache();
    }

    public long getMaxLifetime() {
        return maxLifetime;
    }

    public void setMaxLifetime(long maxLifetime) {
        this.maxLifetime = maxLifetime;
    }

    /**
     * Clears all entries out of cache where the entries are older than the
     * maximum defined age.
     */
    protected synchronized void deleteExpiredEntries() {
        // Check if expiration is turned on.
        if (maxLifetime <= 0) {
            return;
        }

        // Remove all old entries. To do this, we remove objects from the end
        // of the linked list until they are no longer too old. We get to avoid
        // any hash lookups or looking at any more objects than is strictly
        // neccessary.
        LinkedListNode node = ageList.getLast();
        // If there are no entries in the age list, return.
        if (node == null) {
            return;
        }

        // Determine the expireTime, which is the moment in time that elements
        // should expire from cache. Then, we can do an easy check to see
        // if the expire time is greater than the expire time.
        long expireTime = System.currentTimeMillis() - maxLifetime;

        while (expireTime > node.timestamp) {
            if (remove(node.object, true) == null) {
                System.err.println("Error attempting to remove(" + node.object.toString() +
                ") - cacheObject not found in cache!");
                // remove from the ageList
                node.remove();
            }

            // Get the next node.
            node = ageList.getLast();
            // If there are no more entries in the age list, return.
            if (node == null) {
                return;
            }
        }
    }

    /**
     * Removes the least recently used elements if the cache size is greater than
     * or equal to the maximum allowed size until the cache is at least 10% empty.
     */
    protected synchronized void cullCache() {
        // Check if a max cache size is defined.
        if (maxCacheSize < 0) {
            return;
        }

        // See if the cache is too big. If so, clean out cache until it's 10% free.
        if (map.size() > maxCacheSize) {
            // First, delete any old entries to see how much memory that frees.
            deleteExpiredEntries();
            // Next, delete the least recently used elements until 10% of the cache
            // has been freed.
            int desiredSize = (int) (maxCacheSize * .90);
            for (int i=map.size(); i>desiredSize; i--) {
                // Get the key and invoke the remove method on it.
                if (remove(lastAccessedList.getLast().object, true) == null) {
                    System.err.println("Error attempting to cullCache with remove(" +
                            lastAccessedList.getLast().object.toString() + ") - " +
                            "cacheObject not found in cache!");
                    lastAccessedList.getLast().remove();
                }
            }
        }
    }

    /**
     * Wrapper for all objects put into cache. It's primary purpose is to maintain
     * references to the linked lists that maintain the creation time of the object
     * and the ordering of the most used objects.
     *
     * This class is optimized for speed rather than strictly correct encapsulation.
     */
    private static class CacheObject {

       /**
        * Underlying object wrapped by the CacheObject.
        */
        public Object object;

        /**
         * A reference to the node in the cache order list. We keep the reference
         * here to avoid linear scans of the list. Every time the object is
         * accessed, the node is removed from its current spot in the list and
         * moved to the front.
         */
        public LinkedListNode lastAccessedListNode;

        /**
         * A reference to the node in the age order list. We keep the reference
         * here to avoid linear scans of the list. The reference is used if the
         * object has to be deleted from the list.
         */
        public LinkedListNode ageListNode;

        /**
         * A count of the number of times the object has been read from cache.
         */
        public int readCount = 0;

        /**
         * Creates a new cache object wrapper.
         *
         * @param object the underlying Object to wrap.
         */
        public CacheObject(Object object) {
            this.object = object;
        }

        public boolean equals(Object o) {
            if (this == o) {
                return true;
            }
            if (!(o instanceof CacheObject)) {
                return false;
            }

            final CacheObject cacheObject = (CacheObject) o;

            if (!object.equals(cacheObject.object)) {
                return false;
            }

            return true;
        }

        public int hashCode() {
            return object.hashCode();
        }
    }

    /**
     * Simple LinkedList implementation. The main feature is that list nodes
     * are public, which allows very fast delete operations when one has a
     * reference to the node that is to be deleted.<p>
     */
    private static class LinkedList {

        /**
         * The root of the list keeps a reference to both the first and last
         * elements of the list.
         */
        private LinkedListNode head = new LinkedListNode("head", null, null);

        /**
         * Creates a new linked list.
         */
        public LinkedList() {
            head.next = head.previous = head;
        }

        /**
         * Returns the first linked list node in the list.
         *
         * @return the first element of the list.
         */
        public LinkedListNode getFirst() {
            LinkedListNode node = head.next;
            if (node == head) {
                return null;
            }
            return node;
        }

        /**
         * Returns the last linked list node in the list.
         *
         * @return the last element of the list.
         */
        public LinkedListNode getLast() {
            LinkedListNode node = head.previous;
            if (node == head) {
                return null;
            }
            return node;
        }

        /**
         * Adds a node to the beginning of the list.
         *
         * @param node the node to add to the beginning of the list.
         */
        public LinkedListNode addFirst(LinkedListNode node) {
            node.next = head.next;
            node.previous = head;
            node.previous.next = node;
            node.next.previous = node;
            return node;
        }

        /**
         * Adds an object to the beginning of the list by automatically creating a
         * a new node and adding it to the beginning of the list.
         *
         * @param object the object to add to the beginning of the list.
         * @return the node created to wrap the object.
         */
        public LinkedListNode addFirst(Object object) {
            LinkedListNode node = new LinkedListNode(object, head.next, head);
            node.previous.next = node;
            node.next.previous = node;
            return node;
        }

        /**
         * Adds an object to the end of the list by automatically creating a
         * a new node and adding it to the end of the list.
         *
         * @param object the object to add to the end of the list.
         * @return the node created to wrap the object.
         */
        public LinkedListNode addLast(Object object) {
            LinkedListNode node = new LinkedListNode(object, head, head.previous);
            node.previous.next = node;
            node.next.previous = node;
            return node;
        }

        /**
         * Erases all elements in the list and re-initializes it.
         */
        public void clear() {
            //Remove all references in the list.
            LinkedListNode node = getLast();
            while (node != null) {
                node.remove();
                node = getLast();
            }

            //Re-initialize.
            head.next = head.previous = head;
        }

        /**
         * Returns a String representation of the linked list with a comma
         * delimited list of all the elements in the list.
         *
         * @return a String representation of the LinkedList.
         */
        public String toString() {
            LinkedListNode node = head.next;
            StringBuilder buf = new StringBuilder();
            while (node != head) {
                buf.append(node.toString()).append(", ");
                node = node.next;
            }
            return buf.toString();
        }
    }

    /**
     * Doubly linked node in a LinkedList. Most LinkedList implementations keep the
     * equivalent of this class private. We make it public so that references
     * to each node in the list can be maintained externally.
     *
     * Exposing this class lets us make remove operations very fast. Remove is
     * built into this class and only requires two reference reassignments. If
     * remove existed in the main LinkedList class, a linear scan would have to
     * be performed to find the correct node to delete.
     *
     * The linked list implementation was specifically written for the Jive
     * cache system. While it can be used as a general purpose linked list, for
     * most applications, it is more suitable to use the linked list that is part
     * of the Java Collections package.
     */
    private static class LinkedListNode {

        public LinkedListNode previous;
        public LinkedListNode next;
        public Object object;

        /**
         * This class is further customized for the Jive cache system. It
         * maintains a timestamp of when a Cacheable object was first added to
         * cache. Timestamps are stored as long values and represent the number
         * of milliseconds passed since January 1, 1970 00:00:00.000 GMT.<p>
         *
         * The creation timestamp is used in the case that the cache has a
         * maximum lifetime set. In that case, when
         * [current time] - [creation time] > [max lifetime], the object will be
         * deleted from cache.
         */
        public long timestamp;

        /**
         * Constructs a new linked list node.
         *
         * @param object the Object that the node represents.
         * @param next a reference to the next LinkedListNode in the list.
         * @param previous a reference to the previous LinkedListNode in the list.
         */
        public LinkedListNode(Object object, LinkedListNode next,
                LinkedListNode previous)
        {
            this.object = object;
            this.next = next;
            this.previous = previous;
        }

        /**
         * Removes this node from the linked list that it is a part of.
         */
        public void remove() {
            previous.next = next;
            next.previous = previous;
        }

        /**
         * Returns a String representation of the linked list node by calling the
         * toString method of the node's object.
         *
         * @return a String representation of the LinkedListNode.
         */
        public String toString() {
            return object.toString();
        }
    }
}