package org.eclipse.jdt.internal.core.index.impl;

/*
 * (c) Copyright IBM Corp. 2000, 2001.
 * All Rights Reserved.
 */

import java.util.Hashtable;
import java.util.Enumeration;

/**
 * The <code>LRUCache</code> is a hashtable that stores a finite number of elements.  
 * When an attempt is made to add values to a full cache, the least recently used values
 * in the cache are discarded to make room for the new values as necessary.
 * 
 * <p>The data structure is based on the LRU virtual memory paging scheme.
 * 
 * <p>Objects can take up a variable amount of cache space by implementing
 * the <code>ILRUCacheable</code> interface.
 *
 * <p>This implementation is NOT thread-safe.  Synchronization wrappers would
 * have to be added to ensure atomic insertions and deletions from the cache.
 *
 * @see org.eclipse.jdt.internal.core.index.impl.ILRUCacheable
 */
public class LRUCache implements Cloneable {

	/**
	 * This type is used internally by the LRUCache to represent entries 
	 * stored in the cache.
	 * It is static because it does not require a pointer to the cache
	 * which contains it.
	 *
	 * @see LRUCache
	 */
	protected static class LRUCacheEntry {

		/**
		 * Hash table key
		 */
		/* package */
		Object _fKey;

		/**
		 * Hash table value (an LRUCacheEntry object)
		 */
		/* package */
		Object _fValue;

		/**
		 * Time value for queue sorting
		 */
		/* package */
		int _fTimestamp;

		/**
		 * Cache footprint of this entry
		 */
		int _fSpace;

		/**
		 * Previous entry in queue
		 */
		/* package */
		LRUCacheEntry _fPrevious;

		/**
		 * Next entry in queue
		 */
		/* package */
		LRUCacheEntry _fNext;

		/**
		 * Creates a new instance of the receiver with the provided values
		 * for key, value, and space.
		 */
		public LRUCacheEntry(Object key, Object value, int space) {
			_fKey= key;
			_fValue= value;
			_fSpace= space;
		}

		/**
		 * Returns a String that represents the value of this object.
		 */
		public String toString() {

			return "LRUCacheEntry [" + _fKey + "-->" + _fValue + "]"; //$NON-NLS-3$ //$NON-NLS-1$ //$NON-NLS-2$
		}
	}

	/**
	 * Amount of cache space used so far
	 */
	protected int fCurrentSpace;

	/**
	 * Maximum space allowed in cache
	 */
	protected int fSpaceLimit;

	/**
	 * Counter for handing out sequential timestamps
	 */
	protected int fTimestampCounter;

	/**
	 * Hash table for fast random access to cache entries
	 */
	protected Hashtable fEntryTable;

	/**
	 * Start of queue (most recently used entry) 
	 */
	protected LRUCacheEntry fEntryQueue;

	/**
	 * End of queue (least recently used entry)
	 */
	protected LRUCacheEntry fEntryQueueTail;

	/**
	 * Default amount of space in the cache
	 */
	protected static final int DEFAULT_SPACELIMIT= 100;
	/**
	 * Creates a new cache.  Size of cache is defined by 
	 * <code>DEFAULT_SPACELIMIT</code>.
	 */
	public LRUCache() {

		this(DEFAULT_SPACELIMIT);
	}
	/**
	 * Creates a new cache.
	 * @param size Size of Cache
	 */
	public LRUCache(int size) {

		fTimestampCounter= fCurrentSpace= 0;
		fEntryQueue= fEntryQueueTail= null;
		fEntryTable= new Hashtable(size);
		fSpaceLimit= size;
	}
	/**
	 * Returns a new cache containing the same contents.
	 *
	 * @return New copy of object.
	 */
	public Object clone() {

		LRUCache newCache= newInstance(fSpaceLimit);
		LRUCacheEntry qEntry;

		/* Preserve order of entries by copying from oldest to newest */
		qEntry= this.fEntryQueueTail;
		while (qEntry != null) {
			newCache.privateAdd(qEntry._fKey, qEntry._fValue, qEntry._fSpace);
			qEntry= qEntry._fPrevious;
		}
		return newCache;
	}
	/**
	 * Flushes all entries from the cache.
	 */
	public void flush() {

		fCurrentSpace= 0;
		LRUCacheEntry entry= fEntryQueueTail; // Remember last entry
		fEntryTable= new Hashtable(); // Clear it out
		fEntryQueue= fEntryQueueTail= null;
		while (entry != null) { // send deletion notifications in LRU order
			privateNotifyDeletionFromCache(entry);
			entry= entry._fPrevious;
		}
	}
	/**
	 * Flushes the given entry from the cache.  Does nothing if entry does not
	 * exist in cache.
	 *
	 * @param key Key of object to flush
	 */
	public void flush(Object key) {

		LRUCacheEntry entry;

		entry= (LRUCacheEntry) fEntryTable.get(key);

		/* If entry does not exist, return */
		if (entry == null)
			return;

		this.privateRemoveEntry(entry, false);
	}
	/**
	 * Answers the value in the cache at the given key.
	 * If the value is not in the cache, returns null
	 *
	 * @param key Hash table key of object to retrieve
	 * @return Retreived object, or null if object does not exist
	 */
	public Object get(Object key) {

		LRUCacheEntry entry= (LRUCacheEntry) fEntryTable.get(key);
		if (entry == null) {
			return null;
		}

		this.updateTimestamp(entry);
		return entry._fValue;
	}
	/**
	 * Returns the amount of space that is current used in the cache.
	 */
	public int getCurrentSpace() {
		return fCurrentSpace;
	}
	/**
	 * Returns the maximum amount of space available in the cache.
	 */
	public int getSpaceLimit() {
		return fSpaceLimit;
	}
	/**
	 * Returns an Enumeration of the keys currently in the cache.
	 */
	public Enumeration keys() {

		return fEntryTable.keys();
	}
	/**
	 * Returns an enumeration that iterates over all the keys and values 
	 * currently in the cache.
	 */
	public ICacheEnumeration keysAndValues() {
		return new ICacheEnumeration() {

			Enumeration fValues= fEntryTable.elements();
			LRUCacheEntry fEntry;

			public boolean hasMoreElements() {
				return fValues.hasMoreElements();
			}

			public Object nextElement() {
				fEntry= (LRUCacheEntry) fValues.nextElement();
				return fEntry._fKey;
			}

			public Object getValue() {
				if (fEntry == null) {
					throw new java.util.NoSuchElementException();
				}
				return fEntry._fValue;
			}
		};
	}
	/**
	 * Ensures there is the specified amount of free space in the receiver,
	 * by removing old entries if necessary.  Returns true if the requested space was
	 * made available, false otherwise.
	 *
	 * @param space Amount of space to free up
	 */
	protected boolean makeSpace(int space) {

		int limit;

		limit= this.getSpaceLimit();

		/* if space is already available */
		if (fCurrentSpace + space <= limit) {
			return true;
		}

		/* if entry is too big for cache */
		if (space > limit) {
			return false;
		}

		/* Free up space by removing oldest entries */
		while (fCurrentSpace + space > limit && fEntryQueueTail != null) {
			this.privateRemoveEntry(fEntryQueueTail, false);
		}
		return true;
	}
	/**
	 * Returns a new LRUCache instance
	 */
	protected LRUCache newInstance(int size) {
		return new LRUCache(size);
	}
	/**
	 * Adds an entry for the given key/value/space.
	 */
	protected void privateAdd(Object key, Object value, int space) {

		LRUCacheEntry entry;

		entry= new LRUCacheEntry(key, value, space);
		this.privateAddEntry(entry, false);
	}
	/**
	 * Adds the given entry from the receiver.
	 * @param shuffle Indicates whether we are just shuffling the queue 
	 * (i.e., the entry table is left alone).
	 */
	protected void privateAddEntry(LRUCacheEntry entry, boolean shuffle) {

		if (!shuffle) {
			fEntryTable.put(entry._fKey, entry);
			fCurrentSpace += entry._fSpace;
		}

		entry._fTimestamp= fTimestampCounter++;
		entry._fNext= this.fEntryQueue;
		entry._fPrevious= null;

		if (fEntryQueue == null) {
			/* this is the first and last entry */
			fEntryQueueTail= entry;
		} else {
			fEntryQueue._fPrevious= entry;
		}

		fEntryQueue= entry;
	}
	/**
	 * An entry has been removed from the cache, for example because it has 
	 * fallen off the bottom of the LRU queue.  
	 * Subclasses could over-ride this to implement a persistent cache below the LRU cache.
	 */
	protected void privateNotifyDeletionFromCache(LRUCacheEntry entry) {
		// Default is NOP.
	}
	/**
	 * Removes the entry from the entry queue.  
	 * @param shuffle indicates whether we are just shuffling the queue 
	 * (i.e., the entry table is left alone).
	 */
	protected void privateRemoveEntry(LRUCacheEntry entry, boolean shuffle) {

		LRUCacheEntry previous, next;

		previous= entry._fPrevious;
		next= entry._fNext;

		if (!shuffle) {
			fEntryTable.remove(entry._fKey);
			fCurrentSpace -= entry._fSpace;
			privateNotifyDeletionFromCache(entry);
		}

		/* if this was the first entry */
		if (previous == null) {
			fEntryQueue= next;
		} else {
			previous._fNext= next;
		}

		/* if this was the last entry */
		if (next == null) {
			fEntryQueueTail= previous;
		} else {
			next._fPrevious= previous;
		}
	}
	/**
	 * Sets the value in the cache at the given key. Returns the value.
	 *
	 * @param key Key of object to add.
	 * @param value Value of object to add.
	 * @return added value.
	 */
	public Object put(Object key, Object value) {

		int newSpace, oldSpace, newTotal;
		LRUCacheEntry entry;

		/* Check whether there's an entry in the cache */
		newSpace= spaceFor(key, value);
		entry= (LRUCacheEntry) fEntryTable.get(key);

		if (entry != null) {

			/**
			 * Replace the entry in the cache if it would not overflow
			 * the cache.  Otherwise flush the entry and re-add it so as 
			 * to keep cache within budget
			 */
			oldSpace= entry._fSpace;
			newTotal= getCurrentSpace() - oldSpace + newSpace;
			if (newTotal <= getSpaceLimit()) {
				updateTimestamp(entry);
				entry._fValue= value;
				entry._fSpace= newSpace;
				this.fCurrentSpace= newTotal;
				return value;
			} else {
				privateRemoveEntry(entry, false);
			}
		}
		if (makeSpace(newSpace)) {
			privateAdd(key, value, newSpace);
		}
		return value;
	}
	/**
	 * Removes and returns the value in the cache for the given key.
	 * If the key is not in the cache, returns null.
	 *
	 * @param key Key of object to remove from cache.
	 * @return Value removed from cache.
	 */
	public Object removeKey(Object key) {

		LRUCacheEntry entry= (LRUCacheEntry) fEntryTable.get(key);
		if (entry == null) {
			return null;
		}
		Object value= entry._fValue;
		this.privateRemoveEntry(entry, false);
		return value;
	}
	/**
	 * Sets the maximum amount of space that the cache can store
	 *
	 * @param limit Number of units of cache space
	 */
	public void setSpaceLimit(int limit) {
		if (limit < fSpaceLimit) {
			makeSpace(fSpaceLimit - limit);
		}
		fSpaceLimit= limit;
	}
	/**
	 * Returns the space taken by the given key and value.
	 */
	protected int spaceFor(Object key, Object value) {

		if (value instanceof ILRUCacheable) {
			return ((ILRUCacheable) value).getCacheFootprint();
		} else {
			return 1;
		}
	}
	/**
	 * Returns a String that represents the value of this object.  This method
	 * is for debugging purposes only.
	 */
	public String toString() {

		return "LRUCache " + (getCurrentSpace() * 100.0 / getSpaceLimit()) + "% full"; //$NON-NLS-1$ //$NON-NLS-2$
	}
	/**
	 * Updates the timestamp for the given entry, ensuring that the queue is 
	 * kept in correct order.  The entry must exist
	 */
	protected void updateTimestamp(LRUCacheEntry entry) {

		entry._fTimestamp= fTimestampCounter++;
		if (fEntryQueue != entry) {
			this.privateRemoveEntry(entry, true);
			this.privateAddEntry(entry, true);
		}
		return;
	}
}