/*******************************************************************************
 * Copyright (c) 2000, 2016 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/epl-v10.html
 *
 *******************************************************************************/
package org.eclipse.dltk.internal.core;

import java.util.Enumeration;
import java.util.Iterator;

import org.eclipse.dltk.core.DLTKCore;
import org.eclipse.dltk.internal.core.util.LRUCache;
import org.eclipse.dltk.internal.core.util.Messages;

/**
 * The <code>OverflowingLRUCache</code> is an LRUCache which attempts to
 * maintain a size equal or less than its <code>fSpaceLimit</code> by removing
 * the least recently used elements.
 * 
 * <p>
 * The cache will remove elements which successfully close and all elements
 * which are explicitly removed.
 * 
 * <p>
 * If the cache cannot remove enough old elements to add new elements it will
 * grow beyond <code>fSpaceLimit</code>. Later, it will attempt to shink back
 * to the maximum space limit.
 * 
 * The method <code>close</code> should attempt to close the element. If the
 * element is successfully closed it will return true and the element will be
 * removed from the cache. Otherwise the element will remain in the cache.
 * 
 * <p>
 * The cache implicitly attempts shrinks on calls to <code>put</code>and
 * <code>setSpaceLimit</code>. Explicitly calling the <code>shrink</code>
 * method will also cause the cache to attempt to shrink.
 * 
 * <p>
 * The cache calculates the used space of all elements which implement
 * <code>ILRUCacheable</code>. All other elements are assumed to be of size
 * one.
 * 
 * <p>
 * Use the <code>#peek(Object)</code> and <code>#disableTimestamps()</code>
 * method to circumvent the timestamp feature of the cache. This feature is
 * intended to be used only when the <code>#close(LRUCacheEntry)</code> method
 * causes changes to the cache. For example, if a parent closes its children
 * when </code>#close(LRUCacheEntry)</code> is called, it should be careful
 * not to change the LRU linked list. It can be sure it is not causing problems
 * by calling <code>#peek(Object)</code> instead of <code>#get(Object)</code>
 * method.
 * 
 * @see LRUCache
 */
public abstract class OverflowingLRUCache extends LRUCache {
	/**
	 * Indicates if the cache has been over filled and by how much.
	 */
	protected int fOverflow = 0;
	/**
	 * Indicates whether or not timestamps should be updated
	 */
	protected boolean fTimestampsOn = true;
	/**
	 * Indicates how much space should be reclaimed when the cache overflows.
	 * Inital load factor of one third.
	 */
	protected double fLoadFactor = 0.333;

	/**
	 * Creates a OverflowingLRUCache.
	 * 
	 * @param size
	 *            Size limit of cache.
	 */
	public OverflowingLRUCache(int size) {
		this(size, 0);
	}

	/**
	 * Creates a OverflowingLRUCache.
	 * 
	 * @param size
	 *            Size limit of cache.
	 * @param overflow
	 *            Size of the overflow.
	 */
	public OverflowingLRUCache(int size, int overflow) {
		super(size);
		fOverflow = overflow;
	}

	/**
	 * Returns a new cache containing the same contents.
	 * 
	 * @return New copy of this object.
	 */
	@Override
	public Object clone() {

		OverflowingLRUCache newCache = (OverflowingLRUCache) newInstance(
				fSpaceLimit, fOverflow);
		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;
	}

	/**
	 * Returns true if the element is successfully closed and removed from the
	 * cache, otherwise false.
	 * 
	 * <p>
	 * NOTE: this triggers an external remove from the cache by closing the
	 * obejct.
	 * 
	 */
	protected abstract boolean close(LRUCacheEntry entry);

	/**
	 * Returns an enumerator of the values in the cache with the most recently
	 * used first.
	 */
	public Enumeration elements() {
		if (fEntryQueue == null)
			return new LRUCacheEnumerator(null);
		LRUCacheEnumerator.LRUEnumeratorElement head = new LRUCacheEnumerator.LRUEnumeratorElement(
				fEntryQueue._fValue);
		LRUCacheEntry currentEntry = fEntryQueue._fNext;
		LRUCacheEnumerator.LRUEnumeratorElement currentElement = head;
		while (currentEntry != null) {
			currentElement.fNext = new LRUCacheEnumerator.LRUEnumeratorElement(
					currentEntry._fValue);
			currentElement = currentElement.fNext;

			currentEntry = currentEntry._fNext;
		}
		return new LRUCacheEnumerator(head);
	}

	@Override
	public double fillingRatio() {
		return (fCurrentSpace + fOverflow) * 100.0 / fSpaceLimit;
	}

	/**
	 * For internal testing only. This method exposed only for testing purposes!
	 * 
	 * @return Hashtable of entries
	 */
	public java.util.Hashtable getEntryTable() {
		return fEntryTable;
	}

	/**
	 * Returns the load factor for the cache. The load factor determines how
	 * much space is reclaimed when the cache exceeds its space limit.
	 * 
	 * @return double
	 */
	public double getLoadFactor() {
		return fLoadFactor;
	}

	/**
	 * @return The space by which the cache has overflown.
	 */
	public int getOverflow() {
		return fOverflow;
	}

	/**
	 * 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. May not be able to free enough space
	 * since some elements cannot be removed until they are saved.
	 * 
	 * @param space
	 *            Amount of space to free up
	 */
	@Override
	protected boolean makeSpace(int space) {

		int limit = fSpaceLimit;
		if (fOverflow == 0 && fCurrentSpace + space <= limit) {
			/* if space is already available */
			return true;
		}

		/* Free up space by removing oldest entries */
		int spaceNeeded = (int) ((1 - fLoadFactor) * limit);
		spaceNeeded = (spaceNeeded > space) ? spaceNeeded : space;
		LRUCacheEntry entry = fEntryQueueTail;

		try {
			// disable timestamps update while making space so that the previous
			// and next links are not changed
			// (by a call to get(Object) for example)
			fTimestampsOn = false;

			while (fCurrentSpace + spaceNeeded > limit && entry != null) {
				this.privateRemoveEntry(entry, false, false);
				entry = entry._fPrevious;
			}
		} finally {
			fTimestampsOn = true;
		}

		/* check again, since we may have aquired enough space */
		if (fCurrentSpace + space <= limit) {
			fOverflow = 0;
			return true;
		}

		/* update fOverflow */
		fOverflow = fCurrentSpace + space - limit;
		return false;
	}

	/**
	 * Returns a new instance of the reciever.
	 */
	protected abstract LRUCache newInstance(int size, int overflow);

	/**
	 * For testing purposes only
	 */
	public void printStats() {
		int forwardListLength = 0;
		LRUCacheEntry entry = fEntryQueue;
		while (entry != null) {
			forwardListLength++;
			entry = entry._fNext;
		}
		if (DLTKCore.VERBOSE) {
			System.out.println("Forward length: " + forwardListLength); //$NON-NLS-1$
		}

		int backwardListLength = 0;
		entry = fEntryQueueTail;
		while (entry != null) {
			backwardListLength++;
			entry = entry._fPrevious;
		}

		if (DLTKCore.VERBOSE) {
			System.out.println("Backward length: " + backwardListLength); //$NON-NLS-1$
		}

		Enumeration keys = fEntryTable.keys();
		class Temp {
			public Class fClass;
			public int fCount;

			public Temp(Class aClass) {
				fClass = aClass;
				fCount = 1;
			}

			@Override
			public String toString() {
				return "Class: " + fClass + " has " + fCount + " entries."; //$NON-NLS-2$ //$NON-NLS-3$ //$NON-NLS-1$
			}
		}
		java.util.HashMap h = new java.util.HashMap();
		while (keys.hasMoreElements()) {
			entry = (LRUCacheEntry) fEntryTable.get(keys.nextElement());
			Class key = entry._fValue.getClass();
			Temp t = (Temp) h.get(key);
			if (t == null) {
				h.put(key, new Temp(key));
			} else {
				t.fCount++;
			}
		}

		if (DLTKCore.VERBOSE) {
			for (Iterator iter = h.keySet().iterator(); iter.hasNext();) {
				System.out.println(h.get(iter.next()));
			}
		}
	}

	/**
	 * Removes the entry from the entry queue. Calls
	 * <code>privateRemoveEntry</code> with the external functionality
	 * enabled.
	 * 
	 * @param shuffle
	 *            indicates whether we are just shuffling the queue (in which
	 *            case, the entry table is not modified).
	 */
	@Override
	protected void privateRemoveEntry(LRUCacheEntry entry, boolean shuffle) {
		privateRemoveEntry(entry, shuffle, true);
	}

	/**
	 * Removes the entry from the entry queue. If <i>external</i> is true, the
	 * entry is removed without checking if it can be removed. It is assumed
	 * that the client has already closed the element it is trying to remove (or
	 * will close it promptly).
	 * 
	 * If <i>external</i> is false, and the entry could not be closed, it is
	 * not removed and the pointers are not changed.
	 * 
	 * @param shuffle
	 *            indicates whether we are just shuffling the queue (in which
	 *            case, the entry table is not modified).
	 */
	protected void privateRemoveEntry(LRUCacheEntry entry, boolean shuffle,
			boolean external) {

		if (!shuffle) {
			if (external) {
				fEntryTable.remove(entry._fKey);
				fCurrentSpace -= entry._fSpace;
				privateNotifyDeletionFromCache(entry);
			} else {
				if (!close(entry))
					return;
				// buffer close will recursively call #privateRemoveEntry with
				// external==true
				// thus entry will already be removed if reaching this point.
				if (fEntryTable.get(entry._fKey) == null) {
					return;
				} else {
					// basic removal
					fEntryTable.remove(entry._fKey);
					fCurrentSpace -= entry._fSpace;
					privateNotifyDeletionFromCache(entry);
				}
			}
		}
		LRUCacheEntry previous = entry._fPrevious;
		LRUCacheEntry next = entry._fNext;

		/* 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.
	 */
	@Override
	public Object put(Object key, Object value) {
		/* attempt to rid ourselves of the overflow, if there is any */
		if (fOverflow > 0)
			shrink();

		/* Check whether there's an entry in the cache */
		int newSpace = spaceFor(value);
		LRUCacheEntry 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
			 */
			int oldSpace = entry._fSpace;
			int newTotal = fCurrentSpace - oldSpace + newSpace;
			if (newTotal <= fSpaceLimit) {
				updateTimestamp(entry);
				entry._fValue = value;
				entry._fSpace = newSpace;
				fCurrentSpace = newTotal;
				fOverflow = 0;
				return value;
			} else {
				privateRemoveEntry(entry, false, false);
			}
		}

		// attempt to make new space
		makeSpace(newSpace);

		// add without worring about space, it will
		// be handled later in a makeSpace call
		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 remove(Object key) {
		return removeKey(key);
	}

	/**
	 * Sets the load factor for the cache. The load factor determines how much
	 * space is reclaimed when the cache exceeds its space limit.
	 * 
	 * @param newLoadFactor
	 *            double
	 * @throws IllegalArgumentException
	 *             when the new load factor is not in (0.0, 1.0]
	 */
	public void setLoadFactor(double newLoadFactor)
			throws IllegalArgumentException {
		if (newLoadFactor <= 1.0 && newLoadFactor > 0.0)
			fLoadFactor = newLoadFactor;
		else
			throw new IllegalArgumentException(Messages.cache_invalidLoadFactor);
	}

	/**
	 * Sets the maximum amount of space that the cache can store
	 * 
	 * @param limit
	 *            Number of units of cache space
	 */
	@Override
	public void setSpaceLimit(int limit) {
		if (limit < fSpaceLimit) {
			makeSpace(fSpaceLimit - limit);
		}
		fSpaceLimit = limit;
	}

	/**
	 * Attempts to shrink the cache if it has overflown. Returns true if the
	 * cache shrinks to less than or equal to <code>fSpaceLimit</code>.
	 */
	public boolean shrink() {
		if (fOverflow > 0)
			return makeSpace(0);
		return true;
	}

	/**
	 * Returns a String that represents the value of this object. This method is
	 * for debugging purposes only.
	 */
	@Override
	public String toString() {
		return toStringFillingRation("OverflowingLRUCache ") + //$NON-NLS-1$
				toStringContents();
	}

	/**
	 * Updates the timestamp for the given entry, ensuring that the queue is
	 * kept in correct order. The entry must exist.
	 * 
	 * <p>
	 * This method will do nothing if timestamps have been disabled.
	 */
	@Override
	protected void updateTimestamp(LRUCacheEntry entry) {
		if (fTimestampsOn) {
			entry._fTimestamp = fTimestampCounter++;
			if (fEntryQueue != entry) {
				this.privateRemoveEntry(entry, true);
				this.privateAddEntry(entry, true);
			}
		}
	}
}