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 LRUCacheEntry | |
* @see 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 ["/*nonNLS*/ + _fKey + "-->"/*nonNLS*/ + _fValue + "]"/*nonNLS*/; | |
} | |
} | |
/** | |
* 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 "/*nonNLS*/ + (getCurrentSpace() * 100.0 / getSpaceLimit()) + "% full"/*nonNLS*/; | |
} | |
/** | |
* 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; | |
} | |
} |