org.apache.lucene.util
Class PriorityQueue

java.lang.Object
  extended by org.apache.lucene.util.PriorityQueue
Direct Known Subclasses:
FieldSortedHitQueue, FieldValueHitQueue, FuzzyQuery.ScoreTermQueue

public abstract class PriorityQueue
extends Object

A PriorityQueue maintains a partial ordering of its elements such that the least element can always be found in constant time. Put()'s and pop()'s require log(size) time.

NOTE: This class pre-allocates a full array of length maxSize+1, in initialize(int).


Field Summary
protected  Object[] heap
           
 
Constructor Summary
PriorityQueue()
           
 
Method Summary
 Object add(Object element)
          Adds an Object to a PriorityQueue in log(size) time.
 void adjustTop()
          Deprecated. use updateTop() which returns the new top element and saves an additional call to top().
 void clear()
          Removes all entries from the PriorityQueue.
protected  Object getSentinelObject()
          This method can be overridden by extending classes to return a sentinel object which will be used by initialize(int) to fill the queue, so that the code which uses that queue can always assume it's full and only change the top without attempting to insert any new object.
Those sentinel values should always compare worse than any non-sentinel value (i.e., lessThan(Object, Object) should always favor the non-sentinel values).
By default, this method returns false, which means the queue will not be filled with sentinel values.
protected  void initialize(int maxSize)
          Subclass constructors must call this.
 boolean insert(Object element)
          Deprecated. use insertWithOverflow(Object) instead, which encourages objects reuse.
 Object insertWithOverflow(Object element)
          insertWithOverflow() is the same as insert() except its return value: it returns the object (if any) that was dropped off the heap because it was full.
protected abstract  boolean lessThan(Object a, Object b)
          Determines the ordering of objects in this priority queue.
 Object pop()
          Removes and returns the least element of the PriorityQueue in log(size) time.
 void put(Object element)
          Deprecated. use add(Object) which returns the new top object, saving an additional call to top().
 int size()
          Returns the number of elements currently stored in the PriorityQueue.
 Object top()
          Returns the least element of the PriorityQueue in constant time.
 Object updateTop()
          Should be called when the Object at top changes values.
 
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

heap

protected Object[] heap
Constructor Detail

PriorityQueue

public PriorityQueue()
Method Detail

lessThan

protected abstract boolean lessThan(Object a,
                                    Object b)
Determines the ordering of objects in this priority queue. Subclasses must define this one method.


getSentinelObject

protected Object getSentinelObject()
This method can be overridden by extending classes to return a sentinel object which will be used by initialize(int) to fill the queue, so that the code which uses that queue can always assume it's full and only change the top without attempting to insert any new object.
Those sentinel values should always compare worse than any non-sentinel value (i.e., lessThan(Object, Object) should always favor the non-sentinel values).
By default, this method returns false, which means the queue will not be filled with sentinel values. Otherwise, the value returned will be used to pre-populate the queue. Adds sentinel values to the queue.
If this method is extended to return a non-null value, then the following usage pattern is recommended:
 // extends getSentinelObject() to return a non-null value.
 PriorityQueue pq = new MyQueue(numHits);
 // save the 'top' element, which is guaranteed to not be null.
 MyObject pqTop = (MyObject) pq.top();
 <...>
 // now in order to add a new element, which is 'better' than top (after 
 // you've verified it is better), it is as simple as:
 pqTop.change().
 pqTop = pq.updateTop();
 
NOTE: if this method returns a non-null value, it will be called by initialize(int) size() times, relying on a new object to be returned and will not check if it's null again. Therefore you should ensure any call to this method creates a new instance and behaves consistently, e.g., it cannot return null if it previously returned non-null.

Returns:
the sentinel object to use to pre-populate the queue, or null if sentinel objects are not supported.

initialize

protected final void initialize(int maxSize)
Subclass constructors must call this.


put

public final void put(Object element)
Deprecated. use add(Object) which returns the new top object, saving an additional call to top().

Adds an Object to a PriorityQueue in log(size) time. If one tries to add more objects than maxSize from initialize a RuntimeException (ArrayIndexOutOfBound) is thrown.


add

public final Object add(Object element)
Adds an Object to a PriorityQueue in log(size) time. If one tries to add more objects than maxSize from initialize an ArrayIndexOutOfBoundsException is thrown.

Returns:
the new 'top' element in the queue.

insert

public boolean insert(Object element)
Deprecated. use insertWithOverflow(Object) instead, which encourages objects reuse.

Adds element to the PriorityQueue in log(size) time if either the PriorityQueue is not full, or not lessThan(element, top()).

Parameters:
element -
Returns:
true if element is added, false otherwise.

insertWithOverflow

public Object insertWithOverflow(Object element)
insertWithOverflow() is the same as insert() except its return value: it returns the object (if any) that was dropped off the heap because it was full. This can be the given parameter (in case it is smaller than the full heap's minimum, and couldn't be added), or another object that was previously the smallest value in the heap and now has been replaced by a larger one, or null if the queue wasn't yet full with maxSize elements.


top

public final Object top()
Returns the least element of the PriorityQueue in constant time.


pop

public final Object pop()
Removes and returns the least element of the PriorityQueue in log(size) time.


adjustTop

public final void adjustTop()
Deprecated. use updateTop() which returns the new top element and saves an additional call to top().

Should be called when the Object at top changes values. Still log(n) worst case, but it's at least twice as fast to
 pq.top().change();
 pq.adjustTop();
 
instead of
 o = pq.pop();
 o.change();
 pq.push(o);
 


updateTop

public final Object updateTop()
Should be called when the Object at top changes values. Still log(n) worst case, but it's at least twice as fast to
 pq.top().change();
 pq.updateTop();
 
instead of
 o = pq.pop();
 o.change();
 pq.push(o);
 

Returns:
the new 'top' element.

size

public final int size()
Returns the number of elements currently stored in the PriorityQueue.


clear

public final void clear()
Removes all entries from the PriorityQueue.



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