Package org.apache.lucene.util

Class PriorityQueue<T>

  • All Implemented Interfaces:
    Iterable<T>
    Direct Known Subclasses:
    FieldValueHitQueue, HitQueue
    public abstract class PriorityQueue<T>
extends Object
implements Iterable<T>
    A priority queue 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 but the remove() cost implemented here is linear.

    NOTE: This class pre-allocates an array of length maxSize+1 and pre-fills it with elements if instantiated via the PriorityQueue(int,Supplier) constructor.

    NOTE: Iteration order is not specified.

    NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.

    • Constructor Summary

      Constructors 
      Constructor Description
      PriorityQueue​(int maxSize)
      Create an empty priority queue of the configured size.
      PriorityQueue​(int maxSize, Supplier<T> sentinelObjectSupplier)
      Create a priority queue that is pre-filled with sentinel objects, 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.

    • Constructor Detail

      • PriorityQueue

        public PriorityQueue​(int maxSize)
        Create an empty priority queue of the configured size.

      • PriorityQueue

        public PriorityQueue​(int maxSize,
                     Supplier<T> sentinelObjectSupplier)
        Create a priority queue that is pre-filled with sentinel objects, 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(T, T) should always favor the non-sentinel values).

        By default, the supplier returns null, which means the queue will not be filled with sentinel values. Otherwise, the value returned will be used to pre-populate the queue.

        If this method is extended to return a non-null value, then the following usage pattern is recommended: 

         PriorityQueue<MyObject> pq = new MyQueue<MyObject>(numHits);
 // save the 'top' element, which is guaranteed to not be null.
 MyObject pqTop = 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: the given supplier will be called maxSize 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 and all returned instances must compare equal.

    • Method Detail

      • addAll

        public void addAll​(Collection<T> elements)
        Adds all elements of the collection into the queue. This method should be preferred over calling add(Object) in loop if all elements are known in advance as it builds queue faster.

        If one tries to add more objects than the maxSize passed in the constructor, an ArrayIndexOutOfBoundsException is thrown.

      • lessThan

        protected abstract boolean lessThan​(T a,
                                    T b)
        Determines the ordering of objects in this priority queue. Subclasses must define this one method.
        Returns:
        true iff parameter a is less than parameter b.

      • add

        public final T add​(T 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.

      • insertWithOverflow

        public T insertWithOverflow​(T element)
        Adds an Object to a PriorityQueue in log(size) time. 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 T top()
        Returns the least element of the PriorityQueue in constant time.

      • pop

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

      • updateTop

        public final T 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.

      • updateTop

        public final T updateTop​(T newTop)
        Replace the top of the pq with newTop and run updateTop().

      • 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.

      • remove

        public final boolean remove​(T element)
        Removes an existing element currently stored in the PriorityQueue. Cost is linear with the size of the queue. (A specialization of PriorityQueue which tracks element positions would provide a constant remove time but the trade-off would be extra cost to all additions/insertions)

      • getHeapArray

        protected final Object[] getHeapArray()
        This method returns the internal heap array as Object[].
        NOTE: This API is for internal purposes only and might change in incompatible ways in the next release.