org.apache.commons.collections.buffer

## Class PriorityBuffer

- java.lang.Object
- - java.util.AbstractCollection
  - - org.apache.commons.collections.buffer.PriorityBuffer

- All Implemented Interfaces:
    Serializable, Iterable, Collection, Buffer

      public class PriorityBuffer
      extends AbstractCollection
      implements Buffer, Serializable

  Binary heap implementation of `Buffer` that provides for removal based on `Comparator` ordering.

  The removal order of a binary heap is based on either the natural sort order of its elements or a specified `Comparator`. The `remove()` method always returns the first element as determined by the sort order. (The `ascendingOrder` flag in the constructors can be used to reverse the sort order, in which case `remove()` will always remove the last element.) The removal order is not the same as the order of iteration; elements are returned by the iterator in no particular order.

  The `add(Object)` and `remove()` operations perform in logarithmic time. The `get()` operation performs in constant time. All other operations perform in linear time or worse.

  Note that this implementation is not synchronized. Use `BufferUtils.synchronizedBuffer(Buffer)` or `SynchronizedBuffer.decorate(Buffer)` to provide synchronized access to a `PriorityBuffer`:

       Buffer heap = SynchronizedBuffer.decorate(new PriorityBuffer());

  This class is Serializable from Commons Collections 3.2.

  Since:
    Commons Collections 3.0 (previously BinaryHeap v1.0)
  Version:
    $Revision: 646777 $ $Date: 2008-04-10 14:33:15 +0200 (Thu, 10 Apr 2008) $
  Author:
    Peter Donald, Ram Chidambaram, Michael A. Smith, Paul Jack, Stephen Colebourne, Steve Phelps
  See Also:
    Serialized Form

- - ### Field Summary

    Fields
    Modifier and Type | Field and Description
    --- | ---
    `protected boolean` | `ascendingOrder` If true, the first element as determined by the sort order will be returned.
    `protected Comparator` | `comparator` The comparator used to order the elements
    `protected Object[]` | `elements` The elements in this buffer.
    `protected int` | `size` The number of elements currently in this buffer.

  - ### Constructor Summary

    Constructors
    Constructor and Description
    `PriorityBuffer()` Constructs a new empty buffer that sorts in ascending order by the natural order of the objects added.
    `PriorityBuffer(boolean ascendingOrder)` Constructs a new empty buffer specifying the sort order and using the natural order of the objects added.
    `PriorityBuffer(boolean ascendingOrder, Comparator comparator)` Constructs a new empty buffer specifying the sort order and comparator.
    `PriorityBuffer(Comparator comparator)` Constructs a new empty buffer that sorts in ascending order using the specified comparator.
    `PriorityBuffer(int capacity)` Constructs a new empty buffer that sorts in ascending order by the natural order of the objects added, specifying an initial capacity.
    `PriorityBuffer(int capacity, boolean ascendingOrder)` Constructs a new empty buffer that specifying initial capacity and sort order, using the natural order of the objects added.
    `PriorityBuffer(int capacity, boolean ascendingOrder, Comparator comparator)` Constructs a new empty buffer that specifying initial capacity, sort order and comparator.
    `PriorityBuffer(int capacity, Comparator comparator)` Constructs a new empty buffer that sorts in ascending order using the specified comparator and initial capacity.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `boolean` | `add(Object element)` Adds an element to the buffer.
    `void` | `clear()` Clears all elements from the buffer.
    `Comparator` | `comparator()` Gets the comparator being used for this buffer, null is natural order.
    `protected int` | `compare(Object a, Object b)` Compares two objects using the comparator if specified, or the natural order otherwise.
    `Object` | `get()` Gets the next element to be removed without actually removing it (peek).
    `protected void` | `grow()` Increases the size of the heap to support additional elements
    `boolean` | `isAscendingOrder()` Checks whether the heap is ascending or descending order.
    `protected boolean` | `isAtCapacity()` Tests if the buffer is at capacity.
    `Iterator` | `iterator()` Returns an iterator over this heap's elements.
    `protected void` | `percolateDownMaxHeap(int index)` Percolates element down heap from the position given by the index.
    `protected void` | `percolateDownMinHeap(int index)` Percolates element down heap from the position given by the index.
    `protected void` | `percolateUpMaxHeap(int index)` Percolates element up heap from from the position given by the index.
    `protected void` | `percolateUpMaxHeap(Object element)` Percolates a new element up heap from the bottom.
    `protected void` | `percolateUpMinHeap(int index)` Percolates element up heap from the position given by the index.
    `protected void` | `percolateUpMinHeap(Object element)` Percolates a new element up heap from the bottom.
    `Object` | `remove()` Gets and removes the next element (pop).
    `int` | `size()` Returns the number of elements in this buffer.
    `String` | `toString()` Returns a string representation of this heap.

    - ### Methods inherited from class java.util.AbstractCollection

      `addAll, contains, containsAll, isEmpty, remove, removeAll, retainAll, toArray, toArray`

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, wait, wait, wait`

    - ### Methods inherited from interface java.util.Collection

      `addAll, contains, containsAll, equals, hashCode, isEmpty, remove, removeAll, retainAll, toArray, toArray`

- - ### Field Detail

    - #### elements

          protected Object[] elements

      The elements in this buffer.

    - #### size

          protected int size

      The number of elements currently in this buffer.

    - #### ascendingOrder

          protected boolean ascendingOrder

      If true, the first element as determined by the sort order will be returned. If false, the last element as determined by the sort order will be returned.

    - #### comparator

          protected Comparator comparator

      The comparator used to order the elements

  - ### Constructor Detail

    - #### PriorityBuffer

          public PriorityBuffer()

      Constructs a new empty buffer that sorts in ascending order by the natural order of the objects added.

    - #### PriorityBuffer

          public PriorityBuffer(Comparator comparator)

      Constructs a new empty buffer that sorts in ascending order using the specified comparator.
      Parameters:
        `comparator` - the comparator used to order the elements, null means use natural order

    - #### PriorityBuffer

          public PriorityBuffer(boolean ascendingOrder)

      Constructs a new empty buffer specifying the sort order and using the natural order of the objects added.
      Parameters:
        `ascendingOrder` - if `true` the heap is created as a minimum heap; otherwise, the heap is created as a maximum heap

    - #### PriorityBuffer

          public PriorityBuffer(boolean ascendingOrder,
                        Comparator comparator)

      Constructs a new empty buffer specifying the sort order and comparator.
      Parameters:
        `ascendingOrder` - true to use the order imposed by the given comparator; false to reverse that order
        `comparator` - the comparator used to order the elements, null means use natural order

    - #### PriorityBuffer

          public PriorityBuffer(int capacity)

      Constructs a new empty buffer that sorts in ascending order by the natural order of the objects added, specifying an initial capacity.
      Parameters:
        `capacity` - the initial capacity for the buffer, greater than zero
      Throws:
        `IllegalArgumentException` - if `capacity` is <= `0`

    - #### PriorityBuffer

          public PriorityBuffer(int capacity,
                        Comparator comparator)

      Constructs a new empty buffer that sorts in ascending order using the specified comparator and initial capacity.
      Parameters:
        `capacity` - the initial capacity for the buffer, greater than zero
        `comparator` - the comparator used to order the elements, null means use natural order
      Throws:
        `IllegalArgumentException` - if `capacity` is <= `0`

    - #### PriorityBuffer

          public PriorityBuffer(int capacity,
                        boolean ascendingOrder)

      Constructs a new empty buffer that specifying initial capacity and sort order, using the natural order of the objects added.
      Parameters:
        `capacity` - the initial capacity for the buffer, greater than zero
        `ascendingOrder` - if `true` the heap is created as a minimum heap; otherwise, the heap is created as a maximum heap.
      Throws:
        `IllegalArgumentException` - if `capacity` is `<= 0`

    - #### PriorityBuffer

          public PriorityBuffer(int capacity,
                        boolean ascendingOrder,
                        Comparator comparator)

      Constructs a new empty buffer that specifying initial capacity, sort order and comparator.
      Parameters:
        `capacity` - the initial capacity for the buffer, greater than zero
        `ascendingOrder` - true to use the order imposed by the given comparator; false to reverse that order
        `comparator` - the comparator used to order the elements, null means use natural order
      Throws:
        `IllegalArgumentException` - if `capacity` is `<= 0`

  - ### Method Detail

    - #### isAscendingOrder

          public boolean isAscendingOrder()

      Checks whether the heap is ascending or descending order.
      Returns:
        true if ascending order (a min heap)

    - #### comparator

          public Comparator comparator()

      Gets the comparator being used for this buffer, null is natural order.
      Returns:
        the comparator in use, null is natural order

    - #### size

          public int size()

      Returns the number of elements in this buffer.
      Specified by:
        `size` in interface `Collection`
      Specified by:
        `size` in class `AbstractCollection`
      Returns:
        the number of elements in this buffer

    - #### clear

          public void clear()

      Clears all elements from the buffer.
      Specified by:
        `clear` in interface `Collection`
      Overrides:
        `clear` in class `AbstractCollection`

    - #### add

          public boolean add(Object element)

      Adds an element to the buffer.

      The element added will be sorted according to the comparator in use.

      Specified by:
        `add` in interface `Collection`
      Overrides:
        `add` in class `AbstractCollection`
      Parameters:
        `element` - the element to be added
      Returns:
        true always

    - #### get

          public Object get()

      Gets the next element to be removed without actually removing it (peek).
      Specified by:
        `get` in interface `Buffer`
      Returns:
        the next element
      Throws:
        `BufferUnderflowException` - if the buffer is empty

    - #### remove

          public Object remove()

      Gets and removes the next element (pop).
      Specified by:
        `remove` in interface `Buffer`
      Returns:
        the next element
      Throws:
        `BufferUnderflowException` - if the buffer is empty

    - #### isAtCapacity

          protected boolean isAtCapacity()

      Tests if the buffer is at capacity.
      Returns:
        `true` if buffer is full; `false` otherwise.

    - #### percolateDownMinHeap

          protected void percolateDownMinHeap(int index)

      Percolates element down heap from the position given by the index.

      Assumes it is a minimum heap.

      Parameters:
        `index` - the index for the element

    - #### percolateDownMaxHeap

          protected void percolateDownMaxHeap(int index)

      Percolates element down heap from the position given by the index.

      Assumes it is a maximum heap.

      Parameters:
        `index` - the index of the element

    - #### percolateUpMinHeap

          protected void percolateUpMinHeap(int index)

      Percolates element up heap from the position given by the index.

      Assumes it is a minimum heap.

      Parameters:
        `index` - the index of the element to be percolated up

    - #### percolateUpMinHeap

          protected void percolateUpMinHeap(Object element)

      Percolates a new element up heap from the bottom.

      Assumes it is a minimum heap.

      Parameters:
        `element` - the element

    - #### percolateUpMaxHeap

          protected void percolateUpMaxHeap(int index)

      Percolates element up heap from from the position given by the index.

      Assume it is a maximum heap.

      Parameters:
        `index` - the index of the element to be percolated up

    - #### percolateUpMaxHeap

          protected void percolateUpMaxHeap(Object element)

      Percolates a new element up heap from the bottom.

      Assume it is a maximum heap.

      Parameters:
        `element` - the element

    - #### compare

          protected int compare(Object a,
                    Object b)

      Compares two objects using the comparator if specified, or the natural order otherwise.
      Parameters:
        `a` - the first object
        `b` - the second object
      Returns:
        -ve if a less than b, 0 if they are equal, +ve if a greater than b

    - #### grow

          protected void grow()

      Increases the size of the heap to support additional elements

    - #### iterator

          public Iterator iterator()

      Returns an iterator over this heap's elements.
      Specified by:
        `iterator` in interface `Iterable`
      Specified by:
        `iterator` in interface `Collection`
      Specified by:
        `iterator` in class `AbstractCollection`
      Returns:
        an iterator over this heap's elements

    - #### toString

          public String toString()

      Returns a string representation of this heap. The returned string is similar to those produced by standard JDK collections.
      Overrides:
        `toString` in class `AbstractCollection`
      Returns:
        a string representation of this heap

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