PriorityQueue----lucene中工具类

该类的作用同Bobo中的com.browseengine.bobo.util.BoundedPriorityQueue作用一样 。  

1、构造一个存储有序但容量固定的数组，数组中数据按升序排列 。

    可以向该PriorityQueue中插入数据，但这个数据比最小的那个数据小时，则不插入这个数据 。当数据大于最小的那个数据（top（）得到）时候，提出最小的那个数据，然后有序插入到相应的位置 。

源代码以及注释如下 ：

/** 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. */
public abstract class PriorityQueue {
  private int size;     //当前已经存入的数据
  private int maxSize;  //最大容量
  protected Object[] heap;//存储的的数组

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

  /** Subclass constructors must call this. */
  protected final void initialize(int maxSize) {
    size = 0;
    int heapSize;
    if (0 == maxSize)
      // We allocate 1 extra to avoid if statement in top()
      heapSize = 2;
    else
      heapSize = maxSize + 1;
    heap = new Object[heapSize];
    this.maxSize = maxSize;
  }

  /**
   * 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.
   */
  public final void put(Object element) {
    size++;
    heap[size] = element;
    upHeap();
  }

  /**
   * Adds element to the PriorityQueue in log(size) time if either
   * the PriorityQueue is not full, or not lessThan(element, top()).
   * @param element
   * @return true if element is added, false otherwise.
   */
  public boolean insert(Object element) {
    return insertWithOverflow(element) != 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   * heap and now has been replaced by a larger one, or null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.
   */
  public Object insertWithOverflow(Object element) {
    if (size < maxSize) {
      put(element);
      return null;
    } else if (size > 0 && !lessThan(element, heap[1])) {  //如果数组已经存满，而且这个数比第一个要大，则移除第一个然后插入
      Object ret = heap[1];
      heap[1] = element;
      //再排序
      adjustTop();
      return ret;
    } else {
      return element;
    }
  }

  /** Returns the least element of the PriorityQueue in constant time. */
  public final Object top() {
    // We don't need to check size here: if maxSize is 0,
    // then heap is length 2 array with both entries null.
    // If size is 0 then heap[1] is already null.
    return heap[1];
  }

  /** Removes and returns the least element of the PriorityQueue in log(size)
    time. */
  public final Object pop() {
    if (size > 0) {
      Object result = heap[1];              // save first value
      heap[1] = heap[size];              // move last to first
      heap[size] = null;              // permit GC of objects
      size--;
      downHeap();                  // adjust heap
      return result;
    } else
      return null;
  }

  /** Should be called when the Object at top changes values.  Still log(n)
   * worst case, but it's at least twice as fast to <pre>
   *  { pq.top().change(); pq.adjustTop(); }
   * </pre> instead of <pre>
   *  { o = pq.pop(); o.change(); pq.push(o); }
   * </pre>
   */
  public final void adjustTop() {
    downHeap();
  }

  /** Returns the number of elements currently stored in the PriorityQueue. */
  public final int size() {
    return size;
  }

  /** Removes all entries from the PriorityQueue. */
  public final void clear() {
    for (int i = 0; i <= size; i++)
      heap[i] = null;
    size = 0;
  }
  //移除头部的元素，既最小的那个元素
  private final void upHeap() {
    int i = size;
    Object node = heap[i];              // save bottom node
    int j = i >>> 1;
    while (j > 0 && lessThan(node, heap[j])) {
      heap[i] = heap[j];              // shift parents down
      i = j;
      j = j >>> 1;
    }
    heap[i] = node;                  // install saved node
  }
  //移除尾部的元素（最大的那个元素）
  private final void downHeap() {
    int i = 1;
    Object node = heap[i];              // save top node
    int j = i << 1;                  // find smaller child
    int k = j + 1;
    if (k <= size && lessThan(heap[k], heap[j])) {
      j = k;
    }
    while (j <= size && lessThan(heap[j], node)) {
      heap[i] = heap[j];              // shift up child
      i = j;
      j = i << 1;
      k = j + 1;
      if (k <= size && lessThan(heap[k], heap[j])) {
        j = k;
      }
    }
    heap[i] = node;                  // install saved node
  }
}