prioritybuffer.java

初级java程序员如果想要更深一步提高自己的实力

     */
    public Object remove() {
        final Object result = get();
        elements[1] = elements[size--];

        // set the unused element to 'null' so that the garbage collector
        // can free the object if not used anywhere else.(remove reference)
        elements[size + 1] = null;

        if (size != 0) {
            // percolate top element to it's place in tree
            if (ascendingOrder) {
                percolateDownMinHeap(1);
            } else {
                percolateDownMaxHeap(1);
            }
        }

        return result;
    }

    //-----------------------------------------------------------------------
    /**
     * Tests if the buffer is at capacity.
     *
     * @return <code>true</code> if buffer is full; <code>false</code> otherwise.
     */
    protected boolean isAtCapacity() {
        //+1 as element 0 is noop
        return elements.length == size + 1;
    }

    
    /**
     * Percolates element down heap from the position given by the index.
     * <p>
     * Assumes it is a minimum heap.
     *
     * @param index the index for the element
     */
    protected void percolateDownMinHeap(final int index) {
        final Object element = elements[index];
        int hole = index;

        while ((hole * 2) <= size) {
            int child = hole * 2;

            // if we have a right child and that child can not be percolated
            // up then move onto other child
            if (child != size && compare(elements[child + 1], elements[child]) < 0) {
                child++;
            }

            // if we found resting place of bubble then terminate search
            if (compare(elements[child], element) >= 0) {
                break;
            }

            elements[hole] = elements[child];
            hole = child;
        }

        elements[hole] = element;
    }

    /**
     * Percolates element down heap from the position given by the index.
     * <p>
     * Assumes it is a maximum heap.
     *
     * @param index the index of the element
     */
    protected void percolateDownMaxHeap(final int index) {
        final Object element = elements[index];
        int hole = index;

        while ((hole * 2) <= size) {
            int child = hole * 2;

            // if we have a right child and that child can not be percolated
            // up then move onto other child
            if (child != size && compare(elements[child + 1], elements[child]) > 0) {
                child++;
            }

            // if we found resting place of bubble then terminate search
            if (compare(elements[child], element) <= 0) {
                break;
            }

            elements[hole] = elements[child];
            hole = child;
        }

        elements[hole] = element;
    }

    /**
     * Percolates element up heap from the position given by the index.
     * <p>
     * Assumes it is a minimum heap.
     *
     * @param index the index of the element to be percolated up
     */
    protected void percolateUpMinHeap(final int index) {
        int hole = index;
        Object element = elements[hole];
        while (hole > 1 && compare(element, elements[hole / 2]) < 0) {
            // save element that is being pushed down
            // as the element "bubble" is percolated up
            final int next = hole / 2;
            elements[hole] = elements[next];
            hole = next;
        }
        elements[hole] = element;
    }

    /**
     * Percolates a new element up heap from the bottom.
     * <p>
     * Assumes it is a minimum heap.
     *
     * @param element the element
     */
    protected void percolateUpMinHeap(final Object element) {
        elements[++size] = element;
        percolateUpMinHeap(size);
    }

    /**
     * Percolates element up heap from from the position given by the index.
     * <p>
     * Assume it is a maximum heap.
     *
     * @param index the index of the element to be percolated up
     */
    protected void percolateUpMaxHeap(final int index) {
        int hole = index;
        Object element = elements[hole];

        while (hole > 1 && compare(element, elements[hole / 2]) > 0) {
            // save element that is being pushed down
            // as the element "bubble" is percolated up
            final int next = hole / 2;
            elements[hole] = elements[next];
            hole = next;
        }

        elements[hole] = element;
    }

    /**
     * Percolates a new element up heap from the bottom.
     * <p>
     * Assume it is a maximum heap.
     *
     * @param element the element
     */
    protected void percolateUpMaxHeap(final Object element) {
        elements[++size] = element;
        percolateUpMaxHeap(size);
    }

    /**
     * Compares two objects using the comparator if specified, or the
     * natural order otherwise.
     * 
     * @param a  the first object
     * @param b  the second object
     * @return -ve if a less than b, 0 if they are equal, +ve if a greater than b
     */
    protected int compare(Object a, Object b) {
        if (comparator != null) {
            return comparator.compare(a, b);
        } else {
            return ((Comparable) a).compareTo(b);
        }
    }

    /**
     * Increases the size of the heap to support additional elements
     */
    protected void grow() {
        final Object[] array = new Object[elements.length * 2];
        System.arraycopy(elements, 0, array, 0, elements.length);
        elements = array;
    }

    //-----------------------------------------------------------------------
    /**
     * Returns an iterator over this heap's elements.
     *
     * @return an iterator over this heap's elements
     */
    public Iterator iterator() {
        return new Iterator() {

            private int index = 1;
            private int lastReturnedIndex = -1;

            public boolean hasNext() {
                return index <= size;
            }

            public Object next() {
                if (!hasNext()) {
                    throw new NoSuchElementException();
                }
                lastReturnedIndex = index;
                index++;
                return elements[lastReturnedIndex];
            }

            public void remove() {
                if (lastReturnedIndex == -1) {
                    throw new IllegalStateException();
                }
                elements[ lastReturnedIndex ] = elements[ size ];
                elements[ size ] = null;
                size--;  
                if( size != 0 && lastReturnedIndex <= size) {
                    int compareToParent = 0;
                    if (lastReturnedIndex > 1) {
                        compareToParent = compare(elements[lastReturnedIndex], 
                            elements[lastReturnedIndex / 2]);  
                    }
                    if (ascendingOrder) {
                        if (lastReturnedIndex > 1 && compareToParent < 0) {
                            percolateUpMinHeap(lastReturnedIndex); 
                        } else {
                            percolateDownMinHeap(lastReturnedIndex);
                        }
                    } else {  // max heap
                        if (lastReturnedIndex > 1 && compareToParent > 0) {
                            percolateUpMaxHeap(lastReturnedIndex); 
                        } else {
                            percolateDownMaxHeap(lastReturnedIndex);
                        }
                    }          
                }
                index--;
                lastReturnedIndex = -1; 
            }

        };
    }

    /**
     * Returns a string representation of this heap.  The returned string
     * is similar to those produced by standard JDK collections.
     *
     * @return a string representation of this heap
     */
    public String toString() {
        final StringBuffer sb = new StringBuffer();

        sb.append("[ ");

        for (int i = 1; i < size + 1; i++) {
            if (i != 1) {
                sb.append(", ");
            }
            sb.append(elements[i]);
        }

        sb.append(" ]");

        return sb.toString();
    }

}