BinaryHeap

public class BinaryHeap<E> implements Heap<E> {

    private static final int DEFAULT_CAPACITY = 16;
    
    private int size;
    private E[] elements;
    private Comparator<E> comparator;
    
    
    public BinaryHeap() {
        this(null, null);
    }
    
    public BinaryHeap(Comparator<E> comparator) {
        this(null, comparator);
    }
    
    public BinaryHeap(E[] elements) {
        this(elements, null);
    }
    
    public BinaryHeap(E[] elements, Comparator<E> comparator) {
        this.comparator = comparator;
        if (elements == null || elements.length == 0) {
            this.elements = (E[])new Object[DEFAULT_CAPACITY];
        } else {
            int capacity = Math.max(DEFAULT_CAPACITY, elements.length);
            this.elements = (E[])new Object[capacity];
            for (int i = 0; i < elements.length; i++) {
                this.elements[i] = elements[i];
            }
            size = elements.length;
            heapify();
        }
    }

    @Override
    public int size() {
        return size;
    }

    @Override
    public boolean isEmpty() {
        return size == 0;
    }

    @Override
    public void clear() {
        for (int i = 0; i < size; i++) {
            elements[i] = null;
        }
        size = 0;
    }

    @Override
    public void add(E element) {
        elementNotNullCheck(element);
        ensureCapacity(size + 1);
        elements[size++] = element;
        siftUp(size - 1);
    }


    @Override
    public E get() {
        emptyCheck();
        return elements[0];
    }

    @Override
    public E remove() {
        emptyCheck();
        int lastIndex = --size;
        E root = elements[0];
        elements[0] = elements[lastIndex];
        elements[lastIndex] = null;
        siftDown(0);
        return root;
    }

    @Override
    public E replace(E element) {
        elementNotNullCheck(element);
        E root = null;
        if (size == 0) {
            elements[size++] = element;
        } else {
            root = elements[0];
            elements[0] = element;
            siftDown(0);
        }
        return root;
    }
    
    
    /**
     * 批量建堆, 采用自下而上的下滤
     */
    private void heapify() {
        for (int i = (size >> 1) - 1; i >= 0; i--) {
            siftDown(i);
        }
    }
    
    /**
     * 上滤操作
     * @param index
     */
    private void siftUp(int index) {
        E element = elements[index];
        while (index > 0) {
            int parentIndex = (index - 1) >> 1;
            E parent = elements[parentIndex];
            if (compare(element, parent) <= 0) break;
            elements[index] = parent;
            index = parentIndex;
        }
        elements[index] = element;
    }
    
    /**
     * 下滤操作
     * @param index
     */
    private void siftDown(int index) {
        E element = elements[index];
        int half = size >> 1;   // 非叶子节点数量(第一个叶子节点的索引)
        while (index < half) {
            int childIndex = (index << 1) + 1;
            E child = elements[childIndex];
            
            int rightIndex = childIndex + 1;
            if (rightIndex < size && compare(elements[rightIndex], child) > 0) {
                child = elements[childIndex = rightIndex];
            }
            
            if (compare(element, child) >= 0) break;
            elements[index] = child;
            index = childIndex;
        }
        elements[index] = element;
    }
    
    private int compare(E e1, E e2) {
        if (comparator != null) return comparator.compare(e1, e2);
        return ((Comparable)e1).compareTo(e2);
    }
    
    private void ensureCapacity(int capacity) {
        int oldCapacity = elements.length;
        if (oldCapacity >= capacity) return;
        E[] newElements = (E[]) new Object[oldCapacity + (oldCapacity >> 1)];
        for (int i = 0; i < size; i++) {
            newElements[i] = elements[i];
        }
        elements = newElements;
    }
    
    private void emptyCheck() {
        if (size == 0) throw new IndexOutOfBoundsException("Heap is empty");
    }
    
    private void elementNotNullCheck(E element) {
        if (element == null) throw new NullPointerException("Element must not be null");
    }

}