详解Huffman编码 - Java实现

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关于Huffman编码的具体定义可参考《算法导论》P245

Huffman Tree节点定义:

abstract class AbstractNode {
    protected int weight;

    protected abstract AbstractNode setWeight(int weight);

    protected abstract int getWeight();

    protected AbstractNode(int weight) {
        this.weight = weight;
    }
}

class IntlNode extends AbstractNode {
    private AbstractNode leftNode;
    private AbstractNode rightNode;

    public IntlNode(int weight) {
        super(weight);
    }

    public IntlNode(AbstractNode leftNode, AbstractNode rightNode) {
        super(leftNode.getWeight() + rightNode.getWeight());
        this.leftNode = leftNode;
        this.rightNode = rightNode;
    }

    public IntlNode setLeftNode(AbstractNode leftNode) {
        this.leftNode = leftNode;
        return this;
    }

    public IntlNode setRightNode(AbstractNode rightNode) {
        this.rightNode = rightNode;
        return this;
    }

    public AbstractNode getLeftNode() {
        return leftNode;
    }

    public AbstractNode getRightNode() {
        return rightNode;
    }

    @Override
    public IntlNode setWeight(int weight) {
        throw new RuntimeException("Illegal modification");
    }

    @Override
    public int getWeight() {
        return super.weight;
    }

    @Override
    public String toString() {
        return "IntlNode{" +
                "weight=" + super.weight +
                ", leftNode=" + leftNode +
                ", rightNode=" + rightNode +
                '}';
    }
}

class LeafNode extends AbstractNode {
    private char nodeChar;

    public LeafNode(int weight, char nodeChar){
        super(weight);
        this.nodeChar = nodeChar;
    }

    public char getNodeChar() {
        return nodeChar;
    }

    public LeafNode setNodeChar(char nodeChar) {
        this.nodeChar = nodeChar;
        return this;
    }

    @Override
    public LeafNode setWeight(int weight) {
        super.weight = weight;
        return this;
    }

    @Override
    public int getWeight() {
        return super.weight;
    }

    @Override
    public String toString() {
        return "LeafNode{" +
                "weight=" + super.weight +
                ", nodeChar=" + nodeChar +
                '}';
    }
}

获取根节点下某个字符的huffman编码：

    /**
     * @param root huffman树root
     * @param c 字符
     * @return  c 在 root下 的huffmanCode
     */
    private static String getPath(AbstractNode root, char c) {
        return getPath0(root, c, new StringBuilder());
    }

    private static String getPath0(AbstractNode root, char c, StringBuilder builder) {
        if (root instanceof LeafNode) {
            //simple case, root is leaf
            LeafNode leaf = (LeafNode) root;
            if (leaf.getNodeChar() == c) {
                return "";
            }else {
                return null;
            }
        }else {
            //root is intlNode
            IntlNode intl = (IntlNode) root;
            //check left
            String left = getPath(intl.getLeftNode(), c);
            if (left != null) {
                return builder.append(LEFT).append(left).toString();
            }
            //check right
            String right = getPath(intl.getRightNode(), c);
            if (right != null) {
                return builder.append(RIGHT).append(right).toString();
            }
            //subtree failed
            return null;
        }
    }
}

获取根节点下的某个huffman编码对应的字符:

    /**
     * @param root huffman root
     * @param charArray  字符编码数组
     * @return  字符编码对应的字符
     */
    private static char getChar(AbstractNode root, char[] charArray) {
        return getChar0(root, charArray, 0);
    }

    private static char getChar0(AbstractNode root, char[] charArray, int index) {
        if (index == charArray.length) {
            if (root instanceof LeafNode) {
                return ((LeafNode) root).getNodeChar();
            }else {
                throw new RuntimeException("Invalid char array");
            }
        }else {
            if(root instanceof IntlNode) {
                IntlNode intlNode = (IntlNode) root;
                if (charArray[index] == '0') {
                    return getChar0(intlNode.getLeftNode(), charArray, index + 1);
                } else {
                    return getChar0(intlNode.getRightNode(), charArray, index + 1);
                }
            }else {
                throw new RuntimeException("Invalid char array");
            }
        }
    }

建立Huffman树 & 测试：

    public static void main(String[] args) {
        List<LeafNode> leafNodeList = new ArrayList<>();
        leafNodeList.add(makeLeaf(5, 'f'));
        leafNodeList.add(makeLeaf(9, 'e'));
        leafNodeList.add(makeLeaf(12, 'c'));
        leafNodeList.add(makeLeaf(13, 'b'));
        leafNodeList.add(makeLeaf(16, 'd'));
        leafNodeList.add(makeLeaf(28, 't'));
        leafNodeList.add(makeLeaf(45, 'a'));
        leafNodeList.add(makeLeaf(14, 'p'));

		//使用weight作为优先队列的比较器
        Queue<AbstractNode> nodeQueue = new PriorityQueue<>(leafNodeList.size(),
                Comparator.comparingInt(AbstractNode::getWeight));
        nodeQueue.addAll(leafNodeList);

        for (int i = 0, initSize = nodeQueue.size(); i < initSize - 1; i++) {
            AbstractNode left = nodeQueue.poll();
            AbstractNode right = nodeQueue.poll();
            IntlNode combined = new IntlNode(left, right);
            nodeQueue.add(combined);
        }
        //字符'p'对应010
        System.out.println(getPath(nodeQueue.peek(), 'p'));
        System.out.println(getChar(nodeQueue.peek(), "010".toCharArray()));
    }

    private static LeafNode makeLeaf(int weight, char nodeChar) {
        return new LeafNode(weight, nodeChar);
    }

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