Huffman编码介绍
Huffman编码处理的是字符以及字符对应的二进制的编码配对问题，分为编码和解码，目的是压缩字符对应的二进制数据长度。我们知道字符存贮和传输的时候都是二进制的(计算机只认识0/1)，那么就有字符与二进制之间的mapping关系。字符属于字符集(Charset), 字符需要通过编码(encode)为二进制进行存贮和传输，显示的时候需要解码(decode)回字符，字符集与编码方法是一对多关系(Unicode可以用UTF-8,UTF-16等编码)。理解了字符集，编码以及解码，满天飞的乱码问题也就游刃而解了。以英文字母小写a为例, ASCII编码中，十进制为97，二进制为01100001。ASCII的每一个字符都用8个Bit(1Byte)编码，假如有1000个字符要传输，那么就要传输8000个Bit。问题来了，英文中字母e的使用频率为12.702%，而z为0.074%，前者是后者的100多倍，但是确使用相同位数的二进制。可以做得更好，方法就是可变长度编码，指导原则就是频率高的用较短的位数编码，频率低的用较长位数编码。Huffman编码算法就是处理这样的问题。
Huffman编码Java实现
Huffman编码算法主要用到的数据结构是完全二叉树(full binary tree)和优先级队列。后者用的是Java.util.PriorityQueue，前者自己实现(都为内部类)，代码如下:
static class Tree { private Node root; public Node getRoot() { return root; } public void setRoot(Node root) { this.root = root; } } static class Node implements Comparable<Node> { private String chars = ""; private int frequence = 0; private Node parent; private Node leftNode; private Node rightNode; @Override public int compareTo(Node n) { return frequence - n.frequence; } public boolean isLeaf() { return chars.length() == 1; } public boolean isRoot() { return parent == null; } public boolean isLeftChild() { return parent != null && this == parent.leftNode; } public int getFrequence() { return frequence; } public void setFrequence(int frequence) { this.frequence = frequence; } public String getChars() { return chars; } public void setChars(String chars) { this.chars = chars; } public Node getParent() { return parent; } public void setParent(Node parent) { this.parent = parent; } public Node getLeftNode() { return leftNode; } public void setLeftNode(Node leftNode) { this.leftNode = leftNode; } public Node getRightNode() { return rightNode; } public void setRightNode(Node rightNode) { this.rightNode = rightNode; } } 统计数据
既然要按频率来安排编码表，那么首先当然得获得频率的统计信息。我实现了一个方法处理这样的问题。如果已经有统计信息，那么转为Map<Character,Integer>即可。如果你得到的信息是百分比，乘以100或1000，或10000。总是可以转为整数。比如12.702%乘以1000为12702，Huffman编码只关心大小问题。统计方法实现如下：
public static Map<Character, Integer> statistics(char[] charArray) { Map<Character, Integer> map = new HashMap<Character, Integer>(); for (char c : charArray) { Character character = new Character(c); if (map.containsKey(character)) { map.put(character, map.get(character) + 1); } else { map.put(character, 1); } } return map; } 构建树
构建树是Huffman编码算法的核心步骤。思想是把所有的字符挂到一颗完全二叉树的叶子节点，任何一个非页子节点的左节点出现频率不大于右节点。算法为把统计信息转为Node存放到一个优先级队列里面，每一次从队列里面弹出两个最小频率的节点，构建一个新的父Node(非叶子节点), 字符内容刚弹出来的两个节点字符内容之和，频率也是它们的和，最开始的弹出来的作为左子节点，后面一个作为右子节点，并且把刚构建的父节点放到队列里面。重复以上的动作N-1次，N为不同字符的个数(每一次队列里面个数减1)。结束以上步骤，队列里面剩一个节点，弹出作为树的根节点。代码如下:
private static Tree buildTree(Map<Character, Integer> statistics, List<Node> leafs) { Character[] keys = statistics.keySet().toArray(new Character[0]); PriorityQueue<Node> priorityQueue = new PriorityQueue<Node>(); for (Character character : keys) { Node node = new Node(); node.chars = character.toString(); node.frequence = statistics.get(character); priorityQueue.add(node); leafs.add(node); } int size = priorityQueue.size(); for (int i = 1; i <= size - 1; i++) { Node node1 = priorityQueue.poll(); Node node2 = priorityQueue.poll(); Node sumNode = new Node(); sumNode.chars = node1.chars + node2.chars; sumNode.frequence = node1.frequence + node2.frequence; sumNode.leftNode = node1; sumNode.rightNode = node2; node1.parent = sumNode; node2.parent = sumNode; priorityQueue.add(sumNode); } Tree tree = new Tree(); tree.root = priorityQueue.poll(); return tree; } 编码
某个字符对应的编码为，从该字符所在的叶子节点向上搜索，如果该字符节点是父节点的左节点，编码字符之前加0，反之如果是右节点，加1，直到根节点。只要获取了字符和二进制码之间的mapping关系，编码就非常简单。代码如下:
public static String encode(String originalStr, Map<Character, Integer> statistics) { if (originalStr == null || originalStr.equals("")) { return ""; } char[] charArray = originalStr.toCharArray(); List<Node> leafNodes = new ArrayList<Node>(); buildTree(statistics, leafNodes); Map<Character, String> encodInfo = buildEncodingInfo(leafNodes); StringBuffer buffer = new StringBuffer(); for (char c : charArray) { Character character = new Character(c); buffer.append(encodInfo.get(character)); } return buffer.toString(); } private static Map<Character, String> buildEncodingInfo(List<Node> leafNodes) { Map<Character, String> codewords = new HashMap<Character, String>(); for (Node leafNode : leafNodes) { Character character = new Character(leafNode.getChars().charAt(0)); String codeword = ""; Node currentNode = leafNode; do { if (currentNode.isLeftChild()) { codeword = "0" + codeword; } else { codeword = "1" + codeword; } currentNode = currentNode.parent; } while (currentNode.parent != null); codewords.put(character, codeword); } return codewords; } 解码
因为Huffman编码算法能够保证任何的二进制码都不会是另外一个码的前缀，解码非常简单，依次取出二进制的每一位，从树根向下搜索，1向右，0向左，到了叶子节点(命中)，退回根节点继续重复以上动作。代码如下:
public static String decode(String binaryStr, Map<Character, Integer> statistics) { if (binaryStr == null || binaryStr.equals("")) { return ""; } char[] binaryCharArray = binaryStr.toCharArray(); LinkedList<Character> binaryList = new LinkedList<Character>(); int size = binaryCharArray.length; for (int i = 0; i < size; i++) { binaryList.addLast(new Character(binaryCharArray[i])); } List<Node> leafNodes = new ArrayList<Node>(); Tree tree = buildTree(statistics, leafNodes); StringBuffer buffer = new StringBuffer(); while (binaryList.size() > 0) { Node node = tree.root; do { Character c = binaryList.removeFirst(); if (c.charValue() == '0') { node = node.leftNode; } else { node = node.rightNode; } } while (!node.isLeaf()); buffer.append(node.chars); } return buffer.toString(); } 测试以及比较
以下测试Huffman编码的正确性(先编码，后解码，包括中文)，以及Huffman编码与常见的字符编码的二进制字符串比较。代码如下:
public static void main(String[] args) { String oriStr = "Huffman codes compress data very effectively: savings of 20% to 90% are typical, " + "depending on the characteristics of the data being compressed. 中华崛起"; Map<Character, Integer> statistics = statistics(oriStr.toCharArray()); String encodedBinariStr = encode(oriStr, statistics); String decodedStr = decode(encodedBinariStr, statistics); System.out.println("Original sstring: " + oriStr); System.out.println("Huffman encoed binary string: " + encodedBinariStr); System.out.println("decoded string from binariy string: " + decodedStr); System.out.println("binary string of UTF-8: " + getStringOfByte(oriStr, Charset.forName("UTF-8"))); System.out.println("binary string of UTF-16: " + getStringOfByte(oriStr, Charset.forName("UTF-16"))); System.out.println("binary string of US-ASCII: " + getStringOfByte(oriStr, Charset.forName("US-ASCII"))); System.out.println("binary string of GB2312: " + getStringOfByte(oriStr, Charset.forName("GB2312"))); } public static String getStringOfByte(String str, Charset charset) { if (str == null || str.equals("")) { return ""; } byte[] byteArray = str.getBytes(charset); int size = byteArray.length; StringBuffer buffer = new StringBuffer(); for (int i = 0; i < size; i++) { byte temp = byteArray[i]; buffer.append(getStringOfByte(temp)); } return buffer.toString(); } public static String getStringOfByte(byte b) { StringBuffer buffer = new StringBuffer(); for (int i = 7; i >= 0; i--) { byte temp = (byte) ((b >> i) & 0x1); buffer.append(String.valueOf(temp)); } return buffer.toString(); } 以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持武林网。
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