Huffuman Coding (哈夫曼编码)

哈夫曼编码(Huffman Coding)，又称霍夫曼编码，是一种编码方式，哈夫曼编码是可变字长编码(VLC)的一种。Huffman于1952年提出一种编码方法，该方法完全依据字符出现概率来构造异字头的平均长度最短的码字，有时称之为最佳编码，一般就叫做Huffman编码（有时也称为霍夫曼编码）。

#include "stdafx.h"
#include <iostream>
#include<queue>
#include<vector>
#include <string>

using namespace std;

typedef struct 
{
    //内容
    unsigned char    character;
    //频次
    unsigned int    frequency;

} CharNode;

struct MinHeapNode
{
    char data;
    unsigned int freq;
    MinHeapNode *left, *right;
    MinHeapNode(char data, unsigned int freq)
    {
        left = right = NULL;
        this->data = data;
        this->freq = freq;
    }
};
typedef struct MinHeapNode MinHeapNode;

struct compare
{
    bool operator()(MinHeapNode *a, MinHeapNode *b)
    {
        //"a > b" 表示 数值小 的 优先级高, 排最上面
        //反之 "<" 表示大的优先级高
        return (a->freq > b->freq);
    }
};

void get_huffuman_code(MinHeapNode *topNode, string code)
{
    if (NULL == topNode)
    {
        return;
    }
    //表示已到达最末端，含有左右节点的元素中(由while循环中生成)data必为-1
    if (topNode->data != -1)
    {
        cout << topNode->data << " : " << code<<endl;
    }
    get_huffuman_code(topNode->left, code + "0");
    get_huffuman_code(topNode->right, code + "1");
}

int main()
{
         FILE * inputFile = NULL;
         fopen_s(&inputFile,"input.txt", "rb");
         if (!inputFile)
         {
             cout<< "Error: open file failed !" << endl;
             return -1;
         }

         //初始化ASCII码数组
         CharNode nodeArr[256] = { {0,0} };
         while (!feof(inputFile))
         {
              char buf = getc(inputFile);
              cout << buf;
              nodeArr[buf].character = buf;
              nodeArr[buf].frequency++;
         }
         cout << endl;
         //定义一个小顶堆
         priority_queue<MinHeapNode*, vector<MinHeapNode*>, compare > minHeap;
         for (size_t i = 0; i < 256; i++)
         {
             //将数据堆中，与升序排序效果类似，即堆首为小值，堆尾为大值
             if (nodeArr[i].frequency > 0)
             {
                 minHeap.push(new MinHeapNode(nodeArr[i].character, nodeArr[i].frequency));
                 cout << "Node " << i << ": [" << nodeArr[i].character << ", " << nodeArr[i].frequency << "]" << endl;
             }
         }

         MinHeapNode *leftNode = NULL, *rightNode = NULL, *topNode = NULL;
         //从堆首中抛出两个较小节点，生成一个新节点放回堆中并进行重新排序，
         //故，每次操作完成后 堆中会减少一个元素, 且堆顶必为权值最小的2个元素 
         //直至堆中只剩一个元素时,哈夫曼树生成完毕
         while (minHeap.size() > 1)
         {
             //较小放左侧
             leftNode = minHeap.top();
             minHeap.pop();
             //较大的放右侧
             rightNode = minHeap.top();
             minHeap.pop();

             topNode = new MinHeapNode(-1, leftNode->freq + rightNode->freq);
             topNode->left = leftNode;
             topNode->right = rightNode;
             minHeap.push(topNode);
         }
         get_huffuman_code(topNode, "");
         getchar();
         return 0;
}

 对于解码过程，从根节点出发，依次读取文件流的bit 遇到“0”找leftNode，遇到“1”找rightNode，直至 data != -1