本文首先簡要闡述哈夫曼算法的基本思想，然後介紹了使用哈夫曼算法進行文件壓縮和解壓縮的
處理步驟，最後給出了C語言實現的文件壓縮和解壓縮的源代碼。
哈夫曼算法的主要思想是：
①首先遍曆要處理的字符串，得到每個字符的出現的次數；
②將每個字符（以其出現次數為權值）分別構造為二叉樹（注意此時的二叉樹隻有一個節點）；
③取所有二叉樹種種字符出現次數最小的二叉樹合並為一顆新的二叉樹，新二叉樹根節點的權值等於兩個子節點的權值之和，新節點中的字符忽略；
④重複過程③直到所有樹被合並為同一棵二叉樹
⑤遍曆最後得到的二叉樹，自頂向下按路徑編號，指向左節點的邊編號0，指向右節點的邊編號1，從根到葉節點的所有邊上的0和1鏈接起來，就是葉子節點中字符的哈夫曼編碼。
下圖展示了哈夫曼編碼的基本思想。
基於哈夫曼算法的文件壓縮和解壓縮過程分別說明如下：
一、文件壓縮：
①統計詞頻：讀取文件的每個字節，使用整數數組int statistic[MAX_CHARS]統計每個字符出現的次數，
由於一個字節最多表示2^8-1個字符，所以MAX_CHARS=256就足夠了。在統計字符數的時候，
對於每一個byte, 有statistic[(unsigned char)byte]++。
②構造哈夫曼樹：根據statistic數組，基於哈夫曼樹算法造哈夫曼樹，由於構造的過程中每次都要取最小權值的字符，
所以需要用優先隊列來維護每棵樹的根節點。
③生成編碼：深度優先遍曆哈弗曼樹，得到每個葉子節點中的字符的編碼並存入字符串數組char *dictionary[MAX_CHARS];
④存儲詞頻：新建存儲壓縮數據的文件，首先寫入不同字符的個數，然後將每個字符及其對應的詞頻寫入文件。
⑤存儲壓縮數據：再次讀取待壓縮文件的每個字節byte，由dictionary[(unsigned int)byte]得到對應的編碼（注意每個字符
編碼的長度不一），使用位運算一次將編碼中的每個位(BIT)設置到一個char類型的位緩衝中，可能多個編碼才能填滿一個
位緩衝，每填滿一次，將位緩衝區以單個字節的形式寫入文件。當文件遍曆完成的時候，文件的壓縮也就完成了。
二、文件解壓：
①讀取詞頻：讀取壓縮文件，將每個字符的出現次數存入數組statistic
②構造哈夫曼編碼樹：根據statistic數組構造哈夫曼編碼樹
③繼續讀取壓縮文件，對於每個字節，使用位運算得到每個位（BIT）。對於每個BIT，根據BIT從根開始遍曆哈夫曼樹，如果BIT是0
就走左分支，如果BIT是1就走有分支，走到葉子節點的時候，輸出對應的字符。走到葉子節點後，重新從哈夫曼樹根節點開始匹配
每個位。當整個壓縮文件讀取完畢時，文件解壓縮也完成了。
上文介紹了基於哈夫曼算法的文件壓縮和解壓縮，下麵給出基於上述思想的C語言源代碼，一共有5個文件，其中pq.h和pq.c
是優先隊列，compress.h和compress.c是壓縮和解壓縮的實現，main.c是測試文件。
pq.h和pq.c請參見另外一篇文章《優先隊列(priority_queue)的C語言實現》。
另外三個文件內容如下：
/*
* File: compress.h
* Purpose: To compress file using the Haffman algorithm
* Author: puresky
* Date: 2011/05/01
*/
#ifndef _FILE_COMPRESSION_H
#define _FILE_COMPRESSION_H
//Haffuman Tree Node
typedef struct HaffumanTreeNode HTN;
struct HaffumanTreeNode
{
char _ch; //character
int _count; //frequency
struct HaffumanTreeNode *_left; //left child
struct HaffumanTreeNode *_right;//rigth child
};
//FileCompress Struct
#define BITS_PER_CHAR 8 //the number of bits in a char
#define MAX_CHARS 256 //the max number of chars
#define FILE_BUF_SIZE 8192 //the size of Buffer for FILE I/O
typedef struct FileCompressStruct FCS;
struct FileCompressStruct
{
HTN *_haffuman; //A pointer to the root of hafumman tree
unsigned int _charsCount; //To store the number of chars
unsigned int _total; //Total bytes in a file.
char *_dictionary[MAX_CHARS]; //to store the encoding of each character
int _statistic[MAX_CHARS]; //To store the number of each character
};
FCS *fcs_new();
void fcs_compress(FCS *fcs, const char *inFileName, const char *outFileName);
void fcs_decompress(FCS *fcs, const char *inFileName, const char *outFileName);
void fcs_free(FCS *fcs);
#endif
/*
* File: compress.c
* Purpose: To compress file using the Haffman algorithm
* Author: puresky
* Date: 2011/05/01
*/
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include "compress.h"
#include "pq.h"
static const unsigned char mask[8] =
{
0x80, /* 10000000 */
0x40, /* 01000000 */
0x20, /* 00100000 */
0x10, /* 00010000 */
0x08, /* 00001000 */
0x04, /* 00000100 */
0x02, /* 00000010 */
0x01 /* 00000001 */
};
//static functions of HTN
static HTN *htn_new(char ch, int count)
{
HTN *htn = (HTN *)malloc(sizeof(HTN));
htn->_left = NULL;
htn->_right = NULL;
htn->_ch = ch;
htn->_count = count;
return htn;
}
static void htn_print_recursive(HTN *htn, int depth)
{
int i;
if(htn)
{
for(i = 0; i < depth; ++i)
printf(" ");
printf("%d:%d\n", htn->_ch, htn->_count);
htn_print_recursive(htn->_left, depth + 1);
htn_print_recursive(htn->_right, depth + 1);
}
}
static void htn_print(HTN *htn)
{
htn_print_recursive(htn, 0);
}
static void htn_free(HTN *htn)
{
if(htn)
{
htn_free(htn->_left);
htn_free(htn->_right);
free(htn);
}
}
//static functions of FCS
static void fcs_generate_statistic(FCS *fcs, const char *inFileName)
{
int ret, i;
unsigned char buf[FILE_BUF_SIZE];
FILE *pf = fopen(inFileName, "rb");
if(!pf)
{
fprintf(stderr, "can't open file:%s\n", inFileName);
return;
}
while((ret = fread(buf, 1, FILE_BUF_SIZE, pf)) > 0)
{
fcs->_total += ret;
for(i = 0; i < ret; ++i)
{
if(fcs->_statistic[buf[i]] == 0)
fcs->_charsCount++;
fcs->_statistic[buf[i]]++;
}
}
fclose(pf);
}
static void fcs_create_haffuman_tree(FCS *fcs)
{
int i, count;
HTN *htn, *parent, *left, *right;
KeyValue *kv, *kv1, *kv2;
PriorityQueue *pq;
pq = priority_queue_new(PRIORITY_MIN);
for(i = 0; i < MAX_CHARS; ++i)
{
if(fcs->_statistic[i])
{
htn = htn_new((char)i, fcs->_statistic[i]);
kv = key_value_new(fcs->_statistic[i], htn);
priority_queue_enqueue(pq, kv);
}
}
//fprintf(stdout, "the number of haffuman leaf is %d\n", priority_queue_size(pq));
while(!priority_queue_empty(pq))
{
//fprintf(stdout, "priority queue size:%d\n", priority_queue_size(pq));
kv1 = priority_queue_dequeue(pq);
kv2 = priority_queue_dequeue(pq);
if(kv2 == NULL)
{
fcs->_haffuman = kv1->_value;
key_value_free(kv1, NULL);
}
else
{
left = (HTN *)kv1->_value;
right = (HTN *)kv2->_value;
count = left->_count + right->_count;
key_value_free(kv1, NULL);
key_value_free(kv2, NULL);
parent = htn_new(0, count);
parent->_left = left;
parent->_right = right;
kv = key_value_new(count, parent);
priority_queue_enqueue(pq, kv);
}
}
priority_queue_free(pq, NULL);
//htn_print(fcs->_haffuman);
}
static void fcs_generate_dictionary_recursively(HTN *htn, char *dictionary[], char path[], int depth)
{
char *code = NULL;
if(htn)
{
if(htn->_left == NULL && htn->_right == NULL)
{
code = (char *)malloc(sizeof(char) * (depth + 1));
memset(code, 0, sizeof(char) * (depth + 1));
memcpy(code, path, depth);
dictionary[(unsigned char)htn->_ch] = code;
}
if(htn->_left)
{
path[depth] = '0';
fcs_generate_dictionary_recursively(htn->_left, dictionary, path, depth + 1);
}
if(htn->_right)
{
path[depth] = '1';
fcs_generate_dictionary_recursively(htn->_right, dictionary, path, depth + 1);
}
}
}
static void fcs_generate_dictionary(FCS *fcs)
{
char path[32];
fcs_generate_dictionary_recursively(fcs->_haffuman, fcs->_dictionary, path, 0);
//fcs_print_dictionary(fcs);
}
static void fcs_print_dictionary(FCS *fcs)
{
int i;
for(i = 0; i < MAX_CHARS; ++i)
if(fcs->_dictionary[i] != NULL)
fprintf(stdout, "%d:%s\n", i, fcs->_dictionary[i]);
}
static void fcs_write_statistic(FCS *fcs, FILE *pf)
{
int i;
fprintf(pf, "%d\n", fcs->_charsCount);
for(i = 0; i < MAX_CHARS; ++i)
if(fcs->_statistic[i] != 0)
fprintf(pf, "%d %d\n", i, fcs->_statistic[i]);
}
static void fcs_do_compress(FCS *fcs, const char *inFileName, const char* outFileName)
{
int i, j, ret;
char *dictEntry, len;
unsigned int bytes;
char bitBuf;
int bitPos;
unsigned char inBuf[FILE_BUF_SIZE];
FILE *pfIn, *pfOut;
pfIn = fopen(inFileName, "rb");
if(!pfIn)
{
fprintf(stderr, "can't open file:%s\n", inFileName);
return;
}
pfOut = fopen(outFileName, "wb");
if(!pfOut)
{
fclose(pfIn);
fprintf(stderr, "can't open file:%s\n", outFileName);
return;
}
fcs_write_statistic(fcs, pfOut);
bitBuf = 0x00;
bitPos = 0;
bytes = 0;
while((ret = fread(inBuf, 1, FILE_BUF_SIZE, pfIn)) > 0)
{
for(i = 0; i < ret; ++i)
{
len = strlen(fcs->_dictionary[inBuf[i]]);
dictEntry = fcs->_dictionary[inBuf[i]];
//printf("%s\n", dictEntry);
for(j = 0; j < len; ++j)
{
if(dictEntry[j] == '1')
{
bitBuf |= mask[bitPos++];
}
else
{
bitPos++;
}
if(bitPos == BITS_PER_CHAR)
{
fwrite(&bitBuf, 1, sizeof(bitBuf), pfOut);
bitBuf = 0x00;
bitPos = 0;
bytes++;
}
}
}
}
if(bitPos != 0)
{
fwrite(&bitBuf, 1, sizeof(bitBuf), pfOut);
bytes++;
}
fclose(pfIn);
fclose(pfOut);
printf("The compression ratio is:%f%%\n",
(fcs->_total - bytes) * 100.0 / fcs->_total);
}
static void fcs_read_statistic(FCS *fcs, FILE *pf)
{
int i, charsCount = 0;
int ch;
int num;
fscanf(pf, "%d\n", &charsCount);
fcs->_charsCount = charsCount;
for(i = 0; i < charsCount; ++i)
{
fscanf(pf, "%d %d\n", &ch, &num);
fcs->_statistic[(unsigned int)ch] = num;
fcs->_total += num;
}
}
static void fcs_do_decompress(FCS *fcs, FILE *pfIn, const char *outFileName)
{
int i, j, ret;
unsigned char ch;
HTN *htn;
unsigned char buf[FILE_BUF_SIZE];
unsigned char bitCode;
int bitPos;
FILE *pfOut;
pfOut = fopen(outFileName, "wb");
if(!pfOut)
{
fprintf(stderr, "can't open file:%s\n", outFileName);
return;
}
htn = fcs->_haffuman;
bitCode = 0x00;
bitPos = 0;
while((ret = fread(buf, 1, FILE_BUF_SIZE, pfIn)) > 0)
{
for(i = 0; i < ret; ++i)
{
ch = buf[i];
for(j = 0; j < BITS_PER_CHAR; ++j)
{
if(ch & mask[j])
{
htn = htn->_right;
}
else
{
htn = htn->_left;
}
if(htn->_left == NULL && htn->_right == NULL) //leaf
{
if(fcs->_total > 0)
{
fwrite(&htn->_ch, 1, sizeof(char), pfOut);
fcs->_total--;
}
htn = fcs->_haffuman;
}
}
}
}
fclose(pfOut);
}
//FCS functions
FCS *fcs_new()
{
FCS *fcs = (FCS *)malloc(sizeof(FCS));
fcs->_charsCount = 0;
fcs->_total = 0;
memset(fcs->_statistic, 0, sizeof(fcs->_statistic));
memset(fcs->_dictionary, 0, sizeof(fcs->_dictionary));
fcs->_haffuman = NULL;
return fcs;
}
void fcs_free(FCS *fcs)
{
int i;
if(fcs)
{
if(fcs->_haffuman)
htn_free(fcs->_haffuman);
for(i = 0; i < MAX_CHARS; ++i)
free(fcs->_dictionary[i]);
free(fcs);
}
}
void fcs_compress(FCS *fcs, const char *inFileName, const char *outFileName)
{
fprintf(stdout, "To compress file: %s ...\n", inFileName);
fcs_generate_statistic(fcs, inFileName);
fcs_create_haffuman_tree(fcs);
fcs_generate_dictionary(fcs);
fcs_do_compress(fcs, inFileName, outFileName);
fprintf(stdout, "The compressed data of file: %s stored at %s!\n",
inFileName, outFileName);
}
void fcs_decompress(FCS *fcs, const char *inFileName, const char *outFileName)
{
FILE *pfIn;
fprintf(stdout, "To decompress file: %s ...\n", inFileName);
pfIn= fopen(inFileName, "rb");
if(!pfIn)
{
fprintf(stderr, "can't open file: %s\n", inFileName);
return ;
}
fcs_read_statistic(fcs, pfIn);
fcs_create_haffuman_tree(fcs);
fcs_generate_dictionary(fcs);
fcs_do_decompress(fcs, pfIn, outFileName);
fclose(pfIn);
fprintf(stdout, "The decompressed data of file: %s stored at %s\n",
inFileName, outFileName);
}
/*
* File: main.c
* Purpose: testing File Compression
* Author:puresky
* Date: 2011/05/01
*/
#include <stdlib.h>
#include "compress.h"
const int DO_COMPRESS = 1;
const int DO_DECOMPRESS = 1;
const char *InFile = "data.txt"; //The file to compress.
const char *CompressedFile = "data.hfm"; //Compressed data of the file.
const char *OutFile = "data2.txt"; //The decompressed file of the data.
int main(int argc, char **argv)
{
//1. compress file
if(DO_COMPRESS)
{
FCS *fcs1;
fcs1 = fcs_new();
fcs_compress(fcs1, InFile, CompressedFile);
fcs_free(fcs1);
}
//2. decompress file
if(DO_DECOMPRESS)
{
FCS *fcs2;
fcs2 = fcs_new();
fcs_decompress(fcs2, CompressedFile, OutFile);
fcs_free(fcs2);
}
system("pause");
return 0;
}