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/*********************************************************************************************************** LZW.c 本演示程序提供了LZW编码法的压缩和解压缩函数，并实现了对图象 文件的压缩和解压缩 **********************************************************************************************************/ #include <stdio.h> #include <string.h> #include <stdlib.h> #define BITS 12 /* Setting the number of bits to 12, 13*/ #define HASHING_SHIFT BITS-8 /* or 14 affects several constants. */ #define MAX_VALUE (1 << BITS) - 1 /* Note that MS-DOS machines need to */ #define MAX_CODE MAX_VALUE - 1 /* compile their code in large model if*/ /* 14 bits are selected. */ #if BITS == 14 #define TABLE_SIZE 18041 /* The string table size needs to be a */ #endif /* prime number that is somewhat larger*/ #if BITS == 13 /* than 2**BITS. */ #define TABLE_SIZE 9029 #endif #if BITS <= 12 #define TABLE_SIZE 5021 #endif /* 函数原型 */ int LZW_Compression(char *in_filename, char *out_filename); int LZW_Decompression(char *in_filename, char *out_filename); /* 内部函数 */ int find_match(int hash_prefix,unsigned int hash_character); char *decode_string(unsigned char *buffer,unsigned int code); unsigned int input_code(FILE *input); void output_code(FILE *output,unsigned int code); /* 全局变量，编码/解码使用的内存缓冲区 */ int *code_value; /* This is the code value array */ unsigned int *prefix_code; /* This array holds the prefix codes */ unsigned char *append_character; /* This array holds the appended chars */ unsigned char decode_stack[4000]; /* This array holds the decoded string */ /* 主程序 */ void main(int argc, char *argv[]) { printf("LZW compression and decompression utility\n"); if (4 != argc) { printf("\nUsage : lzw -c|d sourcefilename targetfilename\n"); exit(0); } if (! strcmp(argv[1], "-c")) LZW_Compression(argv[2], argv[3]); else if (! strcmp(argv[1], "-d")) LZW_Decompression(argv[2], argv[3]); else printf("\nUnknow command.\n"); } /************************************************************************************************** LZW_Compression() 本函数用LZW算法压缩指定的文件，并将结构存储到新的文件中 ***************************************************************************************************/ int LZW_Compression(char *in_filename, char *out_filename) { unsigned int next_code; unsigned int character; unsigned int string_code; unsigned int index; int i; FILE *input; FILE *output; /* allocate memory for compression */ code_value=malloc(TABLE_SIZE*sizeof(unsigned int)); prefix_code=malloc(TABLE_SIZE*sizeof(unsigned int)); append_character=malloc(TABLE_SIZE*sizeof(unsigned char)); if (code_value==NULL || prefix_code==NULL || append_character==NULL) { printf("Fatal error allocating table space!\n"); return 0; } /* open files */ input=fopen(in_filename,"rb"); output=fopen(out_filename,"wb"); if (input==NULL || output==NULL) { printf("Fatal error opening files.\n"); return 0; }; /* compressing... */ next_code=256; /* Next code is the next available string code*/ for (i=0;i<TABLE_SIZE;i++) /* Clear out the string table before starting */ code_value[i]=-1; i=0; printf("Compressing...\n"); string_code=getc(input); /* Get the first code */ /* ** This is the main loop where it all happens. This loop runs util all of ** the input has been exhausted. Note that it stops adding codes to the ** table after all of the possible codes have been defined. */ while ((character=getc(input)) != (unsigned)EOF) { if (++i==1000) /* Print a * every 1000 */ { /* input characters. This */ i=0; /* is just a pacifier. */ printf("."); } index=find_match(string_code,character);/* See if the string is in */ if (code_value[index] != -1) /* the table. If it is, */ string_code=code_value[index]; /* get the code value. If */ else /* the string is not in the*/ { /* table, try to add it. */ if (next_code <= MAX_CODE) { code_value[index]=next_code++; prefix_code[index]=string_code; append_character[index]=character; } output_code(output,string_code); /* When a string is found */ string_code=character; /* that is not in the table*/ } /* I output the last string*/ } /* after adding the new one*/ /* ** End of the main loop. */ output_code(output,string_code); /* Output the last code */ output_code(output,MAX_VALUE); /* Output the end of buffer code */ output_code(output,0); /* This code flushes the output buffer*/ printf("\n"); /* cleanup... */ fclose(input); fclose(output); free(code_value); free(prefix_code); free(append_character); return 1; } /* ** This is the hashing routine. It tries to find a match for the prefix+char ** string in the string table. If it finds it, the index is returned. If ** the string is not found, the first available index in the string table is ** returned instead. */ int find_match(int hash_prefix,unsigned int hash_character) { int index; int offset; index = (hash_character << HASHING_SHIFT) ^ hash_prefix; if (index == 0) offset = 1; else offset = TABLE_SIZE - index; while (1) { if (code_value[index] == -1) return(index); if ((int)prefix_code[index] == hash_prefix && append_character[index] == hash_character) return(index); index -= offset; if (index < 0) index += TABLE_SIZE; } } /******************************************************************* LZW_Decompression() 用LZW对文件进行解码 ********************************************************************/ int LZW_Decompression(char *in_filename, char *out_filename) { unsigned int next_code; unsigned int new_code; unsigned int old_code; int character; int counter; unsigned char *string; FILE *input; FILE *output; /* allocate memory for decompression */ prefix_code=malloc(TABLE_SIZE*sizeof(unsigned int)); append_character=malloc(TABLE_SIZE*sizeof(unsigned char)); if (prefix_code==NULL || append_character==NULL) { printf("Fatal error allocating table space!\n"); return 0; } /* open files */ input=fopen(in_filename,"rb"); output=fopen(out_filename,"wb"); if (input==NULL || output==NULL) { printf("Fatal error opening files.\n"); return 0; }; /* decompress... */ next_code=256; /* This is the next available code to define */ counter=0; /* Counter is used as a pacifier. */ printf("Decompress...\n"); old_code=input_code(input); /* Read in the first code, initialize the */ character=old_code; /* character variable, and send the first */ putc(old_code,output); /* code to the output file */ /* ** This is the main expansion loop. It reads in characters from the LZW file ** until it sees the special code used to inidicate the end of the data. */ while ((new_code=input_code(input)) != (MAX_VALUE)) { if (++counter==1000) /* This section of code prints out */ { /* an asterisk every 1000 characters */ cou