如何从示例中获取LZW编码结果? [英] How to get LZW encode results as from example?

问题描述

给出来自> http://michael.dipperstein.com/lzw/#上的example1的输入example1 ，我无法获得正确的结果:

Given the input from example1 on page http://michael.dipperstein.com/lzw/#example1, I am unable to get the correct result:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "lzw.h"

void print_hex(unsigned char str[], int len) 
{
    int idx;

    for (idx = 0; idx < len; idx++)
        printf("%02x", str[idx]);
}

int main()
{
    FILE *fpIn;             /* pointer to open input file */
    FILE *fpOut;            /* pointer to open output file */
    FILE *fptr;

    char test_str_lzw[] = { "this_is_his_thing" };
    fptr = fopen("lzw_in_test.txt", "wb");
    fwrite(test_str_lzw, sizeof(char), strlen(test_str_lzw), fptr);
    fclose(fptr);

    fpIn = fopen("lzw_in_test.txt", "rb");
    fpOut = fopen("lzw_out.txt", "wb");

    LZWEncodeFile(fpIn, fpOut);

    fclose(fpIn);
    fclose(fpOut);

    // Getting the results from file
    if ((fptr = fopen("lzw_out.txt", "rb")) == NULL) {
        printf("Error! opening file");
        // Program exits if file pointer returns NULL.
        exit(1);
    }

    unsigned char lzw_out[256];
    memset(lzw_out, 0, 256);

    size_t num;
    num = fread(lzw_out, sizeof(unsigned char), 256, fptr);


    fclose(fptr);

    unsigned int lzw_size = num;
    printf("LZW out size: %d\n", lzw_size);
    printf("LZW out data: \n");
    print_hex(lzw_out, lzw_size);
    printf("\n");

    return(0);
}

十六进制的预期结果:

0x74 0x68 0x69 0x73 0x5F 0x102 0x5F 0x101 0x103 0x100 0x69 0x6E 0x67

结果我进入了十六进制:

Result I'm getting in Hex:

0x74 0x34 0x1A 0x4E 0x65 0xF0 0x15 0x7C 0x03 0x03 0x80 0x5A 0x4D 0xC6 0x70 0x20

有人可以帮我从示例中获取输出文件吗?

Can anyone help me to get output file like from the example?

致谢.

推荐答案

LZW编码器将9位代码字序列编码为8位字节序列.有一种机制可以根据需要用信号通知增加代码字的长度，但是为了简单起见，我们可以忽略它，因为对短输入序列进行编码不需要.

The LZW encoder is encoding a sequence of 9-bit code words as a sequence of 8-bit bytes. There is a mechanism for signalling an increase in the length of the code word as required, but let's ignore that for simplicity as it is not required for encoding short input sequences.

对于OP的示例，前八个9位代码字(以十六进制表示):

For OP's example, the first eight 9-bit code words are (in hex):

0x74 0x68 0x69 0x73 0x5F 0x102 0x5F 0x101

0x74 0x68 0x69 0x73 0x5F 0x102 0x5F 0x101

或以二进制表示:

001110100 001101000 001101001 001110011 001011111 100000010 001011111 100000001

001110100 001101000 001101001 001110011 001011111 100000010 001011111 100000001

编码器将每个9位代码字分成两组-第7位至第0位，然后是第8位:

The encoder splits each 9-bit code word into two groups - bits 7 to 0, followed by bit 8:

01110100 0 01101000 0 01101001 0 01110011 0 01011111 0 00000010 1 01011111 0 00000001 1

01110100 0 01101000 0 01101001 0 01110011 0 01011111 0 00000010 1 01011111 0 00000001 1

然后将其重新组合为8位字节的序列:

This is then regrouped as a sequence of 8-bit bytes:

01110100 00110100 00011010 01001110 01100101 11110000 00010101 01111100 00000011

01110100 00110100 00011010 01001110 01100101 11110000 00010101 01111100 00000011

或以十六进制表示:

0x74 0x34 0x1A 0x4E 0x65 0xF0 0x15 0x7C 0x03

0x74 0x34 0x1A 0x4E 0x65 0xF0 0x15 0x7C 0x03

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