Question

我有一个LZW压缩/解压缩算法的实现，并且大部分都是平方的。但是，我遇到了我正在测试的其中一个文件的问题。以下是所述文件的文本

#include "bits.h"

int check_endianness(){
    int i = 1;

我实现的区域是int i = 1;之前的空格组。下面我分别包含了我的压缩和解压缩循环以及它们的相对调试输出。

压缩循环

i=0;
while(i < input_len && ret == LZW_ERR_OK){
    //get next byte
    char curChar = input_char(f_input, &io_flags);
    i++;

    //not necessary to check for stream end here since the while condition does that
    if(io_flags == STREAM_ERR_READ){
        ret = LZW_ERR_READ;
        break;
    }

    seqset(&temp, &curChar, 1);

    //add bytes to temp until a sequence is found that is not in lookup
    while(i < input_len && dictionary_has_entry(lookup, temp)){
        char curChar = input_char(f_input, &io_flags);
        i++;

        //check for errors / end of file
        if(io_flags != STREAM_ERR_OK){
            if(io_flags == STREAM_ERR_READ)
                ret = LZW_ERR_READ;
            break;
        }

        seqadd(&temp, &curChar, 1);
    }

    if(temp.length > 1){
        #ifdef DEBUG
        printf("Adding entry %d: ", lookup->next_value);
        for(int j = 0; j < temp.length; j++)
            printf("%.4x ", temp.data[j]);
        printf("\n");
        #endif // DEBUG
        dictionary_set_entry(lookup, temp, DICT_SET_FOREVER);
        temp.length--; //if temp.length == 1, then the EOF was probably reached
        i--; //This is important so that the entire file is read
    }

    fseek(f_input, -1, SEEK_CUR);
    output_short(f_output, dictionary_get_entry_byKey(lookup, temp)->value, STREAM_USE_ENDIAN);
    #ifdef DEBUG
    printf("Writing code: %d\n", dictionary_get_entry_byKey(lookup, temp)->value);
    #endif // DEBUG
}

压缩输出

Adding entry 297: 007b 000d
Writing code: 123
Adding entry 298: 000d 000a 0020
Writing code: 273
Adding entry 299: 0020 0020
Writing code: 32
Adding entry 300: 0020 0020 0020
Writing code: 299
Adding entry 301: 0020 0069
Writing code: 32
Adding entry 302: 0069 006e 0074 0020

解压缩循环

i=0;
while(i < input_len && ret == LZW_ERR_OK){
    short code;
    entry *e;
    code = input_short(f_input, &io_flags, STREAM_USE_ENDIAN);
    if(io_flags == STREAM_ERR_READ){
        ret = LZW_ERR_READ;
        break;
    }

    i += 2;

    //e should never be NULL
    printf("Reading code: %d\n", code);
    e = dictionary_get_entry_byValue(lookup, code);
    assert(e != NULL);

    seqset(&temp, e->key.data, e->key.length);

    //requires a slightly different approach to the lookup loop in lzw_encode
    while(i < input_len && e != NULL){
        code = input_short(f_input, &io_flags, STREAM_USE_ENDIAN);
        //check for errors / end of file
        if(io_flags != STREAM_ERR_OK){
            if(io_flags == STREAM_ERR_READ)
                ret = LZW_ERR_READ;
            break;
        }
        i += 2;
        printf("Reading code: %d\n", code);

        //e should never be NULL
        e = dictionary_get_entry_byValue(lookup, code);
        assert(e != NULL); <------------ This is where it is failing

        //start adding bytes to temp
        for(size_t j = 0; j < e->key.length; j++){
            seqadd(&temp, &e->key.data[j], 1);
            if(dictionary_get_entry_byKey(lookup, temp) == NULL){

                //sequence not found, add it to dictionary
                printf("Adding entry %d: ", lookup->next_value);
                dictionary_set_entry(lookup, temp, DICT_SET_FOREVER);
                for(int k = 0; k < temp.length; k++)
                    printf("%.4x ", temp.data[k]);
                printf("\n");
                e = NULL; //to escape from while
                break;
            }
        }
    }

    //if more than one code was read go back by two bytes
    if(e == NULL){
        i -= 2;
        fseek(f_input, -2, SEEK_CUR);
    }

    //only write up to the characters that made a known sequence
    temp.length--;
    for(size_t j = 0; j < temp.length; j++){
        output_char(f_output, temp.data[j]);
        #ifdef DEBUG
        //printf("%c", temp.data[j]);
        #endif // DEBUG
    }
}

减压输出

Reading code: 123
Reading code: 273
Adding entry 297: 007b 000d
Reading code: 273
Reading code: 32
Adding entry 298: 000d 000a 0020
Reading code: 32
Reading code: 299
299, 299 <----error output from dictionary (code asked > next value)
Assertion failed: e != NULL, file lzw.c, line 212

非常感谢任何帮助。

Answer 1

你在Lempel Ziv Welch减压算法中遇到了臭名昭着的KwKwK问题。

来自原始文章A Technique for High-Performance Data Compression，Terry A. Welch，IEEE计算机，1984年6月，第8-19页：

只要输入字符串包含序列KwKwK，就会出现异常情况，其中Kw已出现在压缩器字符串表中。压缩器将解析出Kw，发送CODE（Kw），并将KwK添加到其字符串表中。接下来，它将解析KwK并发送刚刚生成的CODE（KwK）。解压缩器在接收CODE（KwK）时，尚未将该代码添加到其字符串表中，因为它还不知道前一个字符串的扩展字符。

本文解释了如何解决此问题。

Lempel-Ziv-Welch减压不存在指数

1 个答案: