C语言实现短字符串压缩的三种方法详解

2022-11-13 14:11:38 字符串 详解 三种

前言

上一篇探索了LZ4的压缩和解压性能,以及对LZ4和ZSTD的压缩、解压性能进行了横向对比。文末的最后也给了一个彩蛋:任意长度的字符串都可以被ZSTD、LZ4之类的压缩算压缩得很好吗?

本篇我们就来一探究竟。

一、通用算法的短字符压缩

开门见山,我们使用一段比较短的文本:Narrator: It is raining today. So, Peppa and George cannot  play outside.Peppa: Daddy, it's stopped raining.

使用ZSTD与LZ4分别压缩一下上面这段短文本。下面分别是它们的压缩结果。

ZSTD:

LZ4:

对短文本的压缩,zstd的压缩率很低,lz4压缩后的文本长度尽然超过了原有字符串的长度。这是为什么?说实话在这之前我也没想到。

引用两位大佬的名言:

Are you ok?  

What's your problem?

二、短字符串压缩

从上面的结果可以得知,任何压缩算法都有它的使用场景,并不是所有长度的字符串都适合被某种算法压缩。一般原因是通用压缩算法维护了被压缩字符串的,用于字符串还原的相关数据结构,而这些数据结构的长度超过了被压缩短字符串的自身长度。

那么问题来了,“我真的有压缩短字符串的需求,我想体验压缩的极致感,怎么办?”。

短字符压缩算法它来了。这里挑选了3种比较优异的短字符压缩算法,分别是smaz,shoco,以及压轴的unisox2。跟前两章一样,还是从压缩率,压缩和解压缩性能的角度,一起看看他们在短字符压缩场景的各自表现吧。

(1)Smaz

1、Smaz的压缩和解压缩

#include <stdio.h>
#include <string.h>
#include <iOStream>
#include "smaz.h"

using namespace std;

int main()
{
    int buf_len;
    int com_size;
    int decom_size;

    char com_buf[4096] = {0};
    char decom_buf[4096] = {0};

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    com_size = smaz_compress(str_buf, buf_len, com_buf, 4096);

    cout << "text size:" << buf_len << endl;
    cout << "compress text size:" << com_size << endl;
    cout << "compress ratio:" << (float)buf_len / (float)com_size << endl << endl;

    decom_size = smaz_decompress(com_buf, com_size, decom_buf, 4096);
    cout << "decompress text size:" << decom_size << endl;

    if(strncmp(str_buf, decom_buf, buf_len)) {
        cout << "decompress text is not equal to source text" << endl;
    }

    return 0;
}

执行结果如下:

通过smaz压缩后的短字符串长度为77,和源字符串相比,减少了30Byte。

2、Smaz的压缩和解压缩性能

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <sys/time.h>
#include "smaz.h"

using namespace std;

int main()
{
    int cnt = 0;
    int buf_len;
    int com_size;
    int decom_size;

    timeval st, et;

    char *com_ptr = NULL;
    char* decom_ptr = NULL;

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    gettimeofday(&st, NULL);
    while(1) {

        com_ptr = (char *)malloc(buf_len);
        com_size = smaz_compress(str_buf, buf_len, com_ptr, buf_len);

        free(com_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << endl <<"compress per second:" << cnt/10 << " times" << endl;

    cnt = 0;
    com_ptr = (char *)malloc(buf_len);
    com_size = smaz_compress(str_buf, buf_len, com_ptr, buf_len);

    gettimeofday(&st, NULL);
    while(1) {

        // decompress length not more than origin buf length
        decom_ptr = (char *)malloc(buf_len + 1);
        decom_size = smaz_decompress(com_ptr, com_size, decom_ptr, buf_len + 1);

        // check decompress length
        if(buf_len != decom_size) {
            cout << "decom error" << endl;
        }

        free(decom_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << "decompress per second:" << cnt/10 << " times" << endl << endl;

    free(com_ptr);
    return 0;
}

结果如何?

压缩性能在40w条/S,解压在百万级,好像还不错哈!

(2)Shoco

1、Shoco的压缩和解压缩

#include <stdio.h>
#include <string.h>
#include <iostream>
#include "shoco.h"

using namespace std;

int main()
{
    int buf_len;
    int com_size;
    int decom_size;

    char com_buf[4096] = {0};
    char decom_buf[4096] = {0};

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    com_size = shoco_compress(str_buf, buf_len, com_buf, 4096);

    cout << "text size:" << buf_len << endl;
    cout << "compress text size:" << com_size << endl;
    cout << "compress ratio:" << (float)buf_len / (float)com_size << endl << endl;

    decom_size = shoco_decompress(com_buf, com_size, decom_buf, 4096);
    cout << "decompress text size:" << decom_size << endl;

    if(strncmp(str_buf, decom_buf, buf_len)) {
        cout << "decompress text is not equal to source text" << endl;
    }

    return 0;
}

执行结果如下:

通过shoco压缩后的短字符串长度为86,和源字符串相比,减少了21Byte。压缩率比smaz要低。

 2、Shoco的压缩和解压缩性能

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <sys/time.h>
#include "shoco.h"

using namespace std;

int main()
{
    int cnt = 0;
    int buf_len;
    int com_size;
    int decom_size;

    timeval st, et;

    char *com_ptr = NULL;
    char* decom_ptr = NULL;

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    gettimeofday(&st, NULL);
    while(1) {

        com_ptr = (char *)malloc(buf_len);
        com_size = shoco_compress(str_buf, buf_len, com_ptr, buf_len);

        free(com_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << endl <<"compress per second:" << cnt/10 << " times" << endl;

    cnt = 0;
    com_ptr = (char *)malloc(buf_len);
    com_size = shoco_compress(str_buf, buf_len, com_ptr, buf_len);

    gettimeofday(&st, NULL);
    while(1) {

        // decompress length not more than origin buf length
        decom_ptr = (char *)malloc(buf_len + 1);
        decom_size = shoco_decompress(com_ptr, com_size, decom_ptr, buf_len + 1);

        // check decompress length
        if(buf_len != decom_size) {
            cout << "decom error" << endl;
        }

        free(decom_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    cout << "decompress per second:" << cnt/10 << " times" << endl << endl;

    free(com_ptr);
    return 0;
}

执行结果如何呢?

holy shit!压缩和解压缩居然都达到了惊人的百万级。就像算法作者们自己说的一样:“在长字符串压缩领域,shoco不想与通用压缩算法竞争,我们的优势是短字符的快速压缩,虽然压缩率很烂!”。这样说,好像也没毛病。

(3)Unisox2

我们再来看看unisox2呢。

1、Unisox2的压缩和解压缩

#include <stdio.h>
#include <string.h>
#include "unishox2.h"

int main()
{
    int buf_len;
    int com_size;
    int decom_size;

    char com_buf[4096] = {0};
    char decom_buf[4096] = {0};

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    com_size = unishox2_compress_simple(str_buf, buf_len, com_buf);

    printf("text size:%d\n", buf_len);
    printf("compress text size:%d\n", com_size);
    printf("compress ratio:%f\n\n", (float)buf_len / (float)com_size);

    decom_size = unishox2_decompress_simple(com_buf, com_size, decom_buf);

    printf("decompress text size:%d\n", decom_size);

    if(strncmp(str_buf, decom_buf, buf_len)) {
        printf("decompress text is not equal to source text\n");
    }

    return 0;
}

结果如下:

通过Unisox2压缩后的短字符串长度为67,和源字符串相比,减少了40Byte,相当于是打了6折啊!不错不错。

 2、Unisox2的压缩和解压缩性能

Unisox2的压缩能力目前来看是三者中最好的,如果他的压缩和解压性能也不错的话,那就真的就比较完美了。再一起看看Unisox2的压缩和解压性能吧!

#include <stdio.h>
#include <string.h>
#include <malloc.h>
#include <sys/time.h>
#include "unishox2.h"

int main()
{
    int cnt = 0;
    int buf_len;
    int com_size;
    int decom_size;

    struct timeval st, et;

    char *com_ptr = NULL;
    char* decom_ptr = NULL;

    char str_buf[1024] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining.";

    buf_len = strlen(str_buf);
    gettimeofday(&st, NULL);
    while(1) {

        com_ptr = (char *)malloc(buf_len);
        com_size = unishox2_compress_simple(str_buf, buf_len, com_ptr);

        free(com_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    printf("\ncompress per second:%d times\n", cnt/10);

    cnt = 0;
    com_ptr = (char *)malloc(buf_len);
    com_size = unishox2_compress_simple(str_buf, buf_len, com_ptr);

    gettimeofday(&st, NULL);
    while(1) {

        // decompress length not more than origin buf length
        decom_ptr = (char *)malloc(buf_len + 1);
        decom_size = unishox2_decompress_simple(com_ptr, com_size, decom_ptr);

        // check decompress length
        if(buf_len != decom_size) {
            printf("decom error\n");
        }

        free(decom_ptr);
        cnt++;

        gettimeofday(&et, NULL);
        if(et.tv_sec - st.tv_sec >= 10) {
            break;
        }
    }

    printf("decompress per second:%d times\n\n", cnt/10);

    free(com_ptr);
    return 0;
}

执行结果如下:

事与愿违,Unisox2虽然有三个算法中最好的压缩率,可是却也拥有最差的压缩和解压性能。跟前两章分析的不谋而合:有高压缩率,就会损失自身的压缩性能,两者不可兼得。

三、总结

本篇分享了smaz,shoco,unisox2三种短字符串压缩算法,分别探索了它们各自的压缩率与压缩和解压缩性能,结果如下表所示。

表1

shoco的压缩率最低,但是拥有最高的压缩和解压速率;smaz居中;unisox2拥有最高的压缩率,可是它的压缩和解压性能最低。

结论与前两章有关长字符串压缩的分析不谋而合:拥有高压缩率,就会损失自身的压缩性能,两者不可兼得。

实际使用还是看自身需求和环境吧。如果适当压缩就好,那就可以选用shoco,毕竟性能高;想要节约更多的空间,那就选择smaz或者unisox2。

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