C語言字符串壓縮之ZSTD算法詳解
前言
最近項目上有大量的字符串數據需要存儲到內存，並且需要儲存至一定時間，於是自然而然的想到瞭使用字符串壓縮算法對“源串”進行壓縮存儲。由此觸發瞭對一些優秀壓縮算法的調研。
字符串壓縮，我們通常的需求有幾個，一是高壓縮率，二是壓縮速率高，三是解壓速率高。不過高壓縮率與高壓縮速率是魚和熊掌的關系，不可皆得，優秀的算法一般也是采用壓縮率與性能折中的方案。從壓縮率、壓縮速率、解壓速率考慮，zstd與lz4有較好的壓縮與解壓性能，最終選取zstd與lz4進行調研。
zstd是facebook開源的提供高壓縮比的快速壓縮算法（參考https://github.com/facebook/zstd），很想瞭解一下它在壓縮與解壓方面的實際表現。
一、zstd壓縮與解壓
ZSTD_compress屬於ZSTD的Simple API范疇，隻有壓縮級別可以設置。
ZSTD_compress函數原型如下：
size_t ZSTD_compress(void* dst, size_t dstCapacity, const void* src, size_t srcSize, int compressionLevel)
ZSTD_decompress函數原型如下：
size_t ZSTD_decompress( void* dst, size_t dstCapacity, const void* src, size_t compressedSize); 我們先來看看zstd的壓縮與解壓縮示例。
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>
using namespace std;
int main()
{
// compress
size_t com_space_size;
size_t peppa_pig_text_size;
char *com_ptr = NULL;
char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
peppa_pig_text_size = strlen(peppa_pig_buf);
com_space_size= ZSTD_compressBound(peppa_pig_text_size);
com_ptr = (char *)malloc(com_space_size);
if(NULL == com_ptr) {
cout << "compress malloc failed" << endl;
return -1;
}
size_t com_size;
com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);
cout << "peppa pig text size:" << peppa_pig_text_size << endl;
cout << "compress text size:" << com_size << endl;
cout << "compress ratio:" << (float)peppa_pig_text_size / (float)com_size << endl << endl;
// decompress
char* decom_ptr = NULL;
unsigned long long decom_buf_size;
decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);
decom_ptr = (char *)malloc((size_t)decom_buf_size);
if(NULL == decom_ptr) {
cout << "decompress malloc failed" << endl;
return -1;
}
size_t decom_size;
decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
cout << "decompress text size:" << decom_size << endl;
if(strncmp(peppa_pig_buf, decom_ptr, peppa_pig_text_size)) {
cout << "decompress text is not equal peppa pig text" << endl;
}
free(com_ptr);
free(decom_ptr);
return 0;
}
執行結果：
從結果可以發現，壓縮之前的peppa pig文本長度為1827，壓縮後的文本長度為759，壓縮率為2.4，解壓後的長度與壓縮前相等。
另外，上文提到可以調整ZSTD_compress函數的壓縮級別，zstd的默認級別為ZSTD_CLEVEL_DEFAULT = 3，最小值為0，最大值為ZSTD_MAX_CLEVEL = 22。另外也提供一些策略設置，例如 ZSTD_fast, ZSTD_greedy, ZSTD_lazy, ZSTD_lazy2, ZSTD_btlazy2。壓縮級別越高，壓縮率越高，但是壓縮速率越低。
二、ZSTD壓縮與解壓性能探索
上面探索瞭zstd的基礎壓縮與解壓方法，接下來再摸索一下zstd的壓縮與解壓縮性能。
測試方法是，使用ZSTD_compress連續壓縮同一段文本並持續10秒，最後得到每一秒的平均壓縮速率。測試壓縮性能的代碼示例如下：
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>
using namespace std;
int main()
{
int cnt = 0;
size_t com_size;
size_t com_space_size;
size_t peppa_pig_text_size;
char *com_ptr = NULL;
char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
timeval st, et;
peppa_pig_text_size = strlen(peppa_pig_buf);
com_space_size= ZSTD_compressBound(peppa_pig_text_size);
gettimeofday(&st, NULL);
while(1) {
com_ptr = (char *)malloc(com_space_size);
com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, ZSTD_fast);
free(com_ptr);
cnt++;
gettimeofday(&et, NULL);
if(et.tv_sec - st.tv_sec >= 10) {
break;
}
}
cout << "compress per second:" << cnt/10 << " times" << endl;
return 0;
}
執行結果：
結果顯示ZSTD的壓縮性能大概在每秒6-7萬次左右，這個結果其實並不是太理想。需要說明的是壓縮性能與待壓縮文本的長度、字符內容也是有關系的。
我們再來探索一下ZSTD的解壓縮性能。與上面的測試方法類似，先對本文進行壓縮，然後連續解壓同一段被壓縮過的數據並持續10秒，最後得到每一秒的平均解壓速率。測試解壓性能的代碼示例如下：
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>
using namespace std;
int main()
{
int cnt = 0;
size_t com_size;
size_t com_space_size;
size_t peppa_pig_text_size;
timeval st, et;
char *com_ptr = NULL;
char peppa_pig_buf[2048] = "Narrator: It is raining today. So, Peppa and George cannot play outside.Peppa: Daddy, it's stopped raining. Can we go out to play?Daddy: Alright, run along you two.Narrator: Peppa loves jumping in muddy puddles.Peppa: I love muddy puddles.Mummy: Peppa. If you jumping in muddy puddles, you must wear your boots.Peppa: Sorry, Mummy.Narrator: George likes to jump in muddy puddles, too.Peppa: George. If you jump in muddy puddles, you must wear your boots.Narrator: Peppa likes to look after her little brother, George.Peppa: George, let's find some more pud dles.Narrator: Peppa and George are having a lot of fun. Peppa has found a lttle puddle. George hasfound a big puddle.Peppa: Look, George. There's a really big puddle.Narrator: George wants to jump into the big puddle first.Peppa: Stop, George. | must check if it's safe for you. Good. It is safe for you. Sorry, George. It'sonly mud.Narrator: Peppa and George love jumping in muddy puddles.Peppa: Come on, George. Let's go and show Daddy.Daddy: Goodness me.Peppa: Daddy. Daddy. Guess what we' ve been doing.Daddy: Let me think... Have you been wa tching television?Peppa: No. No. Daddy.Daddy: Have you just had a bath?Peppa: No. No.Daddy: | know. You've been jumping in muddy puddles.Peppa: Yes. Yes. Daddy. We've been jumping in muddy puddles.Daddy: Ho. Ho. And look at the mess you're in.Peppa: Oooh....Daddy: Oh, well, it's only mud. Let's clean up quickly before Mummy sees the mess.Peppa: Daddy, when we've cleaned up, will you and Mummy Come and play, too?Daddy: Yes, we can all play in the garden.Narrator: Peppa and George are wearing their boots. Mummy and Daddy are wearing their boots.Peppa loves jumping up and down in muddy puddles. Everyone loves jumping up and down inmuddy puddles.Mummy: Oh, Daddy pig, look at the mess you're in. .Peppa: It's only mud.";
size_t decom_size;
char* decom_ptr = NULL;
unsigned long long decom_buf_size;
peppa_pig_text_size = strlen(peppa_pig_buf);
com_space_size= ZSTD_compressBound(peppa_pig_text_size);
com_ptr = (char *)malloc(com_space_size);
com_size = ZSTD_compress(com_ptr, com_space_size, peppa_pig_buf, peppa_pig_text_size, 1);
gettimeofday(&st, NULL);
decom_buf_size = ZSTD_getFrameContentSize(com_ptr, com_size);
while(1) {
decom_ptr = (char *)malloc((size_t)decom_buf_size);
decom_size = ZSTD_decompress(decom_ptr, decom_buf_size, com_ptr, com_size);
if(decom_size != peppa_pig_text_size) {
cout << "decompress error" << endl;
break;
}
free(decom_ptr);
cnt++;
gettimeofday(&et, NULL);
if(et.tv_sec - st.tv_sec >= 10) {
break;
}
}
cout << "decompress per second:" << cnt/10 << " times" << endl;
free(com_ptr);
return 0;
}
執行結果：
結果顯示ZSTD的解壓縮性能大概在每秒12萬次左右，解壓性能比壓縮性能高。
三、zstd的高級用法
zstd提供瞭一個名為PZSTD的壓縮和解壓工具。PZSTD（parallel zstd），並行壓縮的zstd，是一個使用多線程對待壓縮文本進行切片分段，且進行並行壓縮的命令行工具。
其實高版本（v1.4.0及以上）的zstd也提供瞭指定多線程對文本進行並行壓縮的相關API接口，也就是本小節要介紹的zstd高級API用法。下面我們再來探索一下zstd的多線程壓縮使用方法。
多線程並行壓縮的兩個關鍵API，一個是參數設置API，另一個是壓縮API。
參數設置API的原型是：
size_t ZSTD_CCtx_setParameter(ZSTD_CCtx* cctx, ZSTD_cParameter param, int value)
壓縮API的原型是：
size_t ZSTD_compress2(ZSTD_CCtx* cctx, void* dst, size_t dstCapacity, const void* src, size_t srcSize)
下面給出zstd並行壓縮的示例demo，通過ZSTD_CCtx_setParameter設置線程數為3，即指定宏ZSTD_c_nbWorkers為3，通過ZSTD_compress2壓縮相關文本。另外，為瞭展示zstd確實使用瞭多線程，需要先讀取一個非常大的文件，作為zstd的壓縮文本源，盡量使zstd運行較長時間。
#include <stdio.h>
#include <string.h>
#include <sys/time.h>
#include <malloc.h>
#include <zstd.h>
#include <iostream>
using namespace std;
int main()
{
size_t com_size;
size_t com_space_size;
FILE *fp = NULL;
unsigned int file_len;
char *com_ptr = NULL;
char *file_text_ptr = NULL;
fp = fopen("xxxxxx", "r");
if(NULL == fp){
cout << "file open failed" << endl;
return -1;
}
fseek(fp, 0, SEEK_END);
file_len = ftell(fp);
fseek(fp, 0, SEEK_SET);
cout << "file length:" << file_len << endl;
// malloc space for file content
file_text_ptr = (char *)malloc(file_len);
if(NULL == file_text_ptr) {
cout << "malloc failed" << endl;
return -1;
}
// malloc space for compress space
com_space_size = ZSTD_compressBound(file_len);
com_ptr = (char *)malloc(com_space_size);
if(NULL == com_ptr) {
cout << "malloc failed" << endl;
return -1;
}
// read text from source file
fread(file_text_ptr, 1, file_len, fp);
fclose(fp);
ZSTD_CCtx* cctx;
cctx = ZSTD_createCCtx();
// set multi-thread parameter
ZSTD_CCtx_setParameter(cctx, ZSTD_c_nbWorkers, 3);
ZSTD_CCtx_setParameter(cctx, ZSTD_c_compressionLevel, ZSTD_btlazy2);
com_size = ZSTD_compress2(cctx, com_ptr, com_space_size, file_text_ptr, file_len);
free(com_ptr);
free(file_text_ptr);
return 0;
}
運行上述demo，可見zstd確實啟動瞭3個線程對文本進行瞭並行壓縮。且設置的線程數越多，壓縮時間越短，這裡就不詳細展示瞭，讀者可以自行實驗。
需要說明的是，zstd當前默認編譯單線程的庫文件，要實現多線程的API調用，需要在make的時候指定編譯參數ZSTD_MULTITHREAD。
另外，zstd還支持線程池的方式，線程池的函數原型：
POOL_ctx* ZSTD_createThreadPool(size_t numThreads)
線程池可以避免在多次、連續壓縮場景時頻繁的去創建線程、撤銷線程產生的非必要開銷，使得算力主要開銷在文本壓縮方面。
四、總結
本篇分享瞭zstd壓縮與解壓縮使用的基本方法，對壓縮與解壓的性能進行瞭摸底，最後探索瞭zstd多線程壓縮的使用方法。
從壓縮測試來看，zstd的壓縮比其實已經比較好瞭，比原文所占用空間縮小瞭一半以上，當然壓縮比也跟待壓縮文本的內容有關。
從性能執行結果來看，zstd的壓縮與解壓性能表現比較勉強，我認為zstd在魚（性能）和熊掌（壓縮比）之間更偏向熊掌一些，不過對一些性能要求不太高的，但是要高壓縮比的場景是比較符合的。
多線程並行壓縮，在有大文本需要連續多次壓縮的場景下，結合線程池可以很好的提升壓縮速率。
以上就是C語言字符串壓縮之ZSTD算法詳解的詳細內容，更多關於C語言字符串壓縮的資料請關註WalkonNet其它相關文章！
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