1、背景

本文舉例說明了：專案過程中字串資料傳輸的場景下（檔名列表），如何使用資料壓縮減少頻寬的開銷；

2、相關知識

引用外部的一個手冊進行說明

3、實現方法

deflate壓縮實現，壓縮完成後會提示Z_STREAM_END

int gzcompress(void *data, size_t ndata, void *zdata, size_t *nzdata)
{
        int ret = -1;
        z_stream c_stream;
        if (!data || !ndata) {
                printf("NULL\n");
                return -1;
        }
        c_stream.zalloc = NULL;
        c_stream.zfree  = NULL;
        c_stream.opaque = NULL;
        if (deflateInit2(&c_stream, Z_DEFAULT_COMPRESSION, Z_DEFLATED,
                                         MAX_WBITS + 16, 8, Z_DEFAULT_STRATEGY) != Z_OK) {
                printf("deflateInit2\n");
                return -1;
        }
        c_stream.next_in   = (Bytef *)data;
        c_stream.avail_in  = ndata;
        c_stream.next_out  = (Bytef *)zdata;
        c_stream.avail_out = *nzdata;
        while (c_stream.avail_in != 0 && c_stream.total_out < *nzdata) {
                if (deflate(&c_stream, Z_NO_FLUSH) != Z_OK) {
                        printf("deflate\n");
                        goto end;
                }
        }
        if (c_stream.avail_in != 0) {
                printf("%d\n", c_stream.avail_in);
                return c_stream.avail_in;
        }
        for (;;) {
                ret = deflate(&c_stream, Z_FINISH);
                if (ret == Z_STREAM_END) {
                        printf("deflate: Z_STREAM_END\n");
                        break;
                }
                else if (ret != Z_OK) {
                        printf("deflate: %d\n", ret);
                        break;
                }
        }
end:
        if (deflateEnd(&c_stream) != Z_OK) {
                printf("deflateEnd: Failure\n");
                return -1;
        }
        *nzdata = c_stream.total_out;
        return 0;
}
 

int gzdecompress(void *zdata, size_t nzdata, void *data, size_t *ndata)
{
        int ret = -1;
        z_stream d_stream = {0}; /* decompression stream */
        static char dummy_head[2] = {
                0x8 + 0x7 * 0x10,
                (((0x8 + 0x7 * 0x10) * 0x100 + 30) / 31 * 31) & 0xFF,
        };
        d_stream.zalloc   = NULL;
        d_stream.zfree    = NULL;
        d_stream.opaque   = NULL;
        d_stream.next_in  = (Bytef *)zdata;
        d_stream.avail_in = 0;
        d_stream.next_out = (Bytef *)data;
        if (inflateInit2(&d_stream, MAX_WBITS + 16) != Z_OK) {
                printf("inflateInit2\n");
                return -1;
        }
        while (d_stream.total_out < *ndata && d_stream.total_in < nzdata) {
                d_stream.avail_in = d_stream.avail_out = 1; /* force small buffers */
                ret = inflate(&d_stream, Z_NO_FLUSH);
                switch (ret) {
                case Z_OK:
                        continue;
                case Z_STREAM_END:
                        printf("inflate: Z_STREAM_END\n");
                        goto end;
                case Z_DATA_ERROR:
                        d_stream.next_in = (Bytef *)dummy_head;
                        d_stream.avail_in = sizeof(dummy_head);
                        if ((ret = inflate(&d_stream, Z_NO_FLUSH)) != Z_OK) {
                                printf("inflate failed\n");
                                goto end;
                        }
                        break;
                default:
                        printf("inflate: %d\n", ret);
                        goto end;
                }
        }
end:
        if (inflateEnd(&d_stream) != Z_OK) {
                printf("inflateEnd: Failure\n");
                return -1;
        }
        *ndata = d_stream.total_out;
        return 0;
}
 

int main(int argc, char *argv[])
{
        int ret = -1;
        char input[] = (
                        "/usr/local/vim\n"
                        "/usr/local/vim/share\n"
                        "/usr/local/vim/share/vim\n"
                        "/usr/local/vim/share/vim/print\n"
                        "/usr/local/vim/share/vim/keymap\n"
                        "/usr/local/vim/share/vim/autoload\n"
                        "/usr/local/vim/share/vim/autoload/xml\n"
                        "/usr/local/vim/share/vim/indent\n"
                        "/usr/local/vim/share/vim/macros\n"
                        "/usr/local/vim/share/vim/mang/sk\n"
                        "/usr/local/vim/share/vim/lang/sk/LC_MESSAGES\n"
                        );
        char gzip[1024] = {0};
        char output[1024] = {0};
        size_t ngzip = sizeof(gzip);
        size_t nout = sizeof(output);
        assert((ret = gzcompress(input, sizeof(input), gzip, &ngzip)) == 0);
        assert((ret = gzdecompress(gzip, ngzip, output, &nout)) == 0);
        printf(" input: %zd\n%s\n", sizeof(input), input);
        printf("output: %zd\n%s\n", nout, output);
        printf("Result:\t\t\t\t[%s]\n", ret ? "Failure" : "Success");
        exit(ret ? EXIT_FAILURE : EXIT_SUCCESS);
}
 

執行結果如下：

deflate: Z_STREAM_END
inflate: Z_STREAM_END
 input: 339
/usr/local/vim
/usr/local/vim/share
/usr/local/vim/share/vim
/usr/local/vim/share/vim/print
/usr/local/vim/share/vim/keymap
/usr/local/vim/share/vim/autoload
/usr/local/vim/share/vim/autoload/xml
/usr/local/vim/share/vim/indent
/usr/local/vim/share/vim/macros
/usr/local/vim/share/vim/mang/sk
/usr/local/vim/share/vim/lang/sk/LC_MESSAGES

output: 339
/usr/local/vim
/usr/local/vim/share
/usr/local/vim/share/vim
/usr/local/vim/share/vim/print
/usr/local/vim/share/vim/keymap
/usr/local/vim/share/vim/autoload
/usr/local/vim/share/vim/autoload/xml
/usr/local/vim/share/vim/indent
/usr/local/vim/share/vim/macros
/usr/local/vim/share/vim/mang/sk
/usr/local/vim/share/vim/lang/sk/LC_MESSAGES

Result:                         [Success]

deflate(RFC1951):一種壓縮演算法，使用LZ77和哈弗曼進行編碼；
zlib(RFC1950):一種格式，是對deflate進行了簡單的封裝，他也是一個實現庫(delphi中有zlib,zlibex)
gzip(RFC1952):一種格式，也是對deflate進行的封裝。
gzip = gzip頭 + deflate編碼的實際內容 + gzip尾
zlib = zlib頭 + deflate編碼的實際內容 + zlib尾

以上例子通過一次性將字串資料壓縮轉換成資料流形式，在工程中實踐的意義就是犧牲CPU計算換取頻寬的方法；