C++多线程chap3 多线程异步和通信2

这里，只是记录自己的学习笔记。

顺便和大家分享多线程的基础知识。然后从入门到实战。有代码。

知识点来源：

https://edu.51cto.com/course/26869.html

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C++11 实现base16，并与单线程进行性能测试

base16编码，解码，请看这篇文章：https://www.cnblogs.com/music-liang/p/15656218.html 

/*
C++17 多核并行计算

4.1 手动实现多核 base16编码
4.1.1 实现base16编码
  1.二进制转换为字符串
  2.一个字节8位，拆分为2个4位字节（最大值为16）
  3.拆分后的字节映射到 0123456789abcdef

//不创建线程的情况下通过 foreach 实现多核编码

*/


#include <iostream>
#include <thread>
#include <vector>
#include <chrono>
#include <execution>
using namespace std;

static const char base16[] = "0123456789abcdef";

//base16编码
void Base16Encode(const unsigned char* data, int size, unsigned char* out)
{
    for (int i = 0; i < size; i++) {
        unsigned char d = data[i];
        //1234 5678 >>4  0000 1234   ，右移4位，取到高位字节
        //1234 5678 & 0000 1111   0000 5678，与操作得到低位字节
        char a = base16[d >> 4];
        char b = base16[d & 0x0F];
        out[i * 2] = a;
        out[i * 2 + 1] = b;
    }
}

//C++11 多核base16编码
void Base16EncodeThread(const vector<unsigned char>& data, vector<unsigned char> &out)
{
    int size = data.size();
    int th_count = thread::hardware_concurrency();//系统支持的线程核心数

    //切片数据
    int slice_count = size / th_count;//余数丢弃
    if (size < th_count) { //只切一片
        th_count = 1;
        slice_count = size;
    }

    //准备好线程
    vector<thread> ths;
    ths.resize(th_count);

    //任务分配到各个线程
    for (int i = 0; i < th_count; i++) {
        // 1234 5678 9abc defg hi
        int offset = i * slice_count;
        int count = slice_count;

        //最后一个线程
        if (th_count > 1 && i == th_count - 1) {
            count = slice_count + size % th_count;
        }
        cout << "offset:" << offset << ",count:" << count << endl;

        ths[i] = thread(Base16Encode, data.data() + offset, count, out.data());
    }

    for (auto& th : ths) {
        th.join();//等待所有线程结束
    }
    cout << "over" << endl;
}


int main()
{
    string test_data = "测试base16编码";
    unsigned char out[1024] = { 0 };
    Base16Encode((const unsigned char*)test_data.data(), test_data.size(), out);
    cout << "base16 Encode:" << out << endl;
    cout << "char length:" << sizeof(out) / sizeof(unsigned char) << endl;

    vector<unsigned char> in_data;
    in_data.resize(1024 * 1024 * 20);//20M
    for (int i = 0; i < in_data.size(); i++) {
        in_data[i] = i % 256;
    }
    vector<unsigned char> out_data;
    out_data.resize(in_data.size() * 2);


    //测试单线程 base16 编码效率
    {
        cout << "单线程 base16  开始计算" << endl;

        auto start = chrono::system_clock::now();
        Base16Encode(in_data.data(), in_data.size(), out_data.data());

        auto end = chrono::system_clock::now();
        auto duration = chrono::duration_cast<chrono::milliseconds>(end - start);
        cout << "编码："<<in_data.size()<<"字节数据花费 " << duration.count() <<" 毫秒"<<flush<< endl;
        //cout << "out_data.data():" << out_data.data() << endl;
    }


    //C++11 多线程Base16编码
    {
        cout << "C++11 多线程Base16编码 开始计算" << endl;
        auto start = chrono::system_clock::now();
        Base16EncodeThread(in_data, out_data);
        auto end = chrono::system_clock::now();
        auto duration = chrono::duration_cast<chrono::milliseconds>(end - start);
        cout << "编码：" << in_data.size() << "字节数据花费 " << duration.count() << " 毫秒" << flush << endl;
    }


    //C++17   for_each 写法更加简洁
    {
        cout << "C++17 多线程Base16编码 开始计算" << endl;
        auto start = chrono::system_clock::now();

        // #include <execution> C++17支持
        unsigned char* idata = in_data.data();
        unsigned char* odata = out_data.data();

        //在release模式下，C++17会更快
        std::for_each(std::execution::par,//并行计算 多核
            in_data.begin(), in_data.end(),
            [&](auto& d) { //多线程进入此函数
            char a = base16[(d >> 4)];
            char b = base16[(d & 0x0F)];
            int index = &d - idata ;//计算偏移位置
            odata[index * 2] = a;
            odata[index * 2 + 1] = b;
        }
        );
        
        auto end = chrono::system_clock::now();
        auto duration = chrono::duration_cast<chrono::milliseconds>(end - start);
        cout << "编码：" << in_data.size() << "字节数据花费 " << duration.count() << " 毫秒" << flush << endl;
    }



    getchar();
    return 0;
}