FloatingPointTrick

#include <iostream>
#include <random>
#include <chrono>
#include <limits>
#include <vector>
#include <functional>
#include <cmath>

using namespace std;

int main()
{
    cout << "乱数の生成" << endl;
    int const Size = 0x7ffffff;
    random_device rd;
    mt19937 mt(rd());
    auto value = bind(uniform_real_distribution<float>(1.f, 10.f), mt);
    vector<float> values(Size);
    for(auto i = 0; i < Size; ++i)
    {
        values[i] = value();
    }

    vector<int> int_values(Size);
    cout << "キャストによる変換" << endl;
    auto t1 = chrono::high_resolution_clock::now();
    for(auto i = 0; i < Size; ++i)
    {
        int_values[i] = static_cast<int>(values[i]);
    }
    auto t2 = chrono::high_resolution_clock::now();
    cout << "経過時間 : " << chrono::duration_cast<chrono::milliseconds>(t2 - t1).count() << endl;

    union {
        int i;
        float f;
    } v, bias;

    bias.i = (23 + 127) << 23;

    cout << "共用体による整数への変換" << endl;
    auto t3 = chrono::high_resolution_clock::now();
    for(auto i = 0; i < Size; ++i)
    {
        v.f = values[i];
        v.f += bias.f;
        v.i -= bias.i;
        int_values[i] = v.i;
    }
    auto t4 = chrono::high_resolution_clock::now();
    cout << "経過時間 : " << chrono::duration_cast<chrono::milliseconds>(t4 - t3).count() << endl;

    vector<float> float_values(Size);
    cout << "absによる絶対値への変換" << endl;
    auto t5 = chrono::high_resolution_clock::now();
    for(auto i = 0; i < Size; ++i)
    {
        float_values[i] = abs(values[i]);
    }
    auto t6 = chrono::high_resolution_clock::now();
    cout << "経過時間 : " << chrono::duration_cast<chrono::milliseconds>(t6 - t5).count() << endl;

    cout << "共用体による絶対値への変換" << endl;
    auto t7 = chrono::high_resolution_clock::now();
    for(auto i = 0; i < Size; ++i)
    {
        v.f = values[i];
        v.i &= 0x7fffffff;
        float_values[i] = v.f;
    }
    auto t8 = chrono::high_resolution_clock::now();
    cout << "経過時間 : " << chrono::duration_cast<chrono::milliseconds>(t8 - t7).count() << endl;


}