Округление до 0,5

#include <iostream>
 
double MyRound(const double x)
{
    const int mainPart = static_cast<int>(x);
    if (x < 0)
    {
        const bool toLowerBound = (0.75 <= mainPart - x);
        const bool toUpperBound = (mainPart - x < 0.25);
        return toUpperBound ? mainPart : toLowerBound ? mainPart - 1 : mainPart - 0.5;
    }
    else
    {
        const bool toLowerBound = (x - mainPart < 0.25);
        const bool toUpperBound = (0.75 <= (x - mainPart));
        return toUpperBound ? mainPart + 1 : toLowerBound ? mainPart : mainPart + 0.5;
    }
}
 
int main()
{
    const double arr[] = {4, 4.2, 4.25, 4.3, 4.5, 4.7, 4.75, 4.8};
    for (const double x: arr)
        std::cout << x << "\t=>\t" << MyRound(x) << std::endl;
    for (const double x: arr)
        std::cout << -x << "\t=>\t" << MyRound(-x) << std::endl;
}

constexpr double MyRound(const double x)
{
    return x < 0 ? static_cast<int>(x) - x < 0.25 ? static_cast<int>(x) : 0.75 <= static_cast<int>(x) - x ? static_cast<int>(x) - 1 : static_cast<int>(x) - 0.5 : 0.75 <= x - static_cast<int>(x) ? static_cast<int>(x) + 1 : x - static_cast<int>(x) < 0.25 ? static_cast<int>(x) : static_cast<int>(x) + 0.5;
}

#include <iostream>
#include <cmath>
 
using namespace std;
 
int main()
{
    const double x[] = {0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1};
    for (int i = 0;i < 11;++i){
        cout << x[i] << ", x ~= " << 0.5*floor(2.0*x[i]) + 0.5*floor(4*fmod(x[i],0.5)) << endl;
    }
    return 0;
}