Maze generator SFML

#include <SFML/Graphics.hpp>
#include <vector>
#include <stack>
using namespace sf;
using namespace std;


VideoMode operator/(VideoMode mode, float cef) {
    return { static_cast<unsigned int> (mode.width / cef),
            static_cast<unsigned int> (mode.height / cef) };
}


void eventProcess(RenderWindow& win) {
    Event e;
    while (win.pollEvent(e)) {
        if (e.type == Event::Closed) {
            win.close();
        }
    }
}


// n = maximium empty sections in a column
// m = maximum empty sections in a row
//false == empty section
auto genMaze(unsigned int n, unsigned int m) {
    srand(time(nullptr));
    vector< vector<bool> > maze(2 * n + 1, vector<bool>(2 * m + 1, true));
    maze[1][1] = false;
    stack< pair<int, int> > trail;
    trail.push({ 1, 1 });
    while (!trail.empty()) {
        auto [ci, cj] = trail.top();
        vector< pair<int, int> > next, mb = {
            { ci + 2, cj }, { ci - 2, cj },
            { ci, cj + 2 }, { ci, cj - 2 }
        };
        for (auto [i, j] : mb) {
            if (i >= 0 and j >= 0 and i < 2 * n + 1 and j < 2 * m + 1 and maze[i][j]) {
                next.emplace_back(i, j);
            }
        }
        if (next.empty()) {
            trail.pop();
            continue;
        }
        auto [i, j] = next[rand() % next.size()];
        maze[i][j] = false;
        maze[(i + ci) / 2][(j + cj) / 2] = false;
        trail.emplace(i, j);
    }
    return maze;
}



int main() {
    RenderWindow window(VideoMode::getDesktopMode() / 1.5, "Maze");

    const int N = 18, M = 30;
    const float rectWidth = (float)window.getSize().x / (2 * M + 1);
    const float rectHeight = (float)window.getSize().y / (2 * N + 1);
    auto maze = genMaze(N, M);


    Clock clock;


    while (window.isOpen()) {
        eventProcess(window);

        if (clock.getElapsedTime().asSeconds() >= 7) {
            maze = genMaze(N, M);
            clock.restart();
        }


        window.clear();


        for (int i = 0; i < 2 * N + 1; i++) {
            for (int j = 0; j < 2 * M + 1; j++) {
                RectangleShape rect({ rectWidth, rectHeight });
                rect.setPosition({ rectWidth * j, rectHeight * i });
                rect.setFillColor((maze[i][j] ? Color::White : Color::Black));
                window.draw(rect);
            }
        }


        window.display();
    }
}