// Fixed tickrate gameloop
// Loops through Input and Update at fixed intervals, renders when possible
// Game updated at fixed times, FPS can vary, and will not affect game speed
void Engine::mainloop()
{
    sf::Clock clock;

    double previous = clock.getElapsedTime().asSeconds();
    double metricsCounter = previous;
    double accumulator = 0.0;

    uint16_t tickCounter = 0;
    uint16_t frameCounter = 0;

    while (_isRunning)
    {
        double current = clock.getElapsedTime().asSeconds();
        // calculate time since last frame
        double elapsed = current - previous;
        metricsCounter += elapsed;

        // set previous frame time as now (ready to render a now frame)
        previous = current;

        // add cycle duration to the accumulator.
        // this is the update time + render time from previous cycle
        accumulator += elapsed;

        // when 1 second has passed, reset metrics and timer
        if (metricsCounter >= 1.0)
        {
#ifdef _DEBUG
            // if we're debugging, set window title as the metrics display
            std::string title =
                " FPS: " + std::to_string(frameCounter) +
                " TickRate: " + std::to_string(tickCounter) +
                " Duration: " + std::to_string(clock.getElapsedTime().asSeconds());

            _window.setTitle(title);
#endif
            metricsCounter = 0.0;
            frameCounter = tickCounter = 0;
        }

        // while accumulator larger than current game tickrate duration, do update
        while (accumulator >= GAMETICK_DURATION_MS)
        {
            processInput();
            update(GAMETICK_DURATION_MS);
            tickCounter++;
            accumulator -= GAMETICK_DURATION_MS;
        }

        // Rendering section. Render a frame, then increment frame to FPS counter
        render();
        frameCounter++;
    }
    _window.close();
}