// was:

    if (this->simulator != 0) {
        int deltaTime = min(max(glutGet(GLUT_ELAPSED_TIME) - this->previousTicks, 0), 1000);
        this->accumulator += deltaTime / 1000.0f;
        float timeStep = 1.0f / 62.5f;
        while (this->accumulator > timeStep) {
            this->simulator->step(timeStep);
            this->accumulator -= timeStep;
        }
        Drawer::getInstance()->drawSimulation();

        this->previousTicks = glutGet(GLUT_ELAPSED_TIME);
    }

// should be:

    if (this->simulator != 0) {
        int currentTicks = glutGet(GLUT_ELAPSED_TIME);
        int deltaTime = min(max(currentTicks - this->previousTicks, 0), 1000);

        this->accumulator += deltaTime / 1000.0f;
        float timeStep = 1.0f / 62.5f;
        while (this->accumulator > timeStep) {
            this->simulator->step(timeStep);
            this->accumulator -= timeStep;
        }
        Drawer::getInstance()->drawSimulation();

        this->previousTicks = currentTicks;
    }