float colliderCalculate(COLLIDER *col1, COLLIDER *col2)
{
    //construct difference vector
    float posx = col1->pos->x - col2->pos->x;
    float posy = col1->pos->y - col2->pos->y;
    float velx = col1->vel->x - col2->vel->x;
    float vely = col1->vel->y - col2->vel->y;

    float a = pow(velx,2) + pow(vely,2); //component a of quadratic formula
    float b = 2*posx*velx + 2*posy*vely; //component b of quadratic formula
    float c = pow(posx,2)+ pow(posy,2) - pow(col1->radius + col2->radius, 2); //component c of quadratic formula
    float d = pow(b,2) - 4*a*c; //discriminant of quadratic formula

    if(d < 0) return -1; //no collision detected

    float t = (-b - sqrt(d))/(2*a); //find first collision, ignore second

    if(t >= 0 && t <= 1) return t; //collision occurs during this frame, return time collision occurs
    else return -1; //collision occurs either before or after frame, return -1 signalling no collision detected
}