//Constants
float acceleration, deceleration, maxSpeed;
float V0, G, R, F;

void update(float delta)
{
    int prevDir = signum(velocity.x);
    int newDir = 0;

    if(leftPressed != rightPressed) //if only one of them is pressed
        newDir = leftPressed ? -1 : 1;

    velocity.x -= prevDir * deceleration * delta;

    if(signum(player.velX) != prevDir)
        velocity.x = 0;

    if(abs(velocity.x) < maxSpeed || prevDir != newDir)
    {
        velocity.x += newDir * acceleration * delta;

        if(abs(player.velX) > maxSpeed && prevDir == newDir)
            velocity.x = maxSpeed * prevDir;
    }

    if(jumpPressed)
    {
        if(onGround)
            velocity.y += V0 + min(abs(velocity.x) / maxSpeed, 1) * R;
        velocity.y += F * delta;
    }

    velocity.y -= G * delta;
    position += velocity * delta;
}