How to make this game loop deterministic

Question

I am using the following game loop for my pacman clone:

long prevTime = System.currentTimeMillis();
        while (running) {
            long curTime = System.currentTimeMillis();
            float frameTime = (curTime - prevTime) / 1000f;
            prevTime = curTime;

            while (frameTime > 0.0f) {
                final float deltaTime = Math.min(frameTime, TIME_STEP);
                update(deltaTime);
                frameTime -= deltaTime;
            }

            repaint();
        }

The thing is, I don't always get the same ghost movement every time I run the game (their logic is deterministic), so it must be the game loop. I imagine it's due to the final float deltaTime = Math.min(frameTime, TIME_STEP); line. What's the best way of modifying this to perform the exact same way every time I run it? Also, any further improvements I can make?

Answer 1

What you need to do is use a fixed time step. http://gafferongames.com/game-physics/fix-your-timestep/ has some more details - it looks like you used the loop from before the 'free the physics' section. What you want is one that only updates with a fixed time step. Here's roughly what you should end up with (without spiral of death handling or interpolation):

    while (running) {
        long curTime = System.currentTimeMillis();
        float frameTime = (curTime - prevTime) / 1000f;

        while (frameTime > TIME_STEP) {
            update(TIME_STEP);
            frameTime -= TIME_STEP;
            prevTime = curTime;
        }

        repaint();
    }