Best way to implement game playback?

Question

I'm creating a grid based game in Java and I want to implement game recording and playback. I'm not sure how to do this, although I've considered 2 ideas:

1. Several times every second, I'd record the entire game state. To play it back, I write a renderer to read the states and try to create a visual representation. With this, however, I'd likely have a large save file, and any playback attempts would likely have noticeable lag.

2. I could also write every key press and mouse click into the save file. This would give me a smaller file, and could play back with less lag. However, the slightest error at the start of the game (For example, shooting 1 millisecond later) would result in a vastly different game state several minutes into the game.

What, then, is the best way to implement game playback?

Edit- I'm not sure exactly how deterministic my game is, so I'm not sure the entire game can be pieced together exactly by recording only keystrokes and mouse clicks.

Answer 1

A good playback mechanism is not something that can be simply added to a game without major difiiculties. The best would be do design the game infrastructure with it in mind. The command pattern can be used to achieve such a game infrastructure.

For example:

public interface Command{
    void execute();
}
public class MoveRightCommand implements Command {
   private Grid theGrid;
   private Player thePlayer;

   public MoveRightCommand(Player player, Grid grid){
        this.theGrid = grid;
        this.thePlayer = player;
       }

   public void execute(){
     player.modifyPosition(0, 1, 0, 0);
   } 
}

And then the command can be pushed in an execution queue both when the user presses a keyboard button, moves the mouse or without a trigger with the playback mechanism. The command object can have a time-stamp value (relative to the beginning of the playback) for precise playback...

Answer 2

Shawn Hargreaves had a recent post on his blog about how they implemented replay in MotoGP. Goes over several different approaches and their pros and cons.

http://blogs.msdn.com/shawnhar/archive/2009/03/20/motogp-replays.aspx