Thursday, January 5, 2012

Replicating Game States - Working code!

Before you get too excited - no, there's no multiplayer yet (well, who would have guessed that...). There is, however, working code for undoing and redoing actions (even with multiple, branching histories), and even for replicating the state on multiple virtual machines.

The key to make such a library is that it looks like ordinary Java code from the outside, so that others can use your code without wondering why it looks so weird. Let's take a look:

//Test t = new Test(); is an attribute

//replaced angle with square brackets because of HTML markup
List[StateStamp] states = new ArrayList[StateStamp]();
History h = createHistory(createKey("test"));
try {

    states.add(h.getCurrentState()); // 0

    states.add(h.getCurrentState()); // 1

    states.add(h.getCurrentState()); // 2

    states.add(h.getCurrentState()); // 3

    states.add(h.getCurrentState()); // 4

} finally {

It's probably easy to to guess that print(); prints the current state of the test object. Besides that, the only code that looks different from normal operations is marked in bold. Most of it is only necessary for this test case: storing the previous states so that you can go back. Really mandatory is only the colored code (and the actual functional code of course, but that doesn't count.)

Of course, a history for the modifications must be created. Then, the history is set for the thread: Code running in the thread (that is, everything in this method, and nothing more) can access the history without passing it as a parameter (which would make using the library awkward) or declaring it as a static variable somewhere (which is ugly from an architecture standpoint.) Everything after this is wrapped in try/finally, so that the last method is not skipped in case of an exception: this removes the association of the history with the current Thread.

Of course, implementing setTest(), setA() etc. is different from a usual simple setter. But the point here is that using the code is as simple as shown here. The result is this:

Test@732A54F9[a=0, b=0] not in store
Test@7A6D084B[a=0, b=0] in store
Test@7A6D084B[a=1, b=0] in store
Test@7A6D084B[a=1, b=1] in store
Test@7A6D084B[a=1, b=1] not in store
Test@15301ED8[a=1, b=1] in store
Test@15301ED8[a=1, b=0] in store
Test@15301ED8[a=0, b=0] in store
Test@15301ED8[a=0, b=0] not in store

You see here how the test object runs through different states up to its deletion, then goes back to its initial state. What's not shown here, but is working, is adding another branch to this history. Of course, the actual objects have only one state at a time, but the history can be switched arbitrarily between multiple histories:

This is the actual test case I ran: I went back to the state after creating the test object, changed its a value, and then switched over to the state before, and then after, deleting the test again.

I have code for replication of states going, but it's not yet as clean as for undo: a lot of handling is necessary for the network, which should be transparent to the user. I'll show it once I'm finished, but in the meantime you can look here.

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