Recreating bugs in cocos2d iphone - debugging

Question

I guess someone must have asked a similar question before, but here goes.
It would be useful to be able to record games so that if a bug happened during the game, the recorded play can be reused later with a fixed build to confirm if the bug is fixed or not. I am using box2d as well and from what I remember it seems as if box2d is not really deterministic, but at least being able to recreate most of the state from the first time would be OK in many cases. Recreating the same randomized values would take reinstating the same time etc I assume. Any insight?
I have been fiddling with calabash-ios with various success. I know it's possible to record plays, and playback them there later. I just assume it wouldn't recreate random values.
A quick look at box2d faq and I think box2d is deterministic enough
For the same input, and same binary, Box2D will reproduce any
simulation. Box2D does not use any random numbers nor base any
computation on random events (such as timers, etc).
However, people often want more stringent determinism. People often
want to know if Box2D can produce identical results on different
binaries and on different platforms. The answer is no. The reason for
this answer has to do with how floating point math is implemented in
many compilers and processors. I recommend reading this article if you
are curious:
http://www.yosefk.com/blog/consistency-how-to-defeat-the-purpose-of-ieee-floating-point.html

Answer 1

If you encapsulate the input state the player gives to the world each time step (eg. in a POD struct) then it's pretty straightforward to write that to a file. For example, suppose you have input state like:
struct inputStruct {
bool someButtonPressed;
bool someOtherKeyPressed;
float accelerometerZ;
... etc
};
Then you can do something like this each time step:
inputStruct currentState;
currentState.someButtonPressed = ...; // set contents from live user input
if ( recording )
fwrite( &currentState, sizeof(inputStruct), 1, file );
else if ( replaying ) {
inputStruct tmpState;
int readCount = fread( &tmpState, sizeof(inputStruct), 1, file );
if ( readCount == 1 )
currentState = tmpState; //overwrite live input
}
applyState( currentState ); // apply forces, game logic from input
world->Step( ... ); // step the Box2D world
Please excuse the C++ centric code :~) No doubt there are equivalent ways to do it with Objective-C.
This method lets you regain live control when the input from the file runs out. 'file' is a FILE* that you would have to open in the appropriate mode (rb or wb) when the level was loaded. If the bug you're chasing causes a crash, you might need to fflush after writing to make sure the input state actually gets written before crashing.
As you have noted, this is highly unlikely to work across different platforms. You should not assume that the replay file will reproduce the same result on anything other than the device that recorded it (which should be fine for debugging purposes).
As for random values, you'll need to ensure that anything using random values that may affect the Box2D world go through a deterministic random generator which is not shared with other code, and you'll need to record the seed that was used for each replay. You might like to use one of the many implementations of Mersenne Twister found at http://en.wikipedia.org/wiki/Mersenne_twister
When I say 'not shared', suppose you also use the MT algorithm to generate random directions for particles, purely for rendering purposes - you would not want to use the same generator instance for that as you do for physics-related randomizations.