Improve replay files about xmoto.js HOT 1 OPEN

Non-deterministic behaviour

The solution 2 was implemented but we had a serious issue: Box2D is non-deterministic [1, 2, 3] and some very small differences between simulations created situations where the moto crashed at random different places before finishing the level (usually after about 30 seconds on complicated levels)

[1] http://box2d.org/forum/viewtopic.php?f=3&t=5453
[2] http://www.box2d.org/forum/viewtopic.php?f=3&t=4489
[3] http://www.box2d.org/forum/viewtopic.php?f=3&t=4614

It worked each time only in one specific situation : when creating a replay on Chrome on the first try, saving the replay on its textual representation, and then passing it in params to XMoto. The replay then worked correctly but only on the first run. If the replay was restarted using "ENTER", then the moto crashed.

On Firefox it failed almost each time (except on short-simple levels of course).

The reason is quite simple : Box2D is only deterministic for "one binary, on a specific system and if the action is taken at the exact same moment of the simulation".

In our case, it could be interpreted as "in one browser, on a specific system and only for the first run".

Solution (key-steps)

We implemented the following solution : Every x steps, we save a key-steps containing the following informations of each part of the moto and the rider :

  position:
    x:              object.GetPosition().x
    y:              object.GetPosition().y
  angle:            object.GetAngle()
  linear_velocity:
    x:              object.GetLinearVelocity().x
    y:              object.GetLinearVelocity().y
  angular_velocity: object.GetAngularVelocity()

These informations are concatenated to the inputs informations of the solution 2 and when the replay is run, each part of the moto and rider are positioned manually in the world. This solution prevents moto parts to drift off their original path because of the non-deterministic behavior.

Size: Of course the replay size increases a little bit because of this trick. But it still manageable since the key-steps can be saved with low-frequency like one time every second (60 steps). The total size of the replay (input + key-steps) is about 20ko/minute.

from xmoto.js.