Struct SyncTestSession 

pub struct SyncTestSession<T>
where
    T: Config,
{ /* private fields */ }

A session for verifying that your game logic is deterministic, without any network involvement.

How it works

Every call to advance_frame() performs the following sequence:

1. Save — the current game state is saved.
2. Advance — your game logic runs for one frame with the inputs you provided.
3. Roll back — GGRS rewinds check_distance frames and re-simulates forward, requesting a Load followed by check_distance × Advance requests.
4. Compare — the checksums produced during re-simulation are compared against those saved during the original run. A mismatch means your game is not deterministic: the same inputs from the same state produced different results on separate runs.

If a checksum mismatch is detected, advance_frame() panics — this is intentional. SyncTestSession is a development tool; a mismatch is a bug that must be fixed before shipping.

All players are local

There is no network in a sync test. You must call add_local_input() for every player handle (0 through num_players - 1) before each advance_frame() call, even in a multi-player configuration.

Limitations

• Sparse saving is incompatible with SyncTestSession: the session must save every frame to have checksums available for the full check window.
• The check_distance must be less than max_prediction_window.

Implementations

impl<T: Config> SyncTestSession<T>

pub fn add_local_input( &mut self, player_handle: PlayerHandle, input: T::Input, ) -> Result<(), GgrsError>

Registers local input for a player for the current frame. This should be successfully called for every local player before calling advance_frame(). If this is called multiple times for the same player before advancing the frame, older given inputs will be overwritten. In a sync test, all players are considered to be local, so you need to add input for all of them.

Errors

• Returns InvalidRequest when the given handle is not valid (i.e. not between 0 and num_players).

pub fn advance_frame(&mut self) -> Result<Vec<GgrsRequest<T>>, GgrsError>

In a sync test, this will advance the state by a single frame and afterwards rollback check_distance amount of frames, resimulate and compare checksums with the original states. Returns an order-sensitive Vec<GgrsRequest>. You should fulfill all requests in the exact order they are provided. Failure to do so will cause panics later.

Errors

• Returns MismatchedChecksum if checksums don’t match after resimulation.

pub fn current_frame(&self) -> Frame

Returns the current frame of a session.

pub fn num_players(&self) -> usize

Returns the number of players this session was constructed with.

pub fn max_prediction(&self) -> usize

Returns the maximum prediction window of a session.

pub fn check_distance(&self) -> usize

Returns the check distance set on creation, i.e. the length of the simulated rollbacks