mention fixed timestep issues wrt replicon

REPLICON_INTEGRATION.md

@@ -1,11 +1,11 @@
 ## Integrating bevy_timewarp with bevy_replicon
 
 I'm using [bevy_replicon](https://crates.io/crates/bevy_replicon) in my game, alongside bevy_timewarp.
-You can use custom deserializers with replicon to write updates into the `ServerSnapshot` buffer like this:
+You can use custom deserializers with replicon to write updates into the `ServerSnapshot` buffer.
 
 ### Custom timewarp deserializer example
 
-This is for [bevy_xpbd](https://crates.io/crates/bevy_xpbd_2d)'s `Rotation`` component:
+This is for [bevy_xpbd](https://crates.io/crates/bevy_xpbd_2d)'s `Rotation` component:
 
 ```rust
 pub fn deserialize_rotation_timewarp(
@@ -35,3 +35,41 @@ pub fn serialize_rotation(
     bincode::serialize_into(&mut cursor, &comp.cos())
 }
 ```
+
+### Fixed Timestep
+
+You'll need to set replicon's TickPolicy to manual, and advance the replicon tick on the server in line with your tick counter in your fixed timestep.
+
+```rust
+    #[cfg(feature = "is_server")]
+    {
+        app.add_plugins(
+            ReplicationPlugins
+                .build()
+                .disable::<ClientPlugin>()
+                .set(ServerPlugin::new(TickPolicy::Manual)),
+        );
+        // we set the tick policy to manual, and want to do sending AFTER physics every fixed step,
+        // so clients get the updated-by-physics-systems versions of pos, vel, etc.
+        app.configure_set(
+            FixedUpdate,
+            ServerSet::Send.after(SpacepitSet::AfterPhysics),
+        );
+    }
+```
+
+And scheduled in your fixed timestep:
+
+```rust
+pub fn frame_inc_and_replicon_tick_sync(
+    mut game_clock: ResMut<GameClock>, // your own tick counter
+    mut replicon_tick: ResMut<bevy_replicon::prelude::RepliconTick>,
+) {
+    // advance your tick and replicon's tick in lockstep
+    game_clock.advance(1);
+    let delta = game_clock.frame().saturating_sub(replicon_tick.get());
+    replicon_tick.increment_by(delta);
+}
+```
+
+With this setup, the `RepliconTick` you receive in the deserialize functions will match up with your game's fixed timestep tick.

src/resources.rs

@@ -58,6 +58,13 @@ impl TimewarpConfig {
     pub fn rollback_window(&self) -> FrameNumber {
         self.rollback_window
     }
+    pub fn is_within_rollback_window(
+        &self,
+        current_frame: FrameNumber,
+        target_frame: FrameNumber,
+    ) -> bool {
+        target_frame > current_frame - self.rollback_window
+    }
 }
 
 /// Updated whenever we perform a rollback