Holy shit rewind.

src/bin/flappy/main.rs

@@ -31,6 +31,8 @@ fn main() {
                 (
                     // Only flap when we press the space key
                     flap.run_if(input_just_pressed(KeyCode::Space)),
+                    // Rewinding systems
+                    record.run_if(any_component_changed::<Transform>),
                 )
                     .run_if(in_state(PlayerState::Alive)),
                 // Rewinding systems
@@ -51,6 +53,13 @@ enum PlayerState {
     Dead,
 }
 
+// A tape tracking the bird's state every frame
+#[derive(Component, Default)]
+struct Tape {
+    translations: Vec<Vec3>,
+    rotations: Vec<Quat>,
+}
+
 fn init_bird(
     mut commands: Commands,
     server: Res<AssetServer>,
@@ -74,7 +83,9 @@ fn init_bird(
 
     let force = ExternalForce::default().with_persistence(false);
 
-    commands.spawn((name, mesh, material, mass, t, Bird, force));
+    let tape = Tape::default();
+
+    commands.spawn((name, mesh, material, mass, t, Bird, force, tape));
 }
 
 fn init_ui(
@@ -133,17 +144,33 @@ fn toggle_rewind(keycode: Res<ButtonInput<KeyCode>>, mut next: ResMut<NextState<
     }
 }
 
+fn record(
+    #[cfg(debug_assertions)] state: Res<State<PlayerState>>,
+    mut birds: Query<(&Transform, &mut Tape), With<Bird>>,
+) {
+    debug_assert!(
+        matches!(state.get(), PlayerState::Alive),
+        "Only record in the alive state"
+    );
+
+    birds.iter_mut().for_each(|(transform, mut tape)| {
+        tape.translations.push(transform.translation);
+        tape.rotations.push(transform.rotation);
+    });
+}
+
 fn rewind(
     #[cfg(debug_assertions)] state: Res<State<PlayerState>>,
-    birds: Query<&Transform, With<Bird>>,
+    mut birds: Query<(&mut Transform, &mut Tape), With<Bird>>,
 ) {
     debug_assert!(
         matches!(state.get(), PlayerState::Rewind),
         "Only rewind in the rewinding state"
     );
 
-    birds.iter().for_each(|bird| {
-        info!("Position: {:?}", *bird);
+    birds.iter_mut().for_each(|(mut transform, mut tape)| {
+        transform.translation = tape.translations.pop().unwrap();
+        transform.rotation = tape.rotations.pop().unwrap();
     });
 }