Initial work on rollback and refactor

src/fighter/motion.rs

@@ -0,0 +1,35 @@
+//! Where a fighter is and how it moves. States write [`FighterVelocity`];
+//! this module is what turns that into a position.
+
+use bevy::prelude::*;
+
+#[derive(Component, Clone, Copy)]
+pub struct Grounded;
+
+#[derive(Component, Deref, DerefMut, Default, Debug, Clone, Copy)]
+pub struct FighterVelocity(pub Vec2);
+
+#[derive(Component, Deref, DerefMut, Default, Clone, Copy)]
+pub struct FighterPreviousTranslation(pub Vec2);
+
+#[derive(Component, Deref, DerefMut, Default, Clone, Copy)]
+#[require(FighterPreviousTranslation)]
+pub struct FighterTranslation(pub Vec2);
+
+pub fn integrate_gravity(_fighters: Query<&mut FighterVelocity>, _time: Res<Time<Fixed>>) {}
+
+pub fn apply_motion(
+    fighters: Query<(
+        &FighterVelocity,
+        &mut FighterTranslation,
+        &mut FighterPreviousTranslation,
+    )>,
+    _time: Res<Time<Fixed>>,
+) {
+    for (velocity, mut translation, mut prev_translation) in fighters {
+        prev_translation.0 = translation.0;
+        translation.0 += velocity.0;
+    }
+}
+
+pub fn fighter_movement() {}