Implement Melee-style knockback stacking (#97)

## Summary

- stack residual airborne knockback per axis after the configured launch-frame threshold
- apply DI after combining the launch vectors while retaining the second hit's hitstun
- preserve and decay residual knockback after hitstun
- make stacking state rollback-safe and include it in debug save states
- clear launch state on landing, ledge interactions, KO, and respawn

## Testing

- `cargo check --tests`
- `cargo test` (159 tests passed)
- `git diff --check`

## Asset configuration

For Melee timing, `knockback_stacking_window` should be set to `10`. The local ignored asset package has been updated accordingly.

Reviewed-on: #97
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/debug_tools/save_state.rs

@@ -9,8 +9,9 @@ use crate::{
         FighterTranslation, FighterVelocity,
         animation::FighterAnimationFrame,
         attack::{
-            FighterDamage, FighterKnockbackVelocity, FighterPendingHitReaction, FighterPendingHits,
-            FighterSolvedHurtboxes, FighterStaleMoveQueue,
+            FighterDamage, FighterKnockbackState, FighterKnockbackVelocity,
+            FighterPendingHitReaction, FighterPendingHits, FighterSolvedHurtboxes,
+            FighterStaleMoveQueue,
         },
         baked_animation::FighterBoneMatrices,
         ecb::FighterDesiredECB,
@@ -55,6 +56,7 @@ struct FighterCheckpoint {
     hitstop: FighterHitstop,
     hitstop_shake: FighterHitstopShake,
     knockback_velocity: FighterKnockbackVelocity,
+    knockback_state: FighterKnockbackState,
     stale_moves: FighterStaleMoveQueue,
     hitboxes: FighterHitboxes,
     solved_hurtboxes: FighterSolvedHurtboxes,
@@ -160,6 +162,7 @@ pub(super) fn save_or_restore(world: &mut World) {
         entity.insert(saved.hitstop);
         entity.insert(saved.hitstop_shake);
         entity.insert(saved.knockback_velocity);
+        entity.insert(saved.knockback_state);
         entity.insert(saved.stale_moves.clone());
         entity.insert(saved.hitboxes.clone());
         entity.insert(saved.solved_hurtboxes.clone());
@@ -199,6 +202,7 @@ fn capture_fighter(world: &World, entity: Entity) -> Option<FighterCheckpoint> {
         hitstop: *fighter.get::<FighterHitstop>()?,
         hitstop_shake: *fighter.get::<FighterHitstopShake>()?,
         knockback_velocity: *fighter.get::<FighterKnockbackVelocity>()?,
+        knockback_state: *fighter.get::<FighterKnockbackState>()?,
         stale_moves: fighter.get::<FighterStaleMoveQueue>()?.clone(),
         hitboxes: fighter.get::<FighterHitboxes>()?.clone(),
         solved_hurtboxes: fighter.get::<FighterSolvedHurtboxes>()?.clone(),

src/fighter/attack/directional_influence.rs

@@ -15,6 +15,16 @@ pub fn apply_directional_influence(
     let (sin, cos) = attack_angle.sin_cos();
     let base_launch_direction = FGVec2::new(cos, sin);
 
+    apply_directional_influence_to_direction(receiver_stick, base_launch_direction, di_max_angle)
+}
+
+/// Applies DI to an already-computed launch direction. This is used after
+/// knockback stacking, where the resulting angle may differ from either hit.
+pub fn apply_directional_influence_to_direction(
+    receiver_stick: FGVec2,
+    base_launch_direction: FGVec2,
+    di_max_angle: FGi32,
+) -> FGVec2 {
     let Some(base_launch_direction) = base_launch_direction.normalize() else {
         return FGVec2::ZERO;
     };
@@ -30,14 +40,20 @@ pub fn apply_directional_influence(
         -di_max_angle.to_fixed::<FGWide>(),
         di_max_angle.to_fixed::<FGWide>(),
     );
-    let (sin, cos) = (attack_angle + angle_change.to_fixed::<FGi32>()).sin_cos();
-    FGVec2::new(cos, sin).normalize_or_zero()
+    let (sin, cos) = angle_change.to_fixed::<FGi32>().sin_cos();
+    FGVec2::new(
+        base_launch_direction.x * cos - base_launch_direction.y * sin,
+        base_launch_direction.x * sin + base_launch_direction.y * cos,
+    )
+    .normalize_or_zero()
 }
 
 #[cfg(test)]
 mod test {
     use crate::{
-        fighter::attack::directional_influence::apply_directional_influence,
+        fighter::attack::directional_influence::{
+            apply_directional_influence, apply_directional_influence_to_direction,
+        },
         math::{int::FGi32, vec::FGVec2},
     };
 
@@ -66,4 +82,17 @@ mod test {
         assert!(di.x > FGi32::ZERO);
         assert!(di.y < FGi32::ZERO);
     }
+
+    #[test]
+    fn vector_di_preserves_a_stacked_direction_without_input() {
+        let direction = FGVec2::lit("-0.6", "0.8");
+        let result = apply_directional_influence_to_direction(
+            FGVec2::ZERO,
+            direction,
+            FGi32::lit("18.0"),
+        );
+        const EPSILON: FGi32 = FGi32::from_bits(6);
+        assert!(result.x.abs_diff(direction.x) < EPSILON);
+        assert!(result.y.abs_diff(direction.y) < EPSILON);
+    }
 }

src/fighter/attack/mod.rs

@@ -33,6 +33,75 @@ use crate::{
 #[derive(Deref, Component, Default, Hash, Debug, Copy, Clone)]
 pub struct FighterKnockbackVelocity(pub FGVec2);
 
+/// Lifecycle state for the currently active launch velocity.
+///
+/// This counts frames where knockback was actually applied to movement; hitstop
+/// therefore does not advance it. Melee uses this age to decide whether a
+/// subsequent airborne hit replaces or stacks with the previous launch.
+#[derive(Component, Default, Hash, Debug, Copy, Clone)]
+pub struct FighterKnockbackState {
+    pub frames_since_launch: u32,
+}
+
+impl FighterKnockbackState {
+    pub fn restart(&mut self) {
+        self.frames_since_launch = 0;
+    }
+
+    pub fn clear(&mut self) {
+        self.frames_since_launch = 0;
+    }
+
+    pub fn advance(&mut self) {
+        self.frames_since_launch = self.frames_since_launch.saturating_add(1);
+    }
+
+    pub fn can_stack(&self, minimum_frames: u32) -> bool {
+        self.frames_since_launch >= minimum_frames
+    }
+}
+
+fn stack_knockback_component(previous: FGi32, new: FGi32) -> FGi32 {
+    if previous.is_zero() {
+        return new;
+    }
+    if new.is_zero() {
+        return previous;
+    }
+
+    if previous.signum() != new.signum() {
+        previous + new
+    } else if previous.abs() > new.abs() {
+        previous
+    } else {
+        new
+    }
+}
+
+/// Applies Melee's knockback-stacking rule independently to both axes.
+pub fn stack_knockback(previous: FGVec2, new: FGVec2) -> FGVec2 {
+    FGVec2::new(
+        stack_knockback_component(previous.x, new.x),
+        stack_knockback_component(previous.y, new.y),
+    )
+}
+
+pub fn decay_knockback(
+    velocity: &mut FighterKnockbackVelocity,
+    state: &mut FighterKnockbackState,
+    decay_per_frame: FGi32,
+) {
+    let speed = velocity.length();
+    let next_speed = speed.saturating_sub(decay_per_frame).max(FGi32::ZERO);
+    velocity.0 = velocity.normalize_or_zero() * next_speed;
+
+    if next_speed.is_zero() {
+        state.clear();
+    } else {
+        state.advance();
+    }
+}
+
 #[derive(Debug, Default, Clone, Copy, Serialize, Deserialize, Component, Hash, PartialEq, Eq, PartialOrd, Ord)]
 pub struct FighterDamage(FGi32);
 
@@ -73,6 +142,53 @@ pub struct FighterLastAttackDebug {
     pub trajectory_points: Vec<Vec3>,
 }
 
+#[cfg(test)]
+mod knockback_stacking_tests {
+    use super::*;
+
+    #[test]
+    fn same_direction_keeps_the_stronger_component() {
+        assert_eq!(
+            stack_knockback(FGVec2::lit("3", "8"), FGVec2::lit("5", "4")),
+            FGVec2::lit("5", "8")
+        );
+    }
+
+    #[test]
+    fn opposing_components_add_independently() {
+        assert_eq!(
+            stack_knockback(FGVec2::lit("3", "8"), FGVec2::lit("-5", "-3")),
+            FGVec2::lit("-2", "5")
+        );
+    }
+
+    #[test]
+    fn opposing_components_can_cancel() {
+        assert_eq!(
+            stack_knockback(FGVec2::lit("3", "-4"), FGVec2::lit("-3", "4")),
+            FGVec2::ZERO
+        );
+    }
+
+    #[test]
+    fn zero_components_preserve_the_nonzero_value() {
+        assert_eq!(
+            stack_knockback(FGVec2::lit("0", "2"), FGVec2::lit("-3", "0")),
+            FGVec2::lit("-3", "2")
+        );
+    }
+
+    #[test]
+    fn stacking_threshold_is_inclusive() {
+        let mut state = FighterKnockbackState {
+            frames_since_launch: 9,
+        };
+        assert!(!state.can_stack(10));
+        state.advance();
+        assert!(state.can_stack(10));
+    }
+}
+
 #[derive(Debug, Default, Clone, Deref, DerefMut, Hash, PartialEq, PartialOrd)]
 pub struct HitStop(u32);
 

src/fighter/mod.rs

@@ -41,8 +41,8 @@ use crate::{
     fighter::{
         attack::{
             AttackKind, FighterAttackPlugin, FighterAttackScriptAssets, FighterDamage,
-            FighterKnockbackVelocity, FighterPendingHitReaction, FighterSolvedHurtboxes,
-            FighterStaleMoveQueue,
+            FighterKnockbackState, FighterKnockbackVelocity, FighterPendingHitReaction,
+            FighterSolvedHurtboxes, FighterStaleMoveQueue,
         },
         baked_animation::FighterBoneIndexMap,
         ecb::{FighterDesiredECB, FighterECBBoneIndices},
@@ -212,6 +212,7 @@ pub struct FighterPendingRespawn(bool);
     FighterHitstop,
     FighterLedgeInformation,
     FighterKnockbackVelocity,
+    FighterKnockbackState,
     FighterTranslation,
     FighterInvulnerabilityState,
     FighterInvulnerabilityVisualState
@@ -330,6 +331,8 @@ impl Plugin for FighterPlugin {
             .checksum_component_with_hash::<FighterInvulnerabilityState>()
             .rollback_component_with_copy::<FighterKnockbackVelocity>()
             .checksum_component_with_hash::<FighterKnockbackVelocity>()
+            .rollback_component_with_copy::<FighterKnockbackState>()
+            .checksum_component_with_hash::<FighterKnockbackState>()
             .rollback_component_with_clone::<baked_animation::FighterBoneMatrices>()
             .checksum_component_with_hash::<baked_animation::FighterBoneMatrices>()
             // Debug views.

src/fighter/state/air.rs

@@ -93,6 +93,19 @@ pub fn apply_air_motion(
     translation.0 += velocity.0;
 }
 
+/// Applies ordinary and launch velocity together, then advances launch decay.
+/// Hitstop skips state updates entirely, so neither decay nor launch age moves
+/// while the fighter is frozen.
+pub fn apply_air_motion_with_knockback(ctx: &mut crate::fighter::state::UpdateContext) {
+    let combined_velocity = FighterVelocity(ctx.velocity.0 + ctx.knockback_velocity.0);
+    apply_air_motion(&combined_velocity, ctx.translation, ctx.prev_translation);
+    attack::decay_knockback(
+        ctx.knockback_velocity,
+        ctx.knockback_state,
+        ctx.game_settings.fighter_common.knockback_decay_per_frame,
+    );
+}
+
 pub fn update_standard_motion(
     ctx: &mut crate::fighter::state::UpdateContext,
     airborne_common: &mut AirborneStateCommon,
@@ -109,7 +122,7 @@ pub fn update_standard_motion(
         airborne_common.is_fast_falling,
     );
     apply_air_drift(ctx.velocity, ctx.fighter_attributes, ctx.input);
-    apply_air_motion(ctx.velocity, ctx.translation, ctx.prev_translation);
+    apply_air_motion_with_knockback(ctx);
 }
 
 pub fn air_interrupts_common(
@@ -141,6 +154,8 @@ pub fn collision_interrupt(
     *ctx.desired_ecb = crate::fighter::ecb::FighterDesiredECB::from(*ctx.ecb);
     ctx.presentation_snap.0 = true;
     ctx.surface.land(res.line_id, ctx.ledge_info);
+    ctx.knockback_velocity.0 = FGVec2::ZERO;
+    ctx.knockback_state.clear();
     Some(FighterState::Land(LandingState::create(landing_type)))
 }
 

src/fighter/state/air_dodge.rs

@@ -33,11 +33,7 @@ impl FighterStateImpl for AirDodgeState {
         }
         state_context.velocity.0 *= state_context.fighter_attributes.air_dodge_decay;
 
-        air::apply_air_motion(
-            state_context.velocity,
-            state_context.translation,
-            state_context.prev_translation,
-        );
+        air::apply_air_motion_with_knockback(state_context);
     }
 
     fn check_collision_interrupt(
@@ -59,6 +55,8 @@ impl FighterStateImpl for AirDodgeState {
                         state_context.ecb.bottom = point_in_segment - state_context.translation.0;
                         state_context.ecb.rebase_onto_bottom();
                         state_context.surface.land(StageLineID { polygon: poly_idx, segment: segment_idx }, state_context.ledge_info);
+                        state_context.knockback_velocity.0 = FGVec2::ZERO;
+                        state_context.knockback_state.clear();
                         return Some(FighterState::Land(LandingState::create(LandingType::Special)));
                     }
                 }

src/fighter/state/ground.rs

@@ -260,6 +260,7 @@ pub fn collision_interrupt(ctx: &mut CollisionContext, can_walk_off: bool) -> Op
 #[cfg(test)]
 mod tests {
     use super::*;
+    use crate::fighter::attack::{FighterKnockbackState, FighterKnockbackVelocity};
     use crate::fighter::ecb::{FighterDesiredECB, FighterPreviousECB};
     use crate::fighter::{
         FighterECB, FighterLedgeInformation, FighterPreviousTranslation, FighterSurfaceState,
@@ -388,6 +389,8 @@ mod tests {
         let mut translation = FighterTranslation(FGVec2::lit("11", "0"));
         let previous_translation = FighterPreviousTranslation(translation.0);
         let mut velocity = FighterVelocity::default();
+        let mut knockback_velocity = FighterKnockbackVelocity::default();
+        let mut knockback_state = FighterKnockbackState::default();
         let mut ledge_info = FighterLedgeInformation::default();
         let mut ecb = FighterECB {
             bottom: FGVec2::lit("0.0", "-0.5"),
@@ -403,6 +406,8 @@ mod tests {
             prev_translation: &previous_translation,
             input: &FighterInput::default(),
             velocity: &mut velocity,
+            knockback_velocity: &mut knockback_velocity,
+            knockback_state: &mut knockback_state,
             surface: &mut surface,
             ecb: &mut ecb,
             desired_ecb: &mut desired_ecb,
@@ -453,6 +458,8 @@ mod tests {
         let mut translation = FighterTranslation(FGVec2::lit("11", "0"));
         let previous_translation = FighterPreviousTranslation(translation.0);
         let mut velocity = FighterVelocity::default();
+        let mut knockback_velocity = FighterKnockbackVelocity::default();
+        let mut knockback_state = FighterKnockbackState::default();
         let mut ecb = FighterECB {
             bottom: FGVec2::lit("0.0", "-0.5"),
             top: FGVec2::lit("0.0", "0.5"),
@@ -468,6 +475,8 @@ mod tests {
             prev_translation: &previous_translation,
             input: &FighterInput::default(),
             velocity: &mut velocity,
+            knockback_velocity: &mut knockback_velocity,
+            knockback_state: &mut knockback_state,
             surface: &mut surface,
             ecb: &mut ecb,
             desired_ecb: &mut desired_ecb,

src/fighter/state/hitstun.rs

@@ -1,6 +1,6 @@
 use crate::{
     fighter::{
-        FighterVelocity, animation::AnimKind, attack::{self, FighterHitReaction}, state::{
+        animation::AnimKind, attack::{self, FighterHitReaction}, state::{
             CollisionContext, EnterContext, ExitContext, FighterState, FighterStateImpl,
             InterruptContext, UpdateContext,
             air::{self, AirborneStateCommon},
@@ -9,7 +9,7 @@ use crate::{
             land::LandingType,
             wait::WaitState,
         },
-    }, math::{int::FGi32, vec::FGVec2},
+    }, math::vec::FGVec2,
 };
 
 #[derive(Clone, Debug, Hash)]
@@ -23,14 +23,20 @@ pub struct HitstunState {
     pub hit_reaction: FighterHitReaction,
     pub hitstop_kind: HitstopStateKind,
     knockback_applied: bool,
+    stack_previous_knockback: bool,
 }
 
 impl HitstunState {
-    pub fn new(hit_reaction: FighterHitReaction, hitstop_kind: HitstopStateKind) -> Self {
+    pub fn new(
+        hit_reaction: FighterHitReaction,
+        hitstop_kind: HitstopStateKind,
+        stack_previous_knockback: bool,
+    ) -> Self {
         Self {
             hit_reaction,
             hitstop_kind,
             knockback_applied: false,
+            stack_previous_knockback,
         }
     }
 }
@@ -60,7 +66,12 @@ impl FighterStateImpl for HitstunState {
         state_context.velocity.0 = FGVec2::ZERO;
     }
 
-    fn on_exit(&mut self, _state_context: &mut ExitContext) {}
+    fn on_exit(&mut self, state_context: &mut ExitContext) {
+        if matches!(self.hitstop_kind, HitstopStateKind::Grounded) {
+            state_context.knockback_velocity.0 = FGVec2::ZERO;
+            state_context.knockback_state.clear();
+        }
+    }
 
     fn name(&self) -> String {
         let hitstop_kind = match self.hitstop_kind {
@@ -78,13 +89,27 @@ impl FighterStateImpl for HitstunState {
 
     fn update(&mut self, ctx: &mut UpdateContext) {
         if !self.knockback_applied {
-            let launch_direction = attack::directional_influence::apply_directional_influence(
-                ctx.input.get_last_frame().movement,
+            let new_launch_direction = attack::directional_influence::apply_directional_influence(
+                FGVec2::ZERO,
                 self.hit_reaction.launch_angle,
                 ctx.game_settings.fighter_common.di_max_angle,
             );
+            let new_knockback = new_launch_direction * self.hit_reaction.launch_velocity;
+            let combined_knockback = if self.stack_previous_knockback {
+                attack::stack_knockback(ctx.knockback_velocity.0, new_knockback)
+            } else {
+                new_knockback
+            };
+            let launch_speed = combined_knockback.length();
+            let launch_direction =
+                attack::directional_influence::apply_directional_influence_to_direction(
+                    ctx.input.get_last_frame().movement,
+                    combined_knockback.normalize_or_zero(),
+                    ctx.game_settings.fighter_common.di_max_angle,
+                );
 
-            ctx.knockback_velocity.0 = launch_direction * self.hit_reaction.launch_velocity;
+            ctx.knockback_velocity.0 = launch_direction * launch_speed;
+            ctx.knockback_state.restart();
             self.knockback_applied = true;
         }
 
@@ -92,17 +117,17 @@ impl FighterStateImpl for HitstunState {
             HitstopStateKind::Grounded => {}
             HitstopStateKind::Airborne => {
                 air::integrate_gravity(ctx.velocity, ctx.fighter_attributes, false);
-                let vel_with_kb = FighterVelocity(ctx.velocity.0 + ctx.knockback_velocity.0);
-                air::apply_air_motion(&vel_with_kb, ctx.translation, ctx.prev_translation);
+                air::apply_air_motion_with_knockback(ctx);
             }
         }
 
-        let speed = ctx.knockback_velocity.length();
-        let next_speed = speed
-            .saturating_sub(ctx.game_settings.fighter_common.knockback_decay_per_frame)
-            .max(FGi32::ZERO);
-
-        ctx.knockback_velocity.0 = ctx.knockback_velocity.normalize_or_zero() * next_speed;
+        if matches!(self.hitstop_kind, HitstopStateKind::Grounded) {
+            attack::decay_knockback(
+                ctx.knockback_velocity,
+                ctx.knockback_state,
+                ctx.game_settings.fighter_common.knockback_decay_per_frame,
+            );
+        }
     }
 
     fn check_collision_interrupt(

src/fighter/state/ledge_action.rs

@@ -84,6 +84,7 @@ impl FighterStateImpl for LedgeActionState {
 
         state_context.velocity.0 = FGVec2::ZERO;
         state_context.knockback_velocity.0 = FGVec2::ZERO;
+        state_context.knockback_state.clear();
         let Some(ledge) = state_context.stage_collision.find_ledge_by_vertex(self.ledge_id) else {
             return;
         };

src/fighter/state/ledge_grab.rs

@@ -36,6 +36,8 @@ impl FighterStateImpl for LedgeGrabState {
 
     fn on_enter(&mut self, state_context: &mut EnterContext) {
         state_context.play_animation(AnimKind::LedgeGrab, false);
+        state_context.knockback_velocity.0 = crate::math::vec::FGVec2::ZERO;
+        state_context.knockback_state.clear();
 
         *state_context.vulnerability = FighterInvulnerabilityState::Intangible {
             frames_remaining: None,

src/fighter/state/ledge_wait.rs

@@ -65,6 +65,7 @@ impl FighterStateImpl for LedgeWaitState {
     fn on_enter(&mut self, state_context: &mut EnterContext) {
         state_context.velocity.0 = FGVec2::ZERO;
         state_context.knockback_velocity.0 = FGVec2::ZERO;
+        state_context.knockback_state.clear();
         
         if state_context.ledge_info.grab_count < state_context.game_settings.fighter_common.max_ledge_grab_count {
             *state_context.vulnerability = FighterInvulnerabilityState::Intangible {

src/fighter/state/mod.rs

@@ -7,8 +7,8 @@ use bevy::prelude::*;
 
 use crate::fighter::animation::{AnimKind, FighterAnimationFrame};
 use crate::fighter::attack::{
-    FighterAttackScriptAssets, FighterKnockbackVelocity, FighterPendingHitReaction,
-    FighterStaleMoveQueue, HitStop,
+    FighterAttackScriptAssets, FighterKnockbackState, FighterKnockbackVelocity,
+    FighterPendingHitReaction, FighterStaleMoveQueue, HitStop,
 };
 use crate::fighter::attributes::FighterAttributes;
 use crate::fighter::baked_animation::{BakedFighterAnimations, FighterBoneMatrices};
@@ -167,6 +167,7 @@ pub struct EnterContext<'a> {
     pub velocity: &'a mut FighterVelocity,
     pub translation: &'a mut FighterTranslation,
     pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub stale_move_queue: &'a mut FighterStaleMoveQueue,
     pub facing_direction: &'a mut FighterFacingDirection,
     pub animation_frame: &'a mut FighterAnimationFrame,
@@ -204,6 +205,8 @@ impl EnterContext<'_> {
 pub struct ExitContext<'a> {
     pub input: &'a mut FighterInput,
     pub hitboxes: &'a mut FighterHitboxes,
+    pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub surface: &'a mut FighterSurfaceState,
     pub vulnerability: &'a mut FighterInvulnerabilityState,
 }
@@ -216,6 +219,7 @@ pub struct UpdateContext<'a> {
     pub velocity: &'a mut FighterVelocity,
     pub surface: &'a mut FighterSurfaceState,
     pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub vulnerability: &'a mut FighterInvulnerabilityState,
     pub ledge_info: &'a mut FighterLedgeInformation,
     pub game_settings: &'a GameSettings,
@@ -251,6 +255,8 @@ pub struct CollisionContext<'a> {
     pub prev_translation: &'a FighterPreviousTranslation,
     pub input: &'a FighterInput,
     pub velocity: &'a mut FighterVelocity,
+    pub knockback_velocity: &'a mut FighterKnockbackVelocity,
+    pub knockback_state: &'a mut FighterKnockbackState,
     pub surface: &'a mut FighterSurfaceState,
     pub ecb: &'a mut FighterECB,
     pub desired_ecb: &'a mut FighterDesiredECB,
@@ -335,6 +341,7 @@ pub struct FighterFrameQuery {
     pub velocity: &'static mut FighterVelocity,
     pub surface: &'static mut FighterSurfaceState,
     pub knockback_velocity: &'static mut FighterKnockbackVelocity,
+    pub knockback_state: &'static mut FighterKnockbackState,
     pub stale_move_queue: &'static mut FighterStaleMoveQueue,
     pub ecb: &'static mut FighterECB,
     pub desired_ecb: &'static mut FighterDesiredECB,
@@ -403,6 +410,7 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             hitboxes: &mut self.hitboxes,
             velocity: &mut self.velocity,
             knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             translation: &mut self.translation,
             stale_move_queue: &mut self.stale_move_queue,
             facing_direction: &mut self.facing_direction,
@@ -424,6 +432,8 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
         ExitContext {
             input: &mut self.input,
             hitboxes: &mut self.hitboxes,
+            knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             surface: &mut self.surface,
             vulnerability: &mut self.vulnerability,
         }
@@ -453,6 +463,7 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             velocity: &mut self.velocity,
             surface: &mut self.surface,
             knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             game_settings,
             animation_root_motion_delta,
             animation_root_position,
@@ -483,6 +494,8 @@ impl<'w, 's> FighterFrameQueryItem<'w, 's> {
             presentation_snap: &mut self.presentation_snap,
             input: &self.input,
             velocity: &mut self.velocity,
+            knockback_velocity: &mut self.knockback_velocity,
+            knockback_state: &mut self.knockback_state,
             surface: &mut self.surface,
             ecb: &mut self.ecb,
             desired_ecb: &mut self.desired_ecb,
@@ -655,9 +668,16 @@ pub fn state_interrupt_system(
             }
 
             info!("Entering hitstop kind: {:?}", hitstop_kind);
+            let stack_previous_knockback = !frame.surface.is_grounded()
+                && !frame.knockback_velocity.0.length_squared().is_zero()
+                && frame
+                    .knockback_state
+                    .can_stack(game_settings.fighter_common.knockback_stacking_window);
+
             Some(FighterState::Hitstun(HitstunState::new(
                 reaction,
                 hitstop_kind,
+                stack_previous_knockback,
             )))
         } else if hitstop.remaining_frames > 0 {
             None
@@ -753,6 +773,8 @@ pub fn state_collision_interrupt_system(
 
             // Clear hitstop
             hitstop.remaining_frames = 0;
+            frame.knockback_velocity.0 = FGVec2::ZERO;
+            frame.knockback_state.clear();
 
             // subtract one stock
 
@@ -846,6 +868,8 @@ mod tests {
         let input = FighterInput::default();
         let mut translation = FighterTranslation::default();
         let mut velocity = FighterVelocity::default();
+        let mut knockback_velocity = FighterKnockbackVelocity::default();
+        let mut knockback_state = FighterKnockbackState::default();
         let mut surface = FighterSurfaceState::default();
         let mut ecb = FighterECB::default();
         let mut desired_ecb = FighterDesiredECB {
@@ -865,6 +889,8 @@ mod tests {
             prev_translation: &previous_translation,
             input: &input,
             velocity: &mut velocity,
+            knockback_velocity: &mut knockback_velocity,
+            knockback_state: &mut knockback_state,
             surface: &mut surface,
             ecb: &mut ecb,
             desired_ecb: &mut desired_ecb,

src/game_settings.rs

@@ -18,6 +18,7 @@ pub struct FighterSettingsCommon {
 
     /// Ground friction applied when the fighter is above the walk speed
     pub ground_friction_over_walk_speed_multiplier: FGi32,
+    /// Minimum active launch frames between hits before airborne knockback stacks.
     pub knockback_stacking_window: u32,
     pub knockback_decay_per_frame: FGi32,