Make combat reaction state rollback-safe (#34)

## Summary

- replace the per-fighter received-attack inbox with a typed rollback-registered pending-hit resource
- split combat reaction resolution into its own attack module
- remap entity references during rollback and include pending hits in debug save states

Fixes #21

Reviewed-on: #34
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/attack/reaction.rs

@@ -0,0 +1,145 @@
+use bevy::prelude::*;
+use bevy_ggrs::prelude::GgrsSchedule;
+use fixed_math::SinCos;
+
+use super::*;
+use crate::{
+    fighter::{
+        Fighter, FighterFacingDirection, FighterHitboxes, FighterHitstop,
+        FighterPendingHitReaction, FighterStaleMoveQueue, errors::FighterErrors,
+        manifest::FighterManifest,
+    },
+    game_settings::GameSettings,
+    math::{int::FGi32, vec::FGVec2},
+    player::Player,
+    schedule::GameplaySet,
+    scripting::FighterAttackScript,
+};
+
+pub fn resolve_fighter_hits(
+    mut pending_hits: ResMut<FighterPendingHits>,
+    mut attacker: Query<(
+        &FighterStaleMoveQueue,
+        &FighterFacingDirection,
+        &FighterHitboxes,
+        &Player,
+        &mut FighterHitstop,
+    )>,
+    mut receiver: Query<(&mut FighterDamage, &Fighter, &mut FighterPendingHitReaction)>,
+    attack_scripts: Res<Assets<FighterAttackScript>>,
+    game_settings: Res<GameSettings>,
+    fighter_manifests: Res<Assets<FighterManifest>>,
+) -> Result {
+    let hits = std::mem::take(&mut pending_hits.0);
+    let mut hit_offset = 0;
+
+    while hit_offset < hits.len() {
+        let defender = hits[hit_offset].defender;
+        let next_defender = hits[hit_offset..]
+            .iter()
+            .position(|hit| hit.defender != defender)
+            .map_or(hits.len(), |offset| hit_offset + offset);
+
+        let Ok((mut receiver_damage, fighter, mut pending_reaction)) = receiver.get_mut(defender)
+        else {
+            hit_offset = next_defender;
+            continue;
+        };
+
+        let damage_data = hits[hit_offset..next_defender]
+            .iter()
+            .filter_map(|hit| {
+                let Ok((stale_move_queue, facing_dir, _hitboxes, player, _)) =
+                    attacker.get(hit.attacker)
+                else {
+                    return None;
+                };
+                let attack_script = attack_scripts.get(&hit.attack_script)?;
+                let hitbox = attack_script.hitboxes.get(hit.hitbox_index)?;
+                let damage_received = stale_move_queue
+                    .get_attack_damage_multiplier(hit.attack_kind)
+                    * hitbox.damage.0;
+
+                Some((
+                    hit.attacker,
+                    damage_received,
+                    hitbox,
+                    matches!(facing_dir, FighterFacingDirection::Left),
+                    player.handle,
+                ))
+            })
+            .collect::<Vec<_>>();
+
+        let total_damage_received_this_frame = damage_data
+            .iter()
+            .map(|(_, damage, ..)| damage)
+            .sum::<FGi32>();
+        let fighter_manifest = fighter_manifests
+            .get(&fighter.manifest)
+            .ok_or(FighterErrors::FighterManifestNotFound)?;
+        let mut reactions = Vec::with_capacity(damage_data.len());
+
+        for (attacker_entity, attack_damage, hitbox, facing_left, attacker_slot) in damage_data {
+            let knockback = hitbox.calculate_knockback(
+                &fighter_manifest.attributes,
+                *receiver_damage,
+                FighterDamage(total_damage_received_this_frame),
+                false,
+                false,
+            );
+            let angle = match &hitbox.angle {
+                AttackAngle::Normal(angle) => *angle,
+                AttackAngle::Sakurai => FGi32::lit("32.0"),
+            };
+            let launch_angle = if facing_left {
+                FGi32::lit("180.0") - angle
+            } else {
+                angle
+            };
+            let (sin, cos) = launch_angle.sin_cos();
+            let launch_direction = FGVec2::new(cos, sin);
+            let hitstop = HitStop::calculate(FighterDamage(attack_damage), false);
+            reactions.push((
+                attacker_entity,
+                attacker_slot,
+                FighterHitReaction {
+                    launch_velocity: launch_direction * knockback.get_launch_speed(&game_settings),
+                    hitstun_frames: knockback.get_hitstun_frames(&game_settings),
+                    hitstop_frames: hitstop,
+                    kind: HitReactionKind::Flinch,
+                },
+            ));
+        }
+
+        receiver_damage.0 += total_damage_received_this_frame;
+
+        for (attacker_entity, _, reaction) in &reactions {
+            let Ok((.., mut hitstop)) = attacker.get_mut(*attacker_entity) else {
+                continue;
+            };
+            hitstop.remaining_frames = reaction.hitstop_frames.max(hitstop.remaining_frames);
+        }
+
+        pending_reaction.0 = reactions
+            .into_iter()
+            .min_by(|(_, port_a, _), (_, port_b, _)| port_a.cmp(port_b))
+            .map(|(_, _, reaction)| reaction);
+
+        hit_offset = next_defender;
+    }
+
+    Ok(())
+}
+
+pub struct FighterReactionPlugin;
+
+impl Plugin for FighterReactionPlugin {
+    fn build(&self, app: &mut App) {
+        app.add_systems(
+            GgrsSchedule,
+            resolve_fighter_hits
+                .in_set(GameplaySet::Attacks)
+                .after(super::intersect_attacks_with_hurtboxes),
+        );
+    }
+}