Allow airborne fighters to pass through platforms while holding down (#65)

## Summary
- Ignore soft-platform collision while an airborne fighter holds down.
- Preserve existing momentum and collision with solid floors.
- Add regression coverage for soft and solid platform behavior.

Reviewed-on: #65
Co-authored-by: Álex Román Núñez <eirexe@eirteam.moe>

src/fighter/collision.rs

@@ -4,7 +4,7 @@ use crate::fighter::ecb::FighterPreviousECB;
 use crate::fighter::{FighterECB, FighterPreviousTranslation, FighterTranslation};
 use crate::math::segment::FGSegment2d;
 use crate::math::vec::FGVec2;
-use crate::stage::line::{StageCollision, StageLineID};
+use crate::stage::line::{StageCollision, StageLineID, StagePolyType};
 /*
 pub fn collide_fighter_with_scene(
     fighters: Query<(&FighterECB, &mut FighterVelocity, &mut FighterTranslation)>,
@@ -45,6 +45,7 @@ pub fn air_collide_with_stage(
     ecb: &FighterECB,
     stage_collision: &StageCollision,
     ignored_polys: &[usize],
+    ignore_soft_platforms: bool,
 ) -> Option<AirCollisionWithStageResult> {
     let ray_segment = FGSegment2d::new(
         prev_translation.0 + prev_ecb.get_bottom_point(),
@@ -55,7 +56,10 @@ pub fn air_collide_with_stage(
         .stage_polys
         .iter()
         .enumerate()
-        .filter(|(poly_idx, _)| !ignored_polys.contains(poly_idx))
+        .filter(|(poly_idx, poly)| {
+            !ignored_polys.contains(poly_idx)
+                && !(ignore_soft_platforms && matches!(poly.poly_type, StagePolyType::Platform))
+        })
         .filter_map(|(poly_idx, poly)| {
             poly.intersect_ray(ray_segment).map(|intersect_result| {
                 (
@@ -95,6 +99,18 @@ mod tests {
         )
     }
 
+    fn solid_floor() -> StagePoly {
+        StagePoly::build(
+            StagePolyType::Closed,
+            &[
+                (FGVec2::lit("-2", "0"), StagePolyLineSegmentType::Floor),
+                (FGVec2::lit("2", "0"), StagePolyLineSegmentType::Floor),
+                (FGVec2::lit("2", "-2"), StagePolyLineSegmentType::Wall),
+                (FGVec2::lit("-2", "-2"), StagePolyLineSegmentType::Wall),
+            ],
+        )
+    }
+
     #[test]
     fn airborne_collision_skips_ignored_polygons() {
         let stage_collision = StageCollision {
@@ -122,10 +138,78 @@ mod tests {
             &ecb,
             &stage_collision,
             &[1],
+            false,
         )
         .expect("the non-ignored platform should still be collidable");
 
         assert_eq!(collision.line_id.polygon, 0);
         assert_eq!(collision.hit_position, FGVec2::lit("0", "0"));
     }
+
+    #[test]
+    fn airborne_collision_skips_soft_platforms_while_passing_down() {
+        let stage_collision = StageCollision {
+            stage_polys: vec![horizontal_platform("0")],
+        };
+        let previous_translation = FighterPreviousTranslation(FGVec2::lit("0", "2"));
+        let translation = FighterTranslation(FGVec2::lit("0", "-1"));
+        let previous_ecb = FighterPreviousECB {
+            bottom: FGVec2::ZERO,
+            top: FGVec2::ZERO,
+            left: FGVec2::ZERO,
+            right: FGVec2::ZERO,
+        };
+        let ecb = FighterECB {
+            bottom: FGVec2::ZERO,
+            top: FGVec2::ZERO,
+            left: FGVec2::ZERO,
+            right: FGVec2::ZERO,
+        };
+
+        assert!(air_collide_with_stage(
+            &previous_translation,
+            &translation,
+            &previous_ecb,
+            &ecb,
+            &stage_collision,
+            &[],
+            true,
+        )
+        .is_none());
+    }
+
+    #[test]
+    fn airborne_collision_still_hits_solid_floors_while_passing_down() {
+        let stage_collision = StageCollision {
+            stage_polys: vec![solid_floor()],
+        };
+        let previous_translation = FighterPreviousTranslation(FGVec2::lit("0", "2"));
+        let translation = FighterTranslation(FGVec2::lit("0", "-1"));
+        let previous_ecb = FighterPreviousECB {
+            bottom: FGVec2::ZERO,
+            top: FGVec2::ZERO,
+            left: FGVec2::ZERO,
+            right: FGVec2::ZERO,
+        };
+        let ecb = FighterECB {
+            bottom: FGVec2::ZERO,
+            top: FGVec2::ZERO,
+            left: FGVec2::ZERO,
+            right: FGVec2::ZERO,
+        };
+
+        let collision = air_collide_with_stage(
+            &previous_translation,
+            &translation,
+            &previous_ecb,
+            &ecb,
+            &stage_collision,
+            &[],
+            true,
+        )
+        .expect("solid floors remain collidable while passing down");
+
+        assert_eq!(collision.line_id.polygon, 0);
+        assert_eq!(collision.hit_position, FGVec2::lit("0", "0"));
+    }
 }