From c48066bc5b77547a1f9dc139fa45f1a3fba82e8d Mon Sep 17 00:00:00 2001
From: Eduardo Canellas de Oliveira <eduardo.canellas@bemobi.com>
Date: Fri, 15 Apr 2022 12:49:05 -0300
Subject: [PATCH] improve collide handling

bug fix + refactor. Closes #4478
---
 crates/bevy_sprite/src/collide_aabb.rs | 146 ++++++++++++++++++++++---
 1 file changed, 128 insertions(+), 18 deletions(-)

diff --git a/crates/bevy_sprite/src/collide_aabb.rs b/crates/bevy_sprite/src/collide_aabb.rs
index 05a4e9fe2af1d..9a811166d8f98 100644
--- a/crates/bevy_sprite/src/collide_aabb.rs
+++ b/crates/bevy_sprite/src/collide_aabb.rs
@@ -2,7 +2,7 @@
 
 use bevy_math::{Vec2, Vec3};
 
-#[derive(Debug)]
+#[derive(Debug, PartialEq)]
 pub enum Collision {
     Left,
     Right,
@@ -11,36 +11,52 @@ pub enum Collision {
     Inside,
 }
 
+struct CollisionBox {
+    pub top: f32,
+    pub bottom: f32,
+    pub left: f32,
+    pub right: f32,
+}
+
+impl CollisionBox {
+    pub fn new(pos: Vec3, size: Vec2) -> Self {
+        Self {
+            top: pos.y + size.y / 2.,
+            bottom: pos.y - size.y / 2.,
+            left: pos.x - size.x / 2.,
+            right: pos.x + size.x / 2.,
+        }
+    }
+}
+
 // TODO: ideally we can remove this once bevy gets a physics system
-/// Axis-aligned bounding box collision with "side" detection
+/// Axis-aligned bounding box collision with "side" detection.
+///
+/// The [Collision], in case it occurred, is the side of `b` where `a` hit.
+///
 /// * `a_pos` and `b_pos` are the center positions of the rectangles, typically obtained by
 /// extracting the `translation` field from a `Transform` component
 /// * `a_size` and `b_size` are the dimensions (width and height) of the rectangles.
 pub fn collide(a_pos: Vec3, a_size: Vec2, b_pos: Vec3, b_size: Vec2) -> Option<Collision> {
-    let a_min = a_pos.truncate() - a_size / 2.0;
-    let a_max = a_pos.truncate() + a_size / 2.0;
-
-    let b_min = b_pos.truncate() - b_size / 2.0;
-    let b_max = b_pos.truncate() + b_size / 2.0;
+    let a = CollisionBox::new(a_pos, a_size);
+    let b = CollisionBox::new(b_pos, b_size);
 
     // check to see if the two rectangles are intersecting
-    if a_min.x < b_max.x && a_max.x > b_min.x && a_min.y < b_max.y && a_max.y > b_min.y {
+    if a.left < b.right && a.right > b.left && a.bottom < b.top && a.top > b.bottom {
         // check to see if we hit on the left or right side
-        let (x_collision, x_depth) = if a_min.x < b_min.x && a_max.x > b_min.x && a_max.x < b_max.x
-        {
-            (Collision::Left, b_min.x - a_max.x)
-        } else if a_min.x > b_min.x && a_min.x < b_max.x && a_max.x > b_max.x {
-            (Collision::Right, a_min.x - b_max.x)
+        let (x_collision, x_depth) = if a.left < b.left && a.right > b.left && a.right < b.right {
+            (Collision::Left, b.left - a.right)
+        } else if a.left > b.left && a.left < b.right && a.right > b.right {
+            (Collision::Right, a.left - b.right)
         } else {
             (Collision::Inside, -f32::INFINITY)
         };
 
         // check to see if we hit on the top or bottom side
-        let (y_collision, y_depth) = if a_min.y < b_min.y && a_max.y > b_min.y && a_max.y < b_max.y
-        {
-            (Collision::Bottom, b_min.y - a_max.y)
-        } else if a_min.y > b_min.y && a_min.y < b_max.y && a_max.y > b_max.y {
-            (Collision::Top, a_min.y - b_max.y)
+        let (y_collision, y_depth) = if a.bottom < b.bottom && a.top > b.bottom && a.top < b.top {
+            (Collision::Bottom, b.bottom - a.top)
+        } else if a.bottom > b.bottom && a.bottom < b.top && a.top > b.top {
+            (Collision::Top, a.bottom - b.top)
         } else {
             (Collision::Inside, -f32::INFINITY)
         };
@@ -55,3 +71,97 @@ pub fn collide(a_pos: Vec3, a_size: Vec2, b_pos: Vec3, b_size: Vec2) -> Option<C
         None
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    ///
+    ///
+    ///     ______
+    ///     |    |
+    ///     | A  |
+    /// ----|----|----
+    /// |   |____|   |
+    /// |     B      |
+    /// |____________|
+    ///
+    #[test]
+    fn top_collision() {
+        let a = Vec3::new(0., 30., 0.);
+        let b = Vec3::new(0., 0., 0.);
+
+        check(a, b, Some(Collision::Top));
+    }
+
+    ///
+    ///
+    /// --------------
+    /// |      B     |
+    /// |    _____   |
+    /// |___|____|___|
+    ///     |    |
+    ///     | A  |
+    ///     |    |
+    ///     -----
+    #[test]
+    fn bottom_collision() {
+        let a = Vec3::new(0., -30., 0.);
+        let b = Vec3::new(0., 0., 0.);
+
+        check(a, b, Some(Collision::Bottom));
+    }
+
+    ///
+    ///   ______
+    ///  |   --|-----------
+    ///  |   | |          |
+    ///  | A | |    B     |
+    ///  |   |_|__________|
+    ///  |_____|
+    ///
+    #[test]
+    fn left_collision() {
+        let a = Vec3::new(0., 0., 0.);
+        let b = Vec3::new(30., 0., 0.);
+
+        check(a, b, Some(Collision::Left));
+    }
+
+    ///
+    ///             ______
+    /// -----------|--   |
+    /// |      B   | |   |
+    /// |          | | A |
+    /// |__________|_|   |
+    ///            |_____|
+    #[test]
+    fn right_collision() {
+        let a = Vec3::new(0., 0., 0.);
+        let b = Vec3::new(-30., 0., 0.);
+
+        check(a, b, Some(Collision::Right));
+    }
+
+    ///
+    ///     ______
+    /// ----|----|----
+    /// |   |    |   |
+    /// |   |    | B |
+    /// |___|____|___|
+    ///     | A  |
+    ///     |____|
+    #[test]
+    fn without_corners_on_intersection_area() {
+        let a = Vec3::new(0., 0., 0.);
+        let b = Vec3::new(0., 0., 0.);
+
+        check(a, b, Some(Collision::Inside));
+    }
+
+    fn check(a: Vec3, b: Vec3, expected: Option<Collision>) {
+        let a_size = Vec2::new(30., 50.);
+        let b_size = Vec2::new(50., 30.);
+        assert_eq!(collide(a, a_size, b, b_size), expected);
+    }
+}