[#795] Fix bug in raycasting (#824)

Closes #795 

## Changes:

- Fix raycast bug
- Add test for raycast demonstrating bug
- Rewrite raycasts to be slightly more efficient by avoiding branching
- Add a new method to calculate time to impact in raycasts

src/engine/Collision/BoundingBox.ts

@@ -120,23 +120,46 @@ export class BoundingBox implements ICollidable {
       var tmin = -Infinity;
       var tmax = +Infinity;
 
-      if (ray.dir.x !== 0) {
-         var tx1 = (this.left - ray.pos.x) / ray.dir.x;
-         var tx2 = (this.right - ray.pos.x) / ray.dir.x;
-         tmin = Math.max(tmin, Math.min(tx1, tx2));
-         tmax = Math.min(tmax, Math.max(tx1, tx2));
-      }
-
-      if (ray.dir.y !== 0) {
-         var ty1 = (this.top - ray.pos.y) / ray.dir.y;
-         var ty2 = (this.bottom - ray.pos.y) / ray.dir.y;
-         tmin = Math.max(tmin, Math.min(ty1, ty2));
-         tmax = Math.min(tmax, Math.max(ty1, ty2));
-      }
+      var xinv = ray.dir.x === 0 ? Number.MAX_VALUE : (1 / ray.dir.x);
+      var yinv = ray.dir.y === 0 ? Number.MAX_VALUE : (1 / ray.dir.y);
+
+      var tx1 = (this.left - ray.pos.x) * xinv;
+      var tx2 = (this.right - ray.pos.x) * xinv;
+      tmin = Math.min(tx1, tx2);
+      tmax = Math.max(tx1, tx2);
+
+      var ty1 = (this.top - ray.pos.y) * yinv; 
+      var ty2 = (this.bottom - ray.pos.y) * yinv;
+      tmin = Math.max(tmin, Math.min(ty1, ty2));
+      tmax = Math.min(tmax, Math.max(ty1, ty2));      
 
       return tmax >= Math.max(0, tmin) && tmin < farClipDistance;
    }
 
+   public rayCastTime(ray: Ray, farClipDistance = Infinity): number {
+       // algorithm from https://tavianator.com/fast-branchless-raybounding-box-intersections/ 
+      var tmin = -Infinity;
+      var tmax = +Infinity;
+
+      var xinv = ray.dir.x === 0 ? Number.MAX_VALUE : (1 / ray.dir.x);
+      var yinv = ray.dir.y === 0 ? Number.MAX_VALUE : (1 / ray.dir.y);
+
+      var tx1 = (this.left - ray.pos.x) * xinv;
+      var tx2 = (this.right - ray.pos.x) * xinv;
+      tmin = Math.min(tx1, tx2);
+      tmax = Math.max(tx1, tx2);
+
+      var ty1 = (this.top - ray.pos.y) * yinv; 
+      var ty2 = (this.bottom - ray.pos.y) * yinv;
+      tmin = Math.max(tmin, Math.min(ty1, ty2));
+      tmax = Math.min(tmax, Math.max(ty1, ty2));
+
+      if (tmax >= Math.max(0, tmin) && tmin < farClipDistance) {
+         return tmin;
+      }
+      return -1;
+   }
+
    /**
     * Tests whether a point is contained within the bounding box
     * @param p  The point to test

src/spec/BoundingBoxSpec.ts

@@ -102,6 +102,13 @@ describe('A Bounding Box', () => {
       expect(bb.rayCast(ray)).toBe(false);
    });
 
+   it('ray cast can miss a box far away', () => {
+      var bb = new ex.BoundingBox(1176, 48, 1200, 72);
+
+      var ray = new ex.Ray(new ex.Vector(48, 72), ex.Vector.Down);
+      expect(bb.rayCast(ray)).toBe(false);
+   });
+
    it('ray cast can hit bounding box on the edge', () => {
       var bb = new ex.BoundingBox(0, 0, 10, 10);