starting work on poly poly collisions

collision.go

@@ -0,0 +1,72 @@
+package govector
+
+func Intersection(p1, p2 Point2d, v1, v2 Vector2d) (bool, Point2d) {
+	t := p2.Sub(p1).Cross(v2) / v1.Cross(v2)
+	s := p2.Sub(p1).Cross(v1) / v1.Cross(v2)
+	if t > 0 && t < 1 && s > 0 && s < 1 {
+		return true, p1.Add(v1.Scale(t))
+	}
+
+	return false, Point2d{0, 0}
+}
+
+func RectRectIntersection(r1 AABB2d, r2 AABB2d) bool {
+	return (r1.A.X < r2.B.X && r1.B.X > r2.A.X &&
+		r1.A.Y < r2.B.Y && r1.B.Y > r2.A.Y)
+}
+
+func RectIntersection(r AABB2d, p Point2d, v Vector2d) (bool, bool, Point2d) {
+	collision := false
+	inside := false
+	start_inside := PointInRect(p, r)
+	end_inside := PointInRect(p.Add(v), r)
+	if start_inside && end_inside {
+		inside = true
+		return collision, inside, p
+	} else {
+		wall_left, col1 := Intersection(p, r.A, v, Vector2d{X: 0, Y: r.B.Y - r.A.Y})
+		if wall_left {
+			return wall_left, inside, col1
+		}
+		wall_top, col2 := Intersection(p, r.A, v, Vector2d{X: r.B.X - r.A.X, Y: 0})
+		if wall_top {
+			return wall_top, inside, col2
+		}
+		wall_right, col3 := Intersection(p, r.B, v, Vector2d{X: 0, Y: -r.B.Y + r.A.Y})
+		if wall_right {
+			return wall_right, inside, col3
+		}
+		wall_bottom, col4 := Intersection(p, r.B, v, Vector2d{X: -r.B.X + r.A.X, Y: 0})
+		if wall_bottom {
+			return wall_bottom, inside, col4
+		}
+
+		return false, false, p
+	}
+}
+
+func ProjectPolygon(axis Vector2d, p Polygon2d) (min float32, max float32) {
+	min = axis.Dot(p.Points[0])
+	max = min
+	for _, point := range p.Points {
+		d := axis.Dot(point)
+		if d < min {
+			min = d
+		} else {
+			if d > max {
+				max = d
+			}
+		}
+	}
+	return min, max
+}
+
+func PolygonIntersection(r Polygon2d, p Point2d, v Vector2d) (bool, bool, Point2d) {
+	// TBD
+	return false, false, Point2d{}
+}
+
+func PolyPolyIntersection(p1 Polygon2d, v1 Vector2d, p2 Polygon2d, v2 Vector2d) bool {
+
+	return false
+}