deterministic algorithm to determine whether a point is in a polygon

Author: swipesense

lib/geometry/polygon.rb

@@ -13,13 +13,25 @@ def edges
     def bounding_box
       leftbottom = Point vertices.map(&:x).min, vertices.map(&:y).min
       righttop = Point vertices.map(&:x).max, vertices.map(&:y).max
-      
+
       BoundingBox.new leftbottom, righttop
     end
 
     def contains?(point)
-      point_in_polygon = PointInPolygon.new(point, self)
-      point_in_polygon.inside? || point_in_polygon.on_the_boundary?
+      # Algorithm source:
+      # https://wrf.ecse.rpi.edu//Research/Short_Notes/pnpoly.html
+
+      result = false
+
+      vertices.each_with_index do |vertex, i|
+        previous_vertex = vertices[i - 1] || vertex.last
+        if ((vertex.y > point.y) != (previous_vertex.y > point.y)) &&
+            (point.x < (previous_vertex.x - vertex.x) * (point.y - vertex.y) / (previous_vertex.y - vertex.y) + vertex.x)
+          result = !result
+        end
+      end
+
+      result
     end
 
     def area