Improve intersection test--fails sometimes

src/Geometry/Intersect.hs

@@ -5,6 +5,7 @@ Testing for and finding intersection points.
 module Geometry.Intersect
     where
 import Geometry.Data
+import Geometry.LHS
 import Control.Applicative
 
 import Data.Maybe (isNothing)
@@ -74,6 +75,30 @@ intersectSegLine' (V2 x1 y1) (V2 x2 y2) (V2 x3 y3) (V2 x4 y4)
     den = (x1-x2)*(y3-y4) - (y1-y2)*(x3-x4)
     t'  = (x1-x3)*(y3-y4) - (y1-y3)*(x3-x4)
     --u'  = (y1-y2)*(x1-x3) - (x1-x2)*(y1-y3)
+-- | It is not always necessary to find a point of intersection, sometimes a
+-- test may suffice.
+intersectSegSegTest
+    :: Point2
+    -> Point2
+    -> Point2
+    -> Point2
+    -> Bool
+{-# INLINE intersectSegSegTest #-}
+intersectSegSegTest a' b' c' d' 
+    = f a' b' c' d' && f c' d' a' b'
+  where
+    f a b c d =  (      isLHS a b c  && not (isLHS a b d) ) 
+              || ( not (isLHS a b c) &&      isLHS a b d  )
+intersectSegSegPreTest
+    :: Point2
+    -> Point2
+    -> Point2
+    -> Point2
+    -> Maybe Point2
+{-# INLINE intersectSegSegPreTest #-}
+intersectSegSegPreTest a b c d
+    | intersectSegSegTest a b c d = myIntersectSegSeg a b c d
+    | otherwise = Nothing
 -- | Due to floating point issues, 'intersectSegSeg'' is not always 
 -- accurate---'myIntersectSegSeg'
 -- fixes at least some of