ALGO: Split scanH out and make the graham API more modular

Algorithms/ConvexHull/GrahamScan.hs

@@ -76,28 +76,41 @@ ys = []
 return [(100, 100), (400, 200)]
 =========================================================
 --}
-grahamGetCH :: [PT] -> [PT]
+grahamCH :: [PT] -> [PT]
-grahamGetCH vs =
+grahamCH vs = grahamUCH vs ++ (tailInit . grahamLCH $ vs)
-  scanH uH uHRest ++ tailInit (scanH lH lHRest)
+
+
+-- |Get the lower part of the convex hull.
+grahamLCH :: [PT] -> [PT]
+grahamLCH vs = scanH lH lHRest
   where
     sortedXY     = fmap p2 . sortLex . fmap unp2 $ vs
     (lH, lHRest) = first reverse . splitAt 3 $ sortedXY
+
+
+-- |Get the upper part of the convex hull.
+grahamUCH :: [PT] -> [PT]
+grahamUCH vs = scanH uH uHRest
+  where
+    sortedXY     = fmap p2 . sortLex . fmap unp2 $ vs
     (uH, uHRest) = first reverse . splitAt 3 . reverse $ sortedXY
-    -- This scans only a half of the convex hull. If it's the upper
+
-    -- or lower half depends on the input.
+
-    -- Also, the first list is reversed since we only care about the last
+-- |This scans only a half of the convex hull. If it's the upper
-    -- 3 elements and want to stay efficient.
+-- or lower half depends on the input.
-    scanH :: [PT] -- ^ the first 3 starting points in reversed order
+-- Also, the first list is reversed since we only care about the last
+-- 3 elements and want to stay efficient.
+scanH :: [PT] -- ^ the first 3 starting points in reversed order
       -> [PT] -- ^ the rest of the points
       -> [PT] -- ^ all convex hull points for the half
-    scanH hs@(x:y:z:xs) (r':rs')
+scanH hs@(x:y:z:xs) (r':rs')
   |	notcw z y x     = scanH (r':hs) rs'
   | otherwise       = scanH (x:z:xs) (r':rs')
-    scanH hs@(x:y:z:xs) []
+scanH hs@(x:y:z:xs) []
   |	notcw z y x     = hs
   | otherwise       = scanH (x:z:xs) []
-    scanH hs (r':rs')   = scanH (r':hs) rs'
+scanH hs (r':rs')   = scanH (r':hs) rs'
-    scanH hs _          = hs
+scanH hs _          = hs
 
 
 -- |Compute all steps of the graham scan algorithm to allow

Graphics/Diagram/Plotter.hs

@@ -52,7 +52,7 @@ convexHullPoints = Diag chp
         (repeat dot))
       where
         dot = (circle $ t p :: Diagram Cairo R2) # fc red # lc red
-        vtch = grahamGetCH vt
+        vtch = grahamCH vt
 
 
 -- |Show coordinates as text above the convex hull points.
@@ -64,7 +64,7 @@ convexHullPointsText = Diag chpt
         zip vtchf
             (pointToTextCoord <$> vtchf) # translate (r2 (0, 10))
       where
-        vtchf = grahamGetCH . filter (inRange (dX p) (dY p)) $ vt
+        vtchf = grahamCH . filter (inRange (dX p) (dY p)) $ vt
 
 
 -- |Create a diagram which shows the lines along the convex hull
@@ -76,10 +76,10 @@ convexHullLines = Diag chl
     chl p vt =
       (strokeTrail                           .
           fromVertices                       .
-          flip (++) [head $ grahamGetCH vtf] .
+          flip (++) [head $ grahamCH vtf]    .
-          grahamGetCH                        $
+          grahamCH                           $
           vtf)                       #
-        moveTo (head $ grahamGetCH vtf) #
+        moveTo (head $ grahamCH vtf) #
         lc red
       where
         vtf = filter (inRange (dX p) (dY p)) vt