Newbie Code Critique
Revision as of 15:17, 26 September 2009 by Eric Gesell
This page is a place for new users to post pieces of code that they want analyzed.
Graham Scan Implementation
Descriptions of the problem can be found in Real World Haskell, Chapter 3
--Graham Scan exercise --Direction type data Direction = LeftTurn | RightTurn | Straight deriving (Show, Eq) --Point type data Point = Point (Double, Double) deriving (Show) --some points p0 = Point (2.1,2.0) p1 = Point (4.2,2.0) p2 = Point (0.5,2.5) p3 = Point (3.2,3.5) p4 = Point (1.2,4.0) p5 = Point (0.7,4.7) p6 = Point (1.0,1.0) p7 = Point (3.0,5.2) p8 = Point (4.0,4.0) p9 = Point (3.5,1.5) pA = Point (0.5,1.0) points = [p0,p1,p2,p3,p4,p5,p6,p7,p8,p9,pA] --Get direction of single set of line segments dir :: Point -> Point -> Point -> Direction dir (Point (ax, ay)) (Point (bx, by)) (Point (cx, cy)) = case sign of EQ -> Straight GT -> LeftTurn LT -> RightTurn where sign = compare ((bx - ax) * (cy - ay) - (by - ay) * (cx - ax)) 0 --Get a list of Directions from a list of Points dirlist :: [Point] -> [Direction] dirlist (x:y:z:xs) = dir x y z : dirlist (y:z:xs) dirlist _ =  --Compare Y axes sortByY :: [Point] -> [Point] sortByY xs = sortBy lowestY xs where lowestY (Point(x1,y1)) (Point (x2,y2)) = if y1 == y2 then compare x1 x2 else compare y1 y2 --get COT of line defined by two points and the x-axis pointAngle :: Point -> Point -> Double pointAngle (Point (x1, y1)) (Point (x2, y2)) = (x2 - x1) / (y2 - y1) --compare based on point angle pointOrdering :: Point -> Point -> Ordering pointOrdering a b = compare (pointAngle a b) 0.0 --Sort by angle sortByAngle :: [Point] -> [Point] sortByAngle ps = bottomLeft : sortBy (compareAngles bottomLeft) (tail (sortedPs)) where sortedPs = sortByY ps bottomLeft = head (sortedPs) --Compare angles compareAngles :: Point -> Point -> Point -> Ordering compareAngles base a b = compare (pointAngle base b) (pointAngle base a) --Graham Scan gscan :: [Point] -> [Point] gscan ps = scan (sortByAngle ps) where scan (x:y:z:xs) = if dir x y z == RightTurn then x: scan (z:xs) else x: scan (y:z:xs) scan (x:y:) = (x:y:) scan _ =