# Graham Scan Implementation

### From HaskellWiki

(Difference between revisions)

(Fixed bugs with the recursion) |
(add example where this code doesn't work) |

## Revision as of 03:00, 25 July 2013

Descriptions of this problem can be found in Real World Haskell, Chapter 3

import Data.Ord (comparing) import Data.List (sortBy) --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] -- Actually, I'd leave it as EQ, GT, LT. Then, actually, -- if you wanted to sort points rotationally around a single point, -- sortBy (dir x) would actually work. --wasserman.louis@gmail.com --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)) --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)) = compare (y1,x1) (y2,x2) --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 = comparing . pointAngle --Graham Scan gscan :: [Point] -> [Point] gscan ps = scan (sortByAngle ps) where scan (x:y:z:xs) = if dir x y z == LeftTurn then scan (x:z:xs) else x: scan (y:z:xs) scan [x,y] = [x,y] -- there's no shame in a pattern match -- of this type! scan _ = []

## Testing

The above implementation has an error, which can be found using

import Test.QuickCheck import Control.Monad import Data.List gscan' = map (\(Point x) -> x) . gscan . map Point prop_gscan n = forAll (replicateM n arbitrary) (\xs -> sort (gscan' xs) == sort (gscan' (gscan' xs)))

This leads to a counterexample for lists of 5 elements

> quickCheck (prop_gscan 3) +++ OK, passed 100 tests. > quickCheck (prop_gscan 4) +++ OK, passed 100 tests. > quickCheck (prop_gscan 5) *** Failed! Falsifiable (after 10 tests): [(-2.4444173639942894,13.729627457461254),(65.72912810263666,5.955962930412828),(-34.288098030422404,80.6230134460068),(-6.446932942713564,-11.632835144720378),(2.861905401095031,1.1159493836896193)] > let f = [(-2.4444173639942894,13.729627457461254),(65.72912810263666,5.955962930412828),(-34.288098030422404,80.6230134460068),(-6.446932942713564,-11.632835144720378),(2.861905401095031,1.1159493836896193)] > gscan f [(-6.446932942713564,-11.632835144720378),(-34.288098030422404,80.6230134460068),(-2.4444173639942894,13.729627457461254),(65.72912810263666,5.955962930412828)] > gscan (gscan f) [(-6.446932942713564,-11.632835144720378),(-34.288098030422404,80.6230134460068),(65.72912810263666,5.955962930412828)] > gscan (gscan (gscan f)) [(-6.446932942713564,-11.632835144720378),(-34.288098030422404,80.6230134460068),(65.72912810263666,5.955962930412828)]