Jarvis March in Haskell

Author: jiegillet

chapters/computational_geometry/gift_wrapping/jarvis_march/code/haskell/jarvisMarch.hs

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+import Data.List (sort, maximumBy)
+import Data.Function (on)
+
+type Point = (Double, Double)
+
+angle :: Point -> Point -> Point -> Double
+angle a@(xa, ya) b@(xb, yb) c@(xc, yc)
+  | a==b || c==b = 0
+  | theta<0      = theta+2*pi
+  | otherwise    = theta
+  where thetaA = atan2 (ya-yb) (xa-xb)
+        thetaC = atan2 (yc-yb) (xc-xb)
+        theta = thetaC - thetaA
+
+jarvisMarch :: [Point] -> [Point]
+jarvisMarch [] = []
+jarvisMarch pts = p0 : wrap (x, y-1) p0
+  where p0@(x, y)= minimum pts
+        wrap p1 p2
+          | pm == p0  = []
+          | otherwise = pm : wrap p2 pm
+          where pm = maximumBy (compare `on` angle p1 p2) pts
+
+main = do
+  let pts = filter (\(x,y) -> x^2+y^2<=5^2) [(x,y)|x<-[-5..5], y<-[-5..5]]
+  print $ jarvisMarch pts