Hello all This is my first post to Haskell-cafe so i am not aware of protocols here and pardon me for my stupidity . I am trying to solve this problem [ https://www.spoj.pl/problems/QCJ4 ] but getting wrong answer . I implemented the algorithm by Pr. Chrystal described here [ http://www.personal.kent.edu/~rmuhamma/Compgeometry/MyCG/CG-Applets/Center/c... ] . I tested my convex hull code at this site and its accepted for problem [ http://www.spoj.pl/problems/GARDENHU ] so i think its correct but i am not sure if i have implemented Chrystal's algorithm correctly . Could some please tell me if i have implemented the algorithm correctly and if its possible then why i am getting wrong answer for this problem . In case of indentation problem [ http://ideone.com/perhE ] Thank you Mukesh Tiwari import Data.List import qualified Data.Sequence as DS import Text.Printf data Point a = P a a deriving ( Show , Ord , Eq ) data Turn = S | L | R deriving ( Show , Eq , Ord , Enum ) -- straight left right --start of convex hull http://en.wikipedia.org/wiki/Graham_scan compPoint :: ( Num a , Ord a ) => Point a -> Point a -> Ordering compPoint ( P x1 y1 ) ( P x2 y2 ) | compare x1 x2 == EQ = compare y1 y2 | otherwise = compare x1 x2 findMinx :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] findMinx xs = sortBy ( \x y -> compPoint x y ) xs compAngle ::(Num a , Ord a ) => Point a -> Point a -> Point a -> Ordering compAngle ( P x1 y1 ) ( P x2 y2 ) ( P x0 y0 ) = compare ( ( y1 - y0 ) * ( x2 - x0 ) ) ( ( y2 - y0) * ( x1 - x0 ) ) sortByangle :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] sortByangle (z:xs) = z : sortBy ( \x y -> compAngle x y z ) xs convexHull ::( Num a , Ord a ) => [ Point a ] -> [ Point a ] convexHull [ P x0 y0 ] = [ P x0 y0 ] convexHull xs = reverse . findHull [y,x] $ ys where (x:y:ys) = sortByangle.findMinx $ xs findTurn :: ( Num a , Ord a , Eq a ) => Point a -> Point a -> Point a -

Turn findTurn ( P x0 y0 ) ( P x1 y1 ) ( P x2 y2 ) | ( y1 - y0 ) * ( x2- x0 ) < ( y2 - y0 ) * ( x1 - x0 ) = L | ( y1 - y0 ) * ( x2- x0 ) == ( y2 - y0 ) * ( x1 - x0 ) = S | otherwise = R

findHull :: ( Num a , Ord a ) => [ Point a ] -> [ Point a ] -> [ Point a ] findHull [x] ( z : ys ) = findHull [ z , x ] ys --incase of second point on line from x to z findHull xs [] = xs findHull ( y : x : xs ) ( z:ys ) | findTurn x y z == R = findHull ( x : xs ) ( z:ys ) | findTurn x y z == S = findHull ( x : xs ) ( z:ys ) | otherwise = findHull ( z : y : x : xs ) ys --end of convex hull --start of finding point algorithm http://www.personal.kent.edu/~rmuhamma/Compgeometry/MyCG/CG-Applets/Center/c... Applet’s Algorithm findAngle :: ( Num a , Ord a , Floating a ) => Point a -> Point a -> Point a -> ( Point a , Point a , Point a , a ) findAngle u@(P x0 y0 ) v@(P x1 y1 ) t@(P x2 y2) | u == t || v == t = ( u , v , t , 10 * pi ) -- two points are same so set the angle more than pi | otherwise = ( u , v, t , ang ) where ang = acos ( ( b + c - a ) / ( 2 * sb * sc ) ) where b = ( x0 - x2 ) ^ 2 + ( y0 - y2 ) ^ 2 c = ( x1 - x2 ) ^ 2 + ( y1 - y2 ) ^ 2 a = ( x0 - x1 ) ^ 2 + ( y0 - y1 ) ^ 2 sb = sqrt b sc = sqrt c findPoints :: ( Num a , Ord a , Floating a ) => Point a -> Point a -> [ Point a ] -> ( Point a , Point a , Point a , a ) findPoints u v xs | 2 * theta >= pi = ( a , b , t , theta ) | and [ 2 * alpha <= pi , 2 * beta <= pi ] = ( a , b , t , theta ) | otherwise = if 2 * alpha > pi then findPoints v t xs else findPoints u t xs where ( a , b , t , theta ) = minimumBy ( \(_,_,_, t1 ) ( _ , _ , _ ,t2 ) -

compare t1 t2 ) . map ( findAngle u v ) $ xs ( _ , _ , _ , alpha ) = findAngle v t u --angle between v u t angle subtended at u by v t ( _ , _ , _ , beta ) = findAngle u t v -- angle between u v t angle subtended at v by u t

--end of finding three points algorithm --find the circle through three points http://paulbourke.net/geometry/circlefrom3/ circlePoints :: ( Num a , Ord a , Floating a ) => Point a -> Point a -

Point a -> ( Point a , a ) --( center , radius ) circlePoints u@(P x1 y1 ) v@(P x2 y2 ) t@(P x3 y3 ) | x2 == x1 = circlePoints u t v | x3 == x2 = circlePoints v u t | otherwise = ( P x y , 2 * r ) where m1 = ( y2 - y1 ) / ( x2 - x1 ) m2 = ( y3 - y2 ) / ( x3 - x2 ) x = ( m1 * m2 * ( y1 - y3 ) + m2 * ( x1 + x2 ) - m1 * ( x2 + x3 ) ) / ( 2 * ( m2 - m1 ) ) y = if y2 /= y1 then ( ( x1 + x2 - 2 * x ) / 2 * m1 ) + ( ( y1 + y2 ) / 2.0 ) else ( ( x2 + x3 - 2 * x ) / 2 * m2 ) + ( ( y2 + y3 ) / 2.0 ) r = sqrt $ ( x - x1 ) ^2 + ( y - y1 ) ^ 2

--end of circle through three points --start of SPOJ code format::(Num a , Ord a ) => [[a]] -> [Point a] format xs = map (\[x0 , y0] -> P x0 y0 ) xs readInt ::( Num a , Read a ) => String -> a readInt = read solve :: ( Num a , Ord a , Floating a ) => [ Point a ] -> ( Point a , Point a , Point a , a ) solve [ P x0 y0 ] = ( P x0 y0 , P x0 y0 , P x0 y0 , 0 ) --in case of one point solve [ P x0 y0 , P x1 y1 ] = ( P x0 y0 , P x0 y0 , P x0 y0 , sqrt $ ( x0 - x1 ) ^ 2 + ( y0 - y1 ) ^2 ) -- in case of two points the solve xs = findPoints x y t where t@( x : y : ys ) = convexHull xs final :: ( Num a , Ord a , Floating a ) => ( Point a , Point a , Point a , a ) -> a final ( u , v , t , w ) | w == 0 = 0 | and [ u == v , v == t ] = w | otherwise = r where ( P x y , r ) = circlePoints u v t main = interact $ ( printf "%.2f\n" :: Double -> String ) . final . solve . convexHull . format . map ( map readInt . words ) . tail . lines