Ryan Yates fryguybob at gmail.com
Wed Aug 17 16:48:28 CEST 2011

```I had to stare at this for a while to see the difference.  In the C++
version the comparison's in loop B and loop C are always between areas
of triangles that are off by one in a single index.  In the haskell
calArea though, comparisons are between the passed area and triangles
off by one from the passed coordinates.  Recursive calls under calArea
hold the invariant found in the C++ code, but in looPing area' is the
area of a' b' c', but the recursive call is to looPing a'' b'' c''
with area' so calArea is passed the indexes for a different triangle
then the area it is given.  We can solve this by explicitly passing
around the best result so far:

looPing :: ( Num a , Ord a , Eq a , Floating a ) => Int -> Int -> Int
-> Int -> a -> a -> Array Int ( Point a ) -> a
looPing a b c n area best arr
| a == n = max best area
| otherwise = looPing a'' b'' c'' n area'' (max area' best) arr where
( a' , b' , c' , area' ) = calArea a b c n area arr
a'' = a' + 1
b'' = if a'' == b' then mod ( b' + 1 ) n else b'
c'' = if b'' == c' then mod ( c' + 1 ) n else c'
area'' = caltmp (a'' `mod` n) b'' c'' arr

and solve now applies area twice:  looPing 0 1 2  n area area arr'

This now matches the result from C++ for me.

On Wed, Aug 17, 2011 at 8:09 AM, mukesh tiwari
<mukeshtiwari.iiitm at gmail.com> wrote:
> Hello all
> I am trying implement this algorithm  [
> http://stackoverflow.com/questions/1621364/how-to-find-largest-triangle-in-convex-hull-aside-from-brute-force-search
> ] in Haskell but i am getting wrong answer for some test cases. A c++
> implementation which is accepted for this problem [ http://www.spoj.pl/problems/MTRIAREA
> ] and i   rewrote  the C++ code in Haskell . Running both programs on
> some  random test cases produce different result . For Haskell
> implementation , convex hull part is correct . Both codes [ c++ and
> Haskell ] produced same convex hull so i am skeptical about
> looPing :: ( Num a , Ord a , Eq a , Floating a ) => Int -> Int -> Int -
>> Int -> a -> Array Int ( Point a ) -> a  and calArea :: ( Num a , Ord
> a , Floating a ) => Int -> Int -> Int  -> Int  -> a -> Array Int
> ( Point a ) -> ( Int , Int , Int , a ) function . Could some one
> please tell me what is wrong with these functions. I have posted both
> code on ideone . Haskell code [ http://ideone.com/IlwBv ] and accepted
> c++ for SPOJ problem [ http://ideone.com/vgNnt ] . For test case
> 20
> 886 9383
> 6915 2777
> 8335 7793
> 492 5386
> 1421 6649
> 27 2362
> 59 8690
> 3926 7763
> 3426 540
> 5736 9172
> 5368 5211
> 6429 2567
> 1530 5782
> 5123 2862
> 3135 4067
> 9802 3929
> 3058 4022
> 8167 3069
> 8456 1393
> 8042 5011
> -1
> Haskell produced the convex hull [P 27.0 2362.0,P 3426.0 540.0,P
> 8456.0 1393.0,P 9802.0 3929.0,P 8335.0 7793.0,P 5736.0 9172.0,P 886.0
> 9383.0,P 59.0 8690.0] and maximum triangle area 31466755.50 while C++
> code produced the convex hull
> 27 2362
> 3426 540
> 8456 1393
> 9802 3929
> 8335 7793
> 5736 9172
> 886 9383
> 59 8690
> and maximum area 33642111.00 . Based on these observation , I think my
> convex hull part is correct and I am stuck with looPing and calArea
> function . I tried to write Haskell code many ways but no success so
> it would be great if some one can tell me what is wrong with these two
> functions .
>
> Thank you
>
> _______________________________________________