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  1. type Point = (Int, Int)
  2. type Pair = (Point, Point)
  3. type Node = [Pair]
  4.  
  5. main :: IO ()
  6. main =  do
  7.   print $ length $ solve 8
  8.  
  9. plus                   :: Point -> Point -> Point
  10. plus (x1, y1) (x2, y2) =  (x1+x2, y1+y2)
  11.  
  12. minus                   :: Point -> Point -> Point
  13. minus (x1, y1) (x2, y2) =  (x1-x2, y1-y2)
  14.  
  15. move          :: Pair -> Point -> Pair
  16. move (f, b) p =  (f `plus` p, b `minus` p)
  17.  
  18. moves                   :: Pair -> Pair -> [Pair]
  19. moves (pf, pb) (qf, qb) =  map (move (pf, pb)) [f, r, l]
  20.   where
  21.     (dx, dy) = pf `minus` qf
  22.     f = (dx, dy)
  23.     r = (-dy, dx)
  24.     l = (dy, -dx)
  25.  
  26. nexts            :: Node -> [Node]
  27. nexts (p1:p0:ps) =  [p2:p1:p0:ps | p2 <- moves p1 p0]
  28.  
  29. exists :: Point -> [Pair] -> Bool
  30. exists p = any (either p)
  31.   where
  32.     either p (pf, pb) = p == pf || p == pb
  33.  
  34. solve      :: Int -> [Node]
  35. solve size =  solutions initial
  36.   where
  37.     center :: Point
  38.     center =  (size `div` 2, size `div` 2)
  39.  
  40.     p0 :: Pair
  41.     p0 =  (center, center)
  42.  
  43.     initial :: Node
  44.     initial = [move p0 (0, 1), p0]
  45.  
  46.     atboundary        :: Point -> Bool
  47.     atboundary (x, y) =  (x == 0) || (x == size) || (y == 0) || (y == size)
  48.  
  49.     solutions   :: Node -> [Node]
  50.     solutions n
  51.       | atboundary f = [n]
  52.       | exists f ps  = []
  53.       | otherwise    = [n'' | n' <- nexts n,
  54.                               n'' <- solutions n']
  55.       where
  56.         (f,b):ps = n
  57.  
Success #stdin #stdout 0.65s 4612KB
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https://ideone.com/0gFNU
language:
Haskell (ghc 8.4.4)
created:
12 years ago
visibility:
public

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