46 lines
1.1 KiB
Haskell
46 lines
1.1 KiB
Haskell
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#!/usr/bin/env nix-script
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#!>haskell
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{- Learn you a Haskell exercise -}
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import Control.Monad
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import Data.List
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import Data.Function
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type Pos = (Int, Int)
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chessboard = liftM2 (,) [1..8] [1..8]
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-- | Compose a monadic function with itself n times
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compose :: Monad m => (a -> m a) -> Int -> a -> m a
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compose n = foldr (<=<) return . flip replicate n
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-- | Calculate every possible next move
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moves :: Pos -> [Pos]
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moves (x, y) = filter (`elem` chessboard)
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[ (x+2, y-1), (x+2, y+1)
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, (x-2, y-1), (x-2, y+1)
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, (x+1, y-2), (x+1, y+2)
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, (x-1, y-2), (x-1, y+2) ]
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-- | Positions the knight can reach in `n` moves
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reachIn :: Int -> Pos -> [Pos]
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reachIn = compose moves
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-- | Test for a path between `start` and `end` in `n` moves
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canReachIn :: Int -> Pos -> Pos -> Bool
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canReachIn n start end = end `elem` reachIn n start
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-- | Pretty print for the chessboard
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pprint = putStrLn . show' . groupBy (on (==) fst) where
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show' = concatMap $ (++"\n") . intercalate " " . map show
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main = do
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let
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reached = nub $ reachIn 1 (1,2)
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unreached = chessboard \\ reached
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print (length reached)
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pprint (sort reached)
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print (length unreached)
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pprint (sort unreached)
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