The Knights Tour
The Knight's Tour is a
mathematical problem involving a knight on a chessboard. The knight is
placed on the empty board and, moving according to the rules of chess,
must visit each square exactly once.
Here are some Haskell implementations.
One
--
-- Quick implementation by dmwit on #haskell
-- Faster, shorter, uses less memory than the Python version.
--
import Control.Arrow
import Control.Monad
import Data.List
import Data.Maybe
import Data.Ord
import System.Environment
import qualified Data.Map as M
sortOn f = map snd . sortBy (comparing fst) . map (f &&& id)
clip coord size = coord >= 0 && coord < size
valid size solution xy@(x, y) = and [clip x size, clip y size, isNothing (M.lookup xy solution)]
neighbors size solution xy = length . filter (valid size solution) $ sequence moves xy
moves = do
f <- [(+), subtract]
g <- [(+), subtract]
(x, y) <- [(1, 2), (2, 1)]
[f x *** g y]
solve size solution n xy = do
guard (valid size solution xy)
let solution' = M.insert xy n solution
sortedMoves = sortOn (neighbors size solution) (sequence moves xy)
if n == size * size
then [solution']
else sortedMoves >>= solve size solution' (n+1)
printBoard size solution = board [0..size-1] where
sqSize = size * size
elemSize = length (show sqSize)
separator = intercalate (replicate elemSize '-') (replicate (size + 1) "+")
pad n s = replicate (elemSize - length s) ' ' ++ s
elem xy = pad elemSize . show $ solution M.! xy
line y = concat . intersperseWrap "|" $ [elem (x, y) | x <- [0..size-1]]
board = unlines . intersperseWrap separator . map line
intersperseWrap s ss = s : intersperse s ss ++ [s]
go size = case solve size M.empty 1 (0, 0) of
[] -> "No solution found"
(s:_) -> printBoard size s
main = do
args <- getArgs
name <- getProgName
putStrLn $ case map reads args of
[] -> go 8
[[(size, "")]] -> go size
_ -> "Usage: " ++ name ++ " <size>"
Using Continuations
An efficient version (some 10x faster than the example Python solution) using continuations.
import Control.Applicative ((<$>))
import Control.Monad.Cont
import Control.Monad.ST
import Data.Array.ST
import Data.List
import Data.Ord
import Data.Ix
import Data.Map (Map, lookup, singleton, insert)
import System.Environment
type Square = (Int, Int)
type Board s = STUArray s (Int,Int) Int
type ChessM r s = ContT r (ST s)
successors :: Int -> Board s -> Square -> ChessM r s [Square]
successors n b s = sortWith (fmap length . succs) =<< succs s
where
sortWith f l = map fst <$> sortBy (comparing snd) <$> mapM (\x -> (,) x <$> f x) l
succs (i,j) = filterM (empty b) [ (i', j') | (dx,dy) <- [(1,2),(2,1)]
, i' <- [i+dx,i-dx] , j' <- [j+dy, j-dy]
, inRange ((1,1),(n,n)) (i',j') ]
stop :: Square -> Board s -> ChessM r s Int
stop s b = lift $ readArray b s
empty :: Board s -> Square -> ChessM r s Bool
empty b s = fmap (<1) . lift $ readArray b s
mark :: Square -> Int -> Board s -> ChessM r s ()
mark s k b = lift $ writeArray b s k
tour :: Int -> Int -> (Board s -> ChessM r s ()) -> Square -> Board s -> ChessM r s ()
tour n k exit s b | k > n*n = exit b
| otherwise = do ss <- successors n b s
try ss
where
try [] = return ()
try (x:xs) = do mark x k b
tour n (k+1) exit x b
-- failed
mark x 0 b
try xs
showBoard :: Int -> Board s -> ChessM r s String
showBoard n b = fmap (unlines . map unwords) . sequence . map sequence
$ [ [ fmt `fmap` stop (i,j) b | i <- [1..n] ] | j <- [1..n] ]
where
fmt i | i < 10 = ' ': show i
| otherwise = show i
main = do (n:_) <- map read `fmap` getArgs
s <- stToIO . flip runContT return $
(do b <- lift $ newArray ((1,1),(n,n)) 0
mark (1,1) 1 b
callCC $ \exit -> tour n 2 exit (1,1) b >> fail "No solution!"
showBoard n b)
putStrLn s
LogicT monad
A very short implementation using the LogicT monad
import Control.Monad.Logic
import Prelude hiding (lookup)
import Data.List hiding (lookup, insert)
import Data.Maybe
import Data.Ord
import Data.Ix
import Data.Map (Map, lookup, singleton, insert)
import System.Environment
type Square = (Int, Int)
type Board = Map Square Int
successors :: Int -> Board -> Square -> [Square]
successors n b = sortWith (length . succs) . succs
where
sortWith f = map fst . sortBy (comparing snd) . map (\x -> (x, f x))
succs (i,j) = [ (i', j') | (dx,dy) <- [(1,2),(2,1)]
, i' <- [i+dx,i-dx] , j' <- [j+dy, j-dy]
, empty (i',j') b, inRange ((1,1),(n,n)) (i',j') ]
stop :: Square -> Board -> Maybe Int
stop = lookup
empty :: Square -> Board -> Bool
empty s = isNothing . lookup s
mark :: Square -> Int -> Board -> Board
mark = insert
choose :: MonadPlus m => [a] -> m a
choose = msum . map return
tour :: Int -> Int -> Square -> Board -> Logic Board
tour n k s b | k > n*n = return b
| otherwise = do next <- choose $ successors n b s
tour n (k+1) next (mark next k b)
showBoard :: Int -> Board -> String
showBoard n b = unlines . map unwords
$ [ [ fmt . fromJust $ stop (i,j) b | i <- [1..n] ] | j <- [1..n] ]
where
fmt i | i < 10 = ' ': show i
| otherwise = show i
main = do (n:_) <- map read `fmap` getArgs
let b = observe . tour n 2 (1,1) $ singleton (1,1) 1
putStrLn $ showBoard n b
</haskell>