Haskell Quiz/Chess960/Solution Sjanssen
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import Data.List
import System.Random
import System
bishopRule xs = even i /= even j
where
[i, j] = elemIndices 'B' xs
kingRule xs = i < k && k < j
where
[i, j] = elemIndices 'R' xs
[k] = elemIndices 'K' xs
white = uniq . filter kingRule . filter bishopRule . permutations $ pieces
where
pieces = "KQRRNNBB"
printArrangement x = do
putStrLn x
putStrLn . reverse $ x
main = do
args <- getArgs
case args of
["all"] -> mapM_ printArrangement white
[] -> randomRIO (0, 959) >>= printArrangement . (white!!)
-- Utility functions:
uniq :: Ord a => [a] -> [a]
uniq = map head . group . sort
allInsertions x [] = [[x]]
allInsertions x (y:ys) = [x:y:ys] ++ map (y:) (allInsertions x ys)
permutations [] = [[]]
permutations (x:xs) = concatMap (allInsertions x) . permutations $ xs