# Haskell Quiz/Numeric Maze/Solution Ninju

### From HaskellWiki

module Main where import System.Environment import Data.List data Operator = AddTwo | Double | Halve main :: IO () main = do args <- getArgs if length args == 2 then let [a,b] = map read args in print $ solve a b else putStrLn "Usage: solve START TARGET" apply :: Operator -> Integer -> Integer apply AddTwo x = x + 2 apply Double x = x * 2 apply Halve x = x `div` 2 valid :: Operator -> Integer -> Bool valid Halve x = even x valid _ _ = True solve :: Integer -> Integer -> [Integer] solve a b = solve' [[a]] b [a] solve' :: [[Integer]] -> Integer -> [Integer] -> [Integer] solve' paths target seen = case find ((== target) . last) paths of Just path -> path Nothing -> let newPaths = filter ((`notElem` seen) . last) $ concatMap buildPathsFrom paths newSeen = seen ++ map last newPaths in solve' newPaths target newSeen buildPathsFrom :: [Integer] -> [[Integer]] buildPathsFrom path = let n = last path in [ path ++ [ apply operator n ] | operator <- [AddTwo, Double, Halve], valid operator n ]