# Haskell Quiz/Numeric Maze/Solution Ninju

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< Haskell Quiz | Numeric Maze(Difference between revisions)

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[[Category:Haskell Quiz solutions|Numeric Maze]] | [[Category:Haskell Quiz solutions|Numeric Maze]] | ||

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<haskell> | <haskell> | ||

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import System.Environment | import System.Environment | ||

import Data.List | import Data.List | ||

+ | |||

+ | data Operator = AddTwo | Double | Halve | ||

main :: IO () | main :: IO () | ||

main = do args <- getArgs | main = do args <- getArgs | ||

− | if length args == 2 | + | if length args == 2 |

− | then | + | then let [a,b] = map read args |

− | + | in print $ solve a b | |

else putStrLn "Usage: solve START TARGET" | else putStrLn "Usage: solve START TARGET" | ||

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− | |||

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− | + | 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 :: Integer -> Integer -> [Integer] | ||

− | solve a b = solve' [[a]] b | + | 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 ] | ||

</haskell> | </haskell> |

## Latest revision as of 08:23, 27 November 2009

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 ]