57 lines
1.7 KiB
Haskell
57 lines
1.7 KiB
Haskell
import Data.List (inits, maximumBy)
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import Data.Maybe (fromMaybe)
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repstring :: String -> Maybe String
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-- empty strings are not rep strings
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repstring [] = Nothing
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-- strings with only one character are not rep strings
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repstring [_] = Nothing
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repstring xs
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| any (`notElem` "01") xs = Nothing
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| otherwise = longest xs
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where
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-- length of the original string
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lxs = length xs
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-- half that length
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lq2 = lxs `quot` 2
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-- make a string of same length using repetitions of a part
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-- of the original string, and also return the substring used
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subrepeat x = (x, take lxs $ concat $ repeat x)
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-- check if a repeated string matches the original string
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sndValid (_, ys) = ys == xs
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-- make all possible strings out of repetitions of parts of
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-- the original string, which have max. length lq2
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possible = map subrepeat . take lq2 . tail . inits
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-- filter only valid possibilities, and return the substrings
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-- used for building them
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valid = map fst . filter sndValid . possible
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-- see which string is longer
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compLength a b = compare (length a) (length b)
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-- get the longest substring that, repeated, builds a string
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-- that matches the original string
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longest ys = case valid ys of
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[] -> Nothing
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zs -> Just $ maximumBy compLength zs
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main :: IO ()
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main =
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mapM_ processIO examples
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where
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examples =
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[ "1001110011",
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"1110111011",
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"0010010010",
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"1010101010",
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"1111111111",
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"0100101101",
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"0100100",
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"101",
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"11",
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"00",
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"1"
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]
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process = fromMaybe "Not a rep string" . repstring
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processIO xs = do
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putStr (xs <> ": ")
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putStrLn $ process xs
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