tasks a-s

Task/Self-referential-sequence/Haskell/self-referential-sequence.hs

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+import Data.Set (Set, member, insert, empty)
+import Data.List (group, sort)
+
+step :: String -> String
+step = concatMap (\list -> show (length list) ++ [head list]) . group . sort
+
+findCycle :: (Ord a) => [a] -> [a]
+findCycle = aux empty where
+	aux set (x : xs)
+		| x `member` set = []
+		| otherwise = x : aux (insert x set) xs
+
+select :: [[a]] -> [[a]]
+select = snd . foldl (\(len, acc) xs -> case len `compare` length xs of
+		LT -> (length xs, [xs])
+		EQ -> (len, xs : acc)
+		GT -> (len, acc)) (0, [])
+
+main :: IO ()
+main = mapM_ (mapM_ print) $ -- Print out all the numbers
+	select $ -- find the longest ones
+	map findCycle $ -- run the sequences until there is a repeat
+	map (iterate step) $ -- produce the sequence
+	map show -- turn the numbers into digits
+	[1..1000000] -- The input seeds