Data commit

Task/Van-Eck-sequence/Haskell/van-eck-sequence-1.hs

@@ -0,0 +1,13 @@
+import Data.List (elemIndex)
+import Data.Maybe (maybe)
+
+vanEck :: Int -> [Int]
+vanEck n = reverse $ iterate go [] !! n
+  where
+    go [] = [0]
+    go xxs@(x:xs) = maybe 0 succ (elemIndex x xs) : xxs
+
+main :: IO ()
+main = do
+  print $ vanEck 10
+  print $ drop 990 (vanEck 1000)

Task/Van-Eck-sequence/Haskell/van-eck-sequence-2.hs

@@ -0,0 +1,22 @@
+{-# LANGUAGE TupleSections #-}
+
+import Data.List (mapAccumL)
+import qualified Data.Map.Strict as M hiding (drop)
+import Data.Maybe (maybe)
+
+--------------------- VAN ECK SEQUENCE -------------------
+
+vanEck :: [Int]
+vanEck = 0 : snd (mapAccumL go (0, M.empty) [1 ..])
+  where
+    go (x, dct) i =
+      ((,) =<< (, M.insert x i dct))
+        (maybe 0 (i -) (M.lookup x dct))
+
+--------------------------- TEST -------------------------
+main :: IO ()
+main =
+  mapM_ print $
+    fmap
+      ((drop . subtract 10) <*> flip take vanEck)
+      [10, 1000, 10000, 100000, 1000000]