Data commit

Task/Self-describing-numbers/Haskell/self-describing-numbers-1.hs

@@ -0,0 +1,17 @@
+import Data.Char
+
+count :: Int -> [Int] -> Int
+count x = length . filter (x ==)
+
+isSelfDescribing :: Integer -> Bool
+isSelfDescribing n = nu == f
+  where
+    nu = digitToInt <$> show n
+    f = (`count` nu) <$> [0 .. length nu - 1]
+
+main :: IO ()
+main = do
+  print $
+    isSelfDescribing <$>
+    [1210, 2020, 21200, 3211000, 42101000, 521001000, 6210001000]
+  print $ filter isSelfDescribing [0 .. 4000000]

Task/Self-describing-numbers/Haskell/self-describing-numbers-2.hs

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+import Control.Monad (forM_, replicateM)
+import Data.Char (intToDigit)
+
+count :: Int -> [Int] -> Int
+count x = length . filter (x ==)
+
+-- All the combinations of n digits of base n.
+-- A base-n number is represented as a list of ints,
+-- one per digit
+allBaseNNumsOfLength :: Int -> [[Int]]
+allBaseNNumsOfLength =
+  replicateM
+    <*> (enumFromTo 0 . subtract 1)
+
+isSelfDescribing :: [Int] -> Bool
+isSelfDescribing num =
+  all (\(i, x) -> x == count i num) $
+    zip [0 ..] num
+
+-- Translated back into an integer in base-10
+decimalize :: [Int] -> Int
+decimalize = read . map intToDigit
+
+main :: IO ()
+main =
+  (print . concat) $
+    map decimalize
+      . filter isSelfDescribing
+      . allBaseNNumsOfLength
+      <$> [1 .. 8]