Data commit

Task/Vampire-number/Haskell/vampire-number.hs

@@ -0,0 +1,49 @@
+import Data.List (sort)
+import Control.Arrow ((&&&))
+
+-- VAMPIRE NUMBERS ------------------------------------------------------------
+vampires :: [Int]
+vampires = filter (not . null . fangs) [1 ..]
+
+fangs :: Int -> [(Int, Int)]
+fangs n
+  | odd w = []
+  | otherwise = ((,) <*> quot n) <$> filter isfang (integerFactors n)
+  where
+    ndigit :: Int -> Int
+    ndigit 0 = 0
+    ndigit n = 1 + ndigit (quot n 10)
+    w = ndigit n
+    xmin = 10 ^ (quot w 2 - 1)
+    xmax = xmin * 10
+    isfang x =
+      x > xmin &&
+      x < y &&
+      y < xmax && -- same length
+      (quot x 10 /= 0 || quot y 10 /= 0) && -- not zero-ended
+      sort (show n) == sort (show x ++ show y)
+      where
+        y = quot n x
+
+-- FACTORS --------------------------------------------------------------------
+integerFactors :: Int -> [Int]
+integerFactors n
+  | n < 1 = []
+  | otherwise =
+    lows ++
+    (quot n <$>
+     (if intSquared == n -- A perfect square,
+        then tail -- and cofactor of square root would be redundant.
+        else id)
+       (reverse lows))
+  where
+    (intSquared, lows) =
+      (^ 2) &&& (filter ((0 ==) . rem n) . enumFromTo 1) $
+      floor (sqrt $ fromIntegral n)
+
+-- TEST -----------------------------------------------------------------------
+main :: IO [()]
+main =
+  mapM
+    (print . ((,) <*>) fangs)
+    (take 25 vampires ++ [16758243290880, 24959017348650, 14593825548650])