all tasks

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

@@ -0,0 +1,41 @@
+import Data.List (sort)
+
+primes = 2:filter isPrime [3,5..] where
+    isPrime n = f n primes where
+        f n (p:ps)
+            | p*p > n = True
+            | n`mod`p == 0 = False
+            | otherwise = f n ps
+
+primeFactors n = f n primes where
+    f n (p:ps)
+        | p*p > n = if n == 1 then [] else [(n,1)]
+        | n`mod`p == 0 = (p,e):f res ps
+        | otherwise = f n ps
+        where (e,res) = ppower (n`div`p) p 1
+    ppower n p e
+        | n`mod`p /= 0 = (e,n)
+        | otherwise = ppower (n`div`p) p (e+1)
+
+factors n = comb (primeFactors n) where
+    comb [] = [1]
+    comb ((p,e):others) = [a*b | b<-map (p^) [0 .. e], a<-comb others]
+
+ndigit 0 = 0
+ndigit n = 1 + ndigit (n`div`10)
+
+fangs n
+    | odd w = []
+    | otherwise = map (\x->(x,n`div`x)) $ filter isfang (factors n) where
+        isfang x = x > xmin && x < y && y < ymax &&      -- same length
+            ((x`div`10)/=0 || (y`div`10)/=0) &&          -- not zero-ended
+            sort (show n) == sort ((show x) ++ (show y)) -- same digits
+            where y = n`div`x
+        w = ndigit n
+        xmin = 10^(w`div`2-1)
+        ymax = 10^(w`div`2)
+
+vampires = filter ((0<).length.fangs) [1..]
+
+main = mapM (\n->print (n,fangs n)) $
+        ((take 25 vampires) ++ [16758243290880, 24959017348650, 14593825548650])