Data commit

Task/Amicable-pairs/Haskell/amicable-pairs-1.hs

@@ -0,0 +1,10 @@
+divisors :: (Integral a) => a -> [a]
+divisors n = filter ((0 ==) . (n `mod`)) [1 .. (n `div` 2)]
+
+main :: IO ()
+main = do
+  let range = [1 .. 20000 :: Int]
+      divs = zip range $ map (sum . divisors) range
+      pairs = [(n, m) | (n, nd) <- divs, (m, md) <- divs,
+               n < m, nd == m, md == n]
+  print pairs

Task/Amicable-pairs/Haskell/amicable-pairs-2.hs

@@ -0,0 +1,21 @@
+import Data.Bool (bool)
+
+amicablePairsUpTo :: Int -> [(Int, Int)]
+amicablePairsUpTo n =
+  let sigma = sum . properDivisors
+  in [1 .. n] >>=
+     (\x ->
+         let y = sigma x
+         in bool [] [(x, y)] (x < y && x == sigma y))
+
+properDivisors
+  :: Integral a
+  => a -> [a]
+properDivisors n =
+  let root = (floor . sqrt) (fromIntegral n :: Double)
+      lows = filter ((0 ==) . rem n) [1 .. root]
+  in init $
+     lows ++ drop (bool 0 1 (root * root == n)) (reverse (quot n <$> lows))
+
+main :: IO ()
+main = mapM_ print $ amicablePairsUpTo 20000