Day 15 - takes 2+ minutes to run and froze my computer twice but it works I guess...

15.hs

@@ -0,0 +1,21 @@
+import Data.Int (Int16)
+
+gen :: Int -> Int -> Int -> [Int]
+gen divisor factor seed = iterate (next divisor factor) seed
+    where next d f i = (f * i) `mod` d
+
+judge :: Int -> [Int] -> [Int] -> Int
+judge n a b = length . filter (uncurry eq) . take n $ zip a b
+    where eq i j = (fromIntegral i :: Int16) == (fromIntegral j :: Int16)
+
+divisible :: Int -> Int -> Bool
+divisible d = (== 0) . (`mod` d)
+
+main :: IO ()
+main = do
+    let genA    = gen 2147483647 16807 634
+        genB    = gen 2147483647 48271 301
+        gen4A   = filter (divisible 4) genA
+        gen8B   = filter (divisible 8) genB
+    print $ judge 40000000 genA  genB 
+    print $ judge 5000000  gen4A gen8B