NightRa · October 13, 2015 14:30 · Oct 13, 2015
diff --git a/Fib.hs b/Fib.hs
@@ -0,0 +1,39 @@
+module Fib where
+
+import qualified Data.Vector.Unboxed as V
+import qualified Data.Vector.Fusion.Stream as S
+import qualified Data.Vector.Fusion.Stream.Monadic as SM
+import Data.Vector.Fusion.Stream.Size
+import Data.Vector.Fusion.Util
+{-
+euler2 n = sum $ filter even $ takeWhile (<n) $ fibs
+How can I write this and fuse into what's below?
+-}
+fibsStep :: (Int, Int) -> S.Step (Int,Int) Int
+fibsStep (a,b) = S.Yield a (b, a + b)
+
+fibs :: S.Stream Int
+fibs = SM.Stream (Id . fibsStep) (1,2) Unknown
+
+euler2 :: Int -> Int
+euler2 n = S.foldl' (+) 0 $ S.filter even $ S.takeWhile (<n) fibs
+{-
+Yaay!!!
+This compiles to what's below, a tight loop in cpu registers:
+-}
+{-
+euler2 :: Int -> Int
+euler2 n = go 0 1 2
+           where go sum a b = if a < n
+                              then
+                                if even a
+                                then go (sum + a) b (a + b)
+                                else go sum       b (a + b)
+                              else sum
+-}
+main :: IO ()
+main = do
+  t <- readLn
+  ns <- V.replicateM t readLn
+  V.forM_ ns $ \n ->
+    print $ euler2 n
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