From 74ea0e872306a71b9b48ba0d7cbe499c0a5be3dd Mon Sep 17 00:00:00 2001
From: Bryan O'Sullivan <bos@serpentine.com>
Date: Sat, 19 Sep 2009 22:27:48 +0000
Subject: [PATCH] Time the system PRNG, for the lulz.

---
 tests/T.hs | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/tests/T.hs b/tests/T.hs
index 9ed8aa96..141f8cc8 100644
--- a/tests/T.hs
+++ b/tests/T.hs
@@ -9,7 +9,8 @@ import Control.Monad.ST
 import Data.Time.Clock.POSIX
 import Data.Array.Vector
 import Statistics.RandomVariate
-import System.Random.Mersenne
+import qualified System.Random as R
+import qualified System.Random.Mersenne as M
 
 getTime :: IO Double
 getTime = (fromRational . toRational) `fmap` getPOSIXTime
@@ -21,9 +22,9 @@ time act = do
   e <- ret `seq` getTime
   return (e-s, ret)
 
-count = 250 * 1000000
+count = 100 * 1000000
 
-type T = Word32
+type T = Double
 
 --summ :: [T] -> T
 --summ = foldl' (+) 0
@@ -36,6 +37,18 @@ loop n act = go 0 0
       d <- act
       go (i+1) (s+d)
 
+system :: IO Double
+system = do
+    gen <- R.getStdGen
+    let a :*: g = loop 0 0 gen
+    R.setStdGen g
+    return a
+  where loop !i !a !g | i == k = a :*: g
+                      | otherwise = let (b,h) = R.random g
+                                    in loop (i+1) (a+b) h
+        k = count `div` 100
+                 
+
 mwc :: Word32 -> IO T
 mwc k = return $! runST go where
   go = do
@@ -50,11 +63,13 @@ mwca k = return $! runST go where
 
 mersenne :: IO T
 mersenne = do
-  gen <- getStdGen
-  loop count (random gen)
+  gen <- M.getStdGen
+  loop count (M.random gen)
 
 main = do
   forM_ [1..3] $ \n -> do
+    putStr "system: "
+    print =<< time system
     putStr "mwc: "
     print =<< time (mwc n)
     putStr "mwca: "