Here is a variant that uses mersenne-random-pure64 and works less than
2x slower than C++:
- You don't need to compute samples count an extra time
- You don't need to assemble double pairs from a list
- Notice the strictness in randomDoublePairs: it doubled performance
{-# LANGUAGE BangPatterns #-}
import System.Random.Mersenne.Pure64
import System( getArgs )
import Data.List( foldl' )
isInCircle :: (Double,Double) -> Bool
isInCircle (!x,!y) = sqrt (x*x + y*y) <= 1.0
accumulateHit :: Int -> (Double,Double) -> Int
accumulateHit !hits pair = if isInCircle pair then hits + 1 else hits
monteCarloPi :: Int -> [(Double,Double)] -> Double
monteCarloPi n xs = 4.0 * fromIntegral hits / fromIntegral n
where hits = foldl' accumulateHit 0 . take n $ xs
randomDoublePairs g = let
!(!x,!g') = randomDouble g
!(!y,!g'') = randomDouble g'
in (x,y):randomDoublePairs g''
main = do
samples <- (read . head) `fmap` getArgs
randomNumbers <- randomDoublePairs `fmap` newPureMT
putStrLn . show $ monteCarloPi samples randomNumbers
jkff@*****:~/montecarlo$ time ./mc-hs 10000000
3.1417088
real 0m1.141s
user 0m1.140s
sys 0m0.000s
jkff@*****:~/montecarlo$ time ./mc 10000000
10000000
3.14113
real 0m0.693s
user 0m0.690s
sys 0m0.000s
2009/2/26 Ben Lippmeier
On 26/02/2009, at 9:27 PM, haskell@kudling.de wrote:
Currently i can only imagine to define a data type in order to use unboxed Ints instead of the accumulator tuple.
That would probably help a lot. It would also help to use two separate Double# parameters instead of the tuple.
The thing is that i don't see in the profile output yet what to improve. There are some allocations going on in "main", but i don't know what causes it.
The first thing I would do is replace your isInCircle :: (Floating a, Ord a) => (a,a) -> Bool with isInCircle :: (Double, Double) -> Bool
Can you point me to why that matters?
At the machine level, GHC treats the (Floating a, Ord a) as an extra argument to the function. This argument holds function pointers that tell it how to perform multiplication and <= for the unknown type 'a'. If you use Double instead of 'a', then it's more likely to use the actual machine op.
Ben.
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