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{-
This module implements a (good) random number generator.
The June 1988 (v31 #6) issue of the Communications of the ACM has an
article by Pierre L'Ecuyer called, "Efficient and Portable Combined
Random Number Generators". Here is the Portable Combined Generator of
L'Ecuyer for 32-bit computers. It has a period of roughly 2.30584e18.
Transliterator: Lennart Augustsson
Modified 10/11/97 by Alastair Reid:
Added random and randomIO based on GHC's implementation of the
standard Random library.
-}
module Random( randomIO, random ) where
primitive getRandomSeed :: IO Integer
randomIO :: (Integer, Integer) -> IO [Integer]
randomIO lh = do
seed <- getRandomSeed
return (random lh seed)
random :: (Integer, Integer) -> Integer -> [Integer]
random (l, h) s =
if l > h then error "Random.random: Empty interval" else
if s < 0 then random (l, h) (-s) else
let (q, s1) = s `divMod` 2147483562
s2 = q `mod` 2147483398
k = h-l + 1
b = 2147483561
n = iLogBase b k
f is = let (xs, is') = splitAt n is
in foldr (\ i r -> fromInt i + r * b) 0 xs `mod` k + l : f is'
in f (randomInts (toInt (s1+1)) (toInt (s2+1)))
iLogBase b i = if i < b then 1 else 1 + iLogBase b (i `div` b)
-- Use seeds s1 in 1..2147483562 and s2 in 1..2147483398 to generate
-- an infinite list of random Ints.
randomInts :: Int -> Int -> [Int]
randomInts s1 s2 =
if 1 <= s1 && s1 <= 2147483562 then
if 1 <= s2 && s2 <= 2147483398 then
rands s1 s2
else
error "randomInts: Bad second seed."
else
error "randomInts: Bad first seed."
rands :: Int -> Int -> [Int]
rands s1 s2 = z' : rands s1'' s2''
where z' = if z < 1 then z + 2147483562 else z
z = s1'' - s2''
k = s1 `quot` 53668
s1' = 40014 * (s1 - k * 53668) - k * 12211
s1'' = if s1' < 0 then s1' + 2147483563 else s1'
k' = s2 `quot` 52774
s2' = 40692 * (s2 - k' * 52774) - k' * 3791
s2'' = if s2' < 0 then s2' + 2147483399 else s2'
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