64 bits of entropy doesn't seem like much at all for a RNG. I thought
the idea was StdGen could be fully abstract.
John
On Mon, Feb 6, 2012 at 9:07 AM, Bas van Dijk
Dear Ryan, all,
To quote the documentation of System.Random: "The Show and Read instances of StdGen provide a primitive way to save the state of a random number generator."
Primitive it is indeed. Currently the only way to serialize (using cereal or binary) a StdGen is to convert it to a String and then serialize this String. This takes up many more bytes than are actually needed.
I would like to propose a more efficient way of serializing the state of the random number generator by providing the following two functions:
getStdGenState :: StdGen -> (Int32, Int32) getStdGenState (StdGen s1 s2) = (s1, s2)
stdGenFromState :: (Int32, Int32) -> StdGen stdGenFromState (s1, s2) = StdGen s1 s2
Of course this satisfies: (stdGenFromState . getStdGenState) g == g
Alternatively, instead of using (Int32, Int32) we can use an Int64:
getStdGenState :: StdGen -> Int64 getStdGenState (StdGen s1 s2) = fromIntegral s1 `unsafeShiftL` 32 .|. fromIntegral s2 #if !MIN_VERSION_base(4,5,0) where unsafeShiftL = shiftL #endif
stdGenFromState :: Int64 -> StdGen stdGenFromState s = StdGen (fromIntegral (s `unsafeShiftR` 32)) (fromIntegral s) #if !MIN_VERSION_base(4,5,0) where unsafeShiftR = shiftR #endif
I don't really care which one we use but I think the latter is a bit easier to use.
Oh, I don't really care about the names either.
Regards,
Bas
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