{-# LANGUAGE BangPatterns #-} module Randomish ( randomishInts , randomishDoubles) where import Data.Word import Data.Vector.Unboxed (Vector) import qualified Data.Vector.Unboxed.Mutable as MV import qualified Data.Vector.Unboxed as V import qualified Data.Vector.Generic as G -- | Use the "minimal standard" Lehmer generator to quickly generate some random -- numbers with reasonable statistical properties. By "reasonable" we mean good -- enough for games and test data, but not cryptography or anything where the -- quality of the randomness really matters. -- -- From "Random Number Generators: Good ones are hard to find" -- Stephen K. Park and Keith W. Miller. -- Communications of the ACM, Oct 1988, Volume 31, Number 10. -- randomishInts :: Int -- Length of vector. -> Int -- Minumum value in output. -> Int -- Maximum value in output. -> Int -- Random seed. -> Vector Int -- Vector of random numbers. randomishInts !len !valMin' !valMax' !seed' = let -- a magic number (don't change it) multiplier :: Word64 multiplier = 16807 -- a merzenne prime (don't change it) modulus :: Word64 modulus = 2^(31 :: Integer) - 1 -- if the seed is 0 all the numbers in the sequence are also 0. seed | seed' == 0 = 1 | otherwise = seed' !valMin = fromIntegral valMin' !valMax = fromIntegral valMax' + 1 !range = valMax - valMin {-# INLINE f #-} f x = multiplier * x `mod` modulus in G.create \$ do vec <- MV.new len let go !ix !x | ix == len = return () | otherwise = do let x' = f x MV.write vec ix \$ fromIntegral \$ (x `mod` range) + valMin go (ix + 1) x' go 0 (f \$ f \$ f \$ fromIntegral seed) return vec -- | Generate some randomish doubles with terrible statistical properties. -- This is good enough for test data, but not much else. randomishDoubles :: Int -- Length of vector -> Double -- Minimum value in output -> Double -- Maximum value in output -> Int -- Random seed. -> Vector Double -- Vector of randomish doubles. randomishDoubles !len !valMin !valMax !seed = let range = valMax - valMin mx = 2^(30 :: Integer) - 1 mxf = fromIntegral mx ints = randomishInts len 0 mx seed in V.map (\n -> valMin + (fromIntegral n / mxf) * range) ints