{-# LANGUAGE ScopedTypeVariables, BangPatterns  #-}
module Random
	(randomInts)
where
import qualified Data.Vector.Unboxed.Mutable	as MV
import qualified Data.Vector.Unboxed		as V
import Data.Vector.Unboxed			(Vector)
import qualified Data.Vector.Generic		as G		
import Data.Word
	

-- | 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.
--
randomInts 
	:: Int 		-- Length of vector.
	-> Int 		-- Minumum value in output.
	-> Int 		-- Maximum value in output.
	-> Int 		-- Random seed.	
	-> Vector Int	-- Vector of random numbers.

randomInts !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 - 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 $ fromIntegral seed)
	return vec