-- The Timber compiler <timber-lang.org>
--
-- Copyright 2008-2009 Johan Nordlander <nordland@csee.ltu.se>
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module RandomGenerator where

struct Generator a where
  next :: Request a
  
-- Lehmer/Schrage random number generator. Simple and not too bad.
-- baseGen generates integers in the range [1..m-1] on 32 bit machines.
-- The n-th number produced is (seed * a^n) mod m but with calculations organised to avoid overflow.
-- 7^5 is a primitive root of the prime m, so this generator has period m - 1.

baseGen :: Int -> Class (Generator Int)
baseGen 0 = raise 1
baseGen seed = class
  a = 16807        -- = 7^5
  m = 2147483647   -- = 2^31 - 1
  q = 127773       -- = m `div` a
  r = 2836         -- = m `mod` a

  state := seed
  
  next = request
    tmp = a * (state `mod` q) - r * (state `div` q)
    state := if tmp > 0 then tmp else tmp + m
    result state

  result Generator{..}