Random.hs

module Random (randomListMember, rollDie, makeGeneratorList) where


import System.Random

import Types.Common
import Types.World
import Types.Items

-- select random element from list, undefined on empty lists.
randomListMember :: [a] -> StdGen -> (a, StdGen)
randomListMember [] _ = error "Attempted to pick random member of an empty list in function \"randomListMember\""
randomListMember as gen = (a, newGen)
  where
    (index, newGen) = randomR (0, length as - 1) gen
    a = as !! index
    
-- Roll a dice or multiple die.
rollDie :: Dice -> StdGen -> (Int, StdGen)
rollDie dice gen = (result, lastGen)
  where
    (nofDie, maxDice) = dDie dice
    modifier = dMod dice
    
    (generators, lastGen) = makeGeneratorList gen nofDie

    
    roll = foldl (+) modifier $ map (fst . randomR (1, maxDice)) generators
    
    result = max roll 0 -- no negative rolls if  negative modifiers + low rolls.
    

-- splits a generator into n generators.
makeGeneratorList :: StdGen -> Int -> ([StdGen], StdGen)
makeGeneratorList gen n = makeGeneratorList' ([], gen) n
    
makeGeneratorList' :: ([StdGen], StdGen) -> Int -> ([StdGen], StdGen)
makeGeneratorList' g 0 = g
makeGeneratorList' (genList, gen) n = makeGeneratorList' (gen1:genList, gen2) (n-1)
  where
    (gen1, gen2) = split gen