module Level.Shuffle where

import System.Random
import qualified Data.Vector as V
import Data.Vector ((//), (!))

data Section t = FixedSection [t] | ShuffleableSection [t]

-- This intentionally is biased toward leaving some items in original order.
shuffle :: StdGen -> Section t -> [t]
shuffle _ (FixedSection l) = l
shuffle r (ShuffleableSection l) = swap (length l) r (V.fromList l)
  where
	swap 0 _ v = V.toList v
	swap n g v = 
		let range = (0, V.length v - 1)
		    (p1, g') = randomR range g
		    (p2, g'') = randomR range g'
		in swap (n - 1) g'' (v // [(p1, v ! p2), (p2, v ! p1)])

asis :: Section t -> [t]
asis (FixedSection l) = l
asis (ShuffleableSection l) = l