pills.hs

import System.Random
import Control.Arrow
import Control.Monad
import Text.Printf

run :: Int -> Int -> IO ()
run newPills pillsMin = step newPills pillsMin 1 [] []

step :: Int -> Int -> Int -> [Double] -> [Double] -> IO ()
step newPills pillsMin day pills eaten = do
  -- Morning: half a day has passed and the pills have aged again; also, if
  -- there's less than X pills left, some more pills are added
  pills <- return $ if length pills <= pillsMin
             then map (+0.5) pills ++ replicate newPills 0
             else map (+0.5) pills
  -- Night: half a day has passed, the pills have aged
  pills <- return (map (+0.5) pills)
  -- Eat a pill (chosen randomly)
  i <- randomRIO (0, length pills - 1)
  eaten <- return ((pills !! i) : eaten)
  pills <- return (uncurry (++) $ second tail $ splitAt i pills)
  -- Stats are printed and the next day begins
  when (day `mod` 10 == 0) $
    printf "%d: average = %.2f, max = %.2f\n"
      day (sum eaten / fromIntegral day) (maximum eaten)
  unless (day == 360) $
    step newPills pillsMin (day+1) pills eaten