Main.hs

-- Mark I lazy wheel-sieve.
-- Colin Runciman (colin@cs.york.ac.uk); March 1996.
-- See article "Lazy wheel sieves and spirals of primes" (to appear, JFP).

import System.Environment


data Wheel = Wheel Int [Int]


primes :: [Int]
primes = sieve wheels primes squares

sieve (Wheel s ns:ws) ps qs =
  [n' | o <- s:[s*2,s*3..(head ps-1)*s],
        n <- ns,
        n'<- [n+o], noFactor n'] 
  ++
  sieve ws (tail ps) (tail qs)
  where
  noFactor = if s<=2 then const True else notDivBy ps qs

notDivBy (p:ps) (q:qs) n =
  q > n || n `mod` p > 0 && notDivBy ps qs n

squares :: [Int]
squares = [p*p | p<-primes]

wheels :: [Wheel]
wheels = Wheel 1 [1] : zipWith nextSize wheels primes 

nextSize (Wheel s ns) p =
  Wheel (s*p) ns'
  where
  ns' = [n' | o <- [0,s..(p-1)*s],
              n <- ns,
              n' <- [n+o], n'`mod`p > 0]

main = do
	[arg] <- getArgs
	print (primes!!((read arg) :: Int))