-- The number, 197, is called a circular prime because all rotations of the
-- digits: 197, 971, and 719, are themselves prime.
--
-- There are thirteen such primes below 100: 2, 3, 5, 7, 11, 13, 17, 31, 37,
-- 71, 73, 79, and 97.
--
-- How many circular primes are there below one million?

import Euler
import Data.Array.Unboxed
import Data.Array.ST

rotations n = map (fromDigits . take (length digits))
            $ take (length digits)
            $ tails (cycle digits)
   where digits = toDigits n

main = do
   let limit      = 1000000 - 1
   let isPrimeArr = isPrimeArray limit
   let isPrime n  = (isPrimeArr!n) /= 0
   let circular n = all isPrime $ rotations n
   print $ length $ filter circular [2..limit]