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-- Largest product in a series
-- Problem 8
-- Find the greatest product of five consecutive digits in the 1000-digit number.
--------------------------------------------------------------------------------

import Data.Char
import Data.Bits

n = 
  "73167176531330624919225119674426574742355349194934" ++
  "96983520312774506326239578318016984801869478851843" ++
  "85861560789112949495459501737958331952853208805511" ++
  "12540698747158523863050715693290963295227443043557" ++
  "66896648950445244523161731856403098711121722383113" ++
  "62229893423380308135336276614282806444486645238749" ++
  "30358907296290491560440772390713810515859307960866" ++
  "70172427121883998797908792274921901699720888093776" ++
  "65727333001053367881220235421809751254540594752243" ++
  "52584907711670556013604839586446706324415722155397" ++
  "53697817977846174064955149290862569321978468622482" ++
  "83972241375657056057490261407972968652414535100474" ++
  "82166370484403199890008895243450658541227588666881" ++
  "16427171479924442928230863465674813919123162824586" ++
  "17866458359124566529476545682848912883142607690042" ++
  "24219022671055626321111109370544217506941658960408" ++
  "07198403850962455444362981230987879927244284909188" ++
  "84580156166097919133875499200524063689912560717606" ++
  "05886116467109405077541002256983155200055935729725" ++
  "71636269561882670428252483600823257530420752963450"

-- number of digits in product
m = 5

-- convert known ascii decimal digit to integer
digit_to_integer :: Char -> Integer
digit_to_integer c = fromIntegral $ (ord c) .&. 0xf


solve n pmax =
  let
    d5 = take 5 n
    i5 = map digit_to_integer d5
    p = product i5
  in
    case () of
      _ | (length d5) /= 5 -> pmax
      _ | (p > pmax)       -> solve (tail n) p
      _                    -> solve (tail n) pmax

answer = solve n 0

main = print answer

{-
This is getting easier.
-}


