p008.hs

{- 
 - Solution to Project Euler problem 8
 - By Nayuki Minase
 - 
 - http://nayuki.eigenstate.org/page/project-euler-solutions
 - https://github.com/nayuki/Project-Euler-solutions
 -}

import Data.Char (ord)


main = putStrLn (show ans)
ans = foldl1 max (map product reversedWantedSubstrings)

-- [7, 3, 1, 6, 7, 1, 7, 6, 5, 3, ...]
digits = map (\c -> (ord c) - (ord '0')) number

-- [[], [7], [3,7], [1,3,7], [6,1,3,7], ...]
reversedPrefixes = scanl (\lst n -> n:lst) [] digits

-- [[7,6,1,3,7], [1,7,6,1,3], [7,1,7,6,1], ...]
reversedWantedSubstrings = [take 5 lst | lst <- reversedPrefixes, (length lst) >= 5]

number = "7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450"