Problem8.hs

module ProjectEuler.Problem8.Problem8 where

-- Split a number into a list of its digits
-- https://stackoverflow.com/questions/3963269/split-a-number-into-its-digits-with-haskell
digits :: Integral a => a -> [a]
digits 0 = []
digits x = digits (x `div` 10) ++ [x `mod` 10]

-- Find the k adjacent digits in n with the largest product; return this product
largestProduct :: (Integral a) => a -> Int -> a
largestProduct n = lp' (digits n)

-- Recursive helper function for largestProduct, takes list of digits rather than single number
lp' :: (Integral a) => [a] -> Int -> a
lp' ds k | length ds <= k = product ds
         | otherwise = max (product (take k ds)) (lp' (tail ds) k)

-- The 1000 digit number given on Project Euler
num :: Integer
num = 7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450

main :: IO ()
main = do
    print (largestProduct num 13)