Euler question 8

src/projectEuler/question8.hs

@@ -0,0 +1,66 @@
+{-
+The four adjacent digits in the 1000-digit number that have the greatest product are 9 x 9 x 8 x 9 = 5832.
+
+73167176531330624919225119674426574742355349194934
+96983520312774506326239578318016984801869478851843
+85861560789112949495459501737958331952853208805511
+12540698747158523863050715693290963295227443043557
+66896648950445244523161731856403098711121722383113
+62229893423380308135336276614282806444486645238749
+30358907296290491560440772390713810515859307960866
+70172427121883998797908792274921901699720888093776
+65727333001053367881220235421809751254540594752243
+52584907711670556013604839586446706324415722155397
+53697817977846174064955149290862569321978468622482
+83972241375657056057490261407972968652414535100474
+82166370484403199890008895243450658541227588666881
+16427171479924442928230863465674813919123162824586
+17866458359124566529476545682848912883142607690042
+24219022671055626321111109370544217506941658960408
+07198403850962455444362981230987879927244284909188
+84580156166097919133875499200524063689912560717606
+05886116467109405077541002256983155200055935729725
+71636269561882670428252483600823257530420752963450
+
+Find the thirteen adjacent digits in the 1000-digit number that have the greatest product.
+What is the value of this product?
+-}
+module Main where
+
+import           Data.Char       (ord)
+import           Data.List       (sort)
+import           Data.List.Split (splitOn)
+
+thousandDigitNum :: Integer
+thousandDigitNum = 7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450
+
+digits :: Integer -> [Integer]
+digits = go []
+  where
+    go acc 0 = acc
+    go acc n = go (digit:acc) rest
+      where
+        digit = n `rem` 10
+        rest  = n `div` 10
+
+main :: IO ()
+main = print ans
+
+ans :: [[Integer]]
+ans = (windowsOf 13 . digits) thousandDigitNum
+-- "9878799272442"
+
+-- what dose it mean to have the greatest product in adjacent digits?
+--  why ask it like that???
+--  doesn't have 0
+--  otherwise is the largest number in the larger 1000-digit number
+chunksWithoutZero :: Int -> [String]
+chunksWithoutZero n = (filter (\str -> length str >= n) . filter (/= "") . splitOn "0" . show) thousandDigitNum
+
+windowsOf :: Int -> [a] -> [[a]]
+windowsOf n = drop (n-1) . go []
+  where
+    go acc [] = acc
+    go acc lst = go (lst':acc) (tail lst)
+      where
+        lst' = take n lst