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PureScript solution to Project Euler problem 8

2018-May-12 07:27
purescriptproject-euler

Problem details at Project Euler problem 8 page.

Test

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module Euler008Test (euler8suite) where
 
import Prelude
 
import Data.BigInt (fromString)
import Euler008 (euler8)
import Test.Unit (TestSuite, suite, test)
import Test.Unit.Assert as Assert
 
euler8suite :: forall e. TestSuite e
euler8suite =
  suite "Euler 8" do
    test "Warmup" do
      Assert.equal (fromString "5832") (euler8 4)
    test "Real" do
      Assert.equal (fromString "23514624000") (euler8 13)

Solution

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module Euler008 (euler8) where
 
import Prelude
 
import Data.Array (catMaybes, drop, filter, foldl, length, take, (:))
import Data.BigInt (BigInt, fromInt)
import Data.Char.Unicode (isDigit)
import Data.Foldable (maximum)
import Data.Int (fromString)
import Data.Maybe (Maybe)
import Data.String (singleton, toCharArray)
 
num1000 :: String
num1000 = """
  73167176531330624919225119674426574742355349194934
  96983520312774506326239578318016984801869478851843
  85861560789112949495459501737958331952853208805511
  12540698747158523863050715693290963295227443043557
  66896648950445244523161731856403098711121722383113
  62229893423380308135336276614282806444486645238749
  30358907296290491560440772390713810515859307960866
  70172427121883998797908792274921901699720888093776
  65727333001053367881220235421809751254540594752243
  52584907711670556013604839586446706324415722155397
  53697817977846174064955149290862569321978468622482
  83972241375657056057490261407972968652414535100474
  82166370484403199890008895243450658541227588666881
  16427171479924442928230863465674813919123162824586
  17866458359124566529476545682848912883142607690042
  24219022671055626321111109370544217506941658960408
  07198403850962455444362981230987879927244284909188
  84580156166097919133875499200524063689912560717606
  05886116467109405077541002256983155200055935729725
  71636269561882670428252483600823257530420752963450
"""
 
digits :: Array Int
digits = toCharArray num1000
       # filter isDigit
       # map (fromString <<< singleton)
       # catMaybes
 
groups :: forall a. Int -> Array a -> Array (Array a)
groups n a
  | length a < n = []
  | otherwise = take n a : groups n (drop 1 a)
 
euler8 :: Int -> Maybe BigInt
euler8 n =
  let mula = map fromInt
         >>> foldl (*) (fromInt 1)
  in   groups n digits
     # map mula
     # maximum