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PureScript solution to Project Euler problem 4

2018-Jan-22 06:53
purescriptproject-euler

Problem details at Project Euler problem 4 page.

Test

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module Euler004Test (euler4suite) where
 
import Prelude
 
import Data.Maybe (Maybe(..))
import Euler004 (euler4)
import Test.Unit (TestSuite, suite, test)
import Test.Unit.Assert as Assert
 
euler4suite :: forall e. TestSuite e
euler4suite =
  suite "Euler 4" do
    test "Warmup" do
      Assert.equal (Just 9009) (euler4 2)
    test "Real" do
      Assert.equal (Just 906609) (euler4 3)

Solution

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module Euler004 (euler4, isPalindrome) where
 
import Prelude
 
import Data.Array (filter, range, reverse)
import Data.Foldable (maximum)
import Data.Int (decimal, pow, toStringAs)
import Data.Maybe (Maybe)
import Data.String (fromCharArray, toCharArray)
 
isPalindrome :: Int -> Boolean
isPalindrome n =
  let s = toStringAs decimal n
      rs = toCharArray s
           # reverse
           # fromCharArray
  in s == rs
 
euler4 :: Int -> Maybe Int
euler4 n =
  let low = pow 10 (n - 1)
      high = 10 * low - 1
      nums = range low high
      prods = (*) <$> nums <*> nums
      palins = filter isPalindrome prods
  in maximum palins