{-
https://projecteuler.net/problem=4
A palindromic number reads the same both ways.
The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 x 99.

Find the largest palindrome made from the product of two 3-digit numbers.
-}

import           Debug.Trace (trace)

twoDigitProducts :: [Integer]
twoDigitProducts =  [ n*m | n <- [10..99], m <- [n..99] ]

threeDigitProducts :: [Integer]
threeDigitProducts =  [ n*m | n <- [100..999], m <- [n..999] ]

isPalindrome :: Integer -> Bool
isPalindrome n = all (== True) $ zipWith (==) half half'
  where
    (half, half'') = splitAt l n'
    half' = reverse half''
    l     = length n' `div` 2
    n'    = show n
    -- n' = trace (show n) $ show n

solve :: Integer
solve = maximum $ filter isPalindrome threeDigitProducts

main :: IO ()
main = print solve