module Euler4

imports
  Prelude

where

// A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 × 99.
// Find the largest palindrome made from the product of two 3-digit numbers.

main :: () -> I32
main () = do
  m = new (0 :: W32)
  times 1000
    (\i -> do
      times 1000
        (\j -> do
          x = i * j
          when (is_palindrome x)
            (do
              m <- max @m x
            )
        )
    )
  assert (@m == 906609)
  putW @m
  0

is_palindrome x => rev_digits x == x

rev_digits n => do
  y = new 0
  for n (\x -> x > 0) (\x -> x / 10)
    (\x -> y <- (@y * 10) + (x % 10))
  @y