USING: kernel locals make math math.order math.parser math.ranges
sequences ;
IN: problem4

: pali? ( x -- ? ) dup reverse = ;

: num-pali? ( x -- ? ) number>string pali? ;

:: make-range ( a b -- seq )
    [ a b [a,b]
      [| x | a b  [a,b]
          [| y |
              t [ x y * , ] when
          ] each
      ] each
    ] { } make ;

: problem4 ( -- n )
    100 999 make-range [ num-pali? ] filter 0 [ max ] reduce ;