%!PS

% begin included library code
% see https://github.com/Firedrake/postscript-libraries/
/quicksort.partition {
    3 dict begin
    /pivot arr hi lo add 2 idiv get def
    /i lo 1 sub def
    /j hi 1 add def
    {
        {
            /i i 1 add def
            arr i get pivot ge {
                exit
            } if
        } loop
        {
            /j j 1 sub def
            arr j get pivot le {
                exit
            } if
        } loop
        i j ge {
            j
            exit
        } if
        i j quicksort.swap
    } loop
    end
} bind def

/test.end {
    ( ) print
    test.count 0 gt {
        (Passed ) print
        test.pass (...) cvs print
        (/) print
        test.count (...) cvs print
        ( \() print
        test.pass 100 mul test.count idiv (...) cvs print
        (%\)) print
        (\r\n) print
    } if
} bind def

/quicksort { % [ a c b ] -> [ a b c ]
    1 dict begin
    /arr exch def
    arr length 0 gt {
        0 arr length 1 sub quicksort.main
    } if
    arr
    end
} bind def

/test {
    /test.count test.count 1 add def
    {
        /test.pass test.pass 1 add def
    } {
        ( ) print
        test.count (....) cvs print
        (-fail) print
    } ifelse
} bind def

/quicksort.main { % lo hi -> (null)
    3 dict begin
    /hi exch def
    /lo exch def
    /xit false def
    lo 0 lt {
        /xit true def
    } if
    hi 0 lt {
        /xit true def
    } if
    lo hi ge {
        /xit true def
    } if
    xit not {
        /p quicksort.partition def
        lo p quicksort.main
        p 1 add hi quicksort.main
    } if
    end
} bind def

/quicksort.swap {
    2 dict begin
    /bi exch def
    /ai exch def
    arr ai get
    arr bi get
    arr exch ai exch put
    arr exch bi exch put
    end
} bind def

/reduce { % array proc -> value
    2 dict begin
    /p exch def
    /a exch def
    a 0 get
    1 1 a length 1 sub {
        a exch get
        p
    } for
    end
} bind def

/listmax {
    { max } reduce
} bind def

/keys { % dict -> array of dict keys
    [ exch
      {
          pop
      } forall
    ]
} bind def

/values { % dict -> array of dict values
    [ exch
      {
          exch pop
      } forall
    ]
} bind def

/filter { % array proc(bool) -> array
    1 dict begin
    /p exch def
    [ exch
      {
          dup p not
          {
              pop
          } if
      } forall
    ]
    end
} bind def

/test.start {
    print (:) print
    /test.pass 0 def
    /test.count 0 def
} bind def


% end included library code

/mostfrequenteven {
    2 dict begin
    /l exch def
    /f 0 dict def
    l { 2 mod 0 eq } filter {
        dup f exch known {
            dup f exch get 1 add f 3 1 roll put
        } {
            f exch 1 put
        } ifelse
    } forall
    f length 0 gt {
        /m f values listmax def
        f keys { f exch get m eq } filter
        quicksort 0 get
    } {
        -1
    } ifelse
    end
} bind def

(mostfrequenteven) test.start
[ 1 1 2 6 2 ] mostfrequenteven 2 eq test
[ 1 3 5 7 ] mostfrequenteven -1 eq test
[ 6 4 4 6 1 ] mostfrequenteven 4 eq test
test.end