sub iterator {
   my $box = shift;

   # direction:
   #   0 => right
   #   1 => down
   #   2 => left
   #   3 => up
   my $direction = 0;
   
   my @pos = (0,0);

   my $up = 0;
   my $left = 0;
   my $down = ( scalar @$box ) - 1;
   my $right = ( scalar @{ $box->[0] } ) - 1;

   return sub {
      my $return_element = $box->[ $pos[1] ]->[ $pos[0] ];

      if ( $up > $down or $left > $right ) {
         # reset, so if we call it after undef, it will start over.
         $up = 0;
         $left = 0;
         $down = ( scalar @$box ) - 1;
         $right = ( scalar @{ $box->[0] } ) - 1;
         @pos = ( 0, 0 );
         $direction = 0;

         # return that we are done.
         return undef;
      }

      LABEL: {
         if ( $direction == 0 ) {
            if ( $pos[0] == $right ) {
               # we're traveling right and have hit the right edge.

               # change the direction to down.
               $direction++;

               # move our top edge (because we just got finished with
               # our uppermost remaining row)
               $up++;
               redo LABEL;
            }
         }
         if ( $direction == 1 ) {
            if ( $pos[1] == $down ) {
               # we're traveling down and have hit the bottom edge.

               # change the direction to left.
               $direction++;

               # move our right edge (because we just got finished with
               # our rightmost remaining row)
               $right--;
               redo LABEL;
            }
         }
         if ( $direction == 2 ) {
            if ( $pos[0] == $left ) {
               # we're traveling left and have hit the leftmost edge.

               # change the direction to up.
               $direction++;

               # move our bottom edge (because we just got finished with
               # our bottommost remaining row)
               $down--;
               redo LABEL;
            }
         }
         if ( $direction == 3 ) {
            if ( $pos[1] == $up ) {
               # we're traveling up and have hit the top edge.

               # change the direction to right.
               $direction=0;

               # move our left edge (because we just got finished with
               # our leftmost remaining row)
               $left++;
               redo LABEL;
            }
         }
      };

      # move our position.
      if ( $direction == 0 ) {
         $pos[ 0 ]++;
      }
      elsif ( $direction == 1 ) {
         $pos[ 1 ]++;
      }
      elsif ( $direction == 2 ) {
         $pos[ 0 ]--;
      }
      elsif ( $direction == 3 ) {
         $pos[ 1 ]--;
      }

      return $return_element;
   }
}