Re: Bidirectional iterators: signed distance?

As an intellectual exercise, I've implemented an STL-esque List<> and
List<>::Iterator. Now, I would like a signed distance between two
iterators which corresponds to their relative position in the list.
For instance, if I did something like distance(list.end(),
list.begin()), I would get -list.size(). The STL's iterator distance
function amounts to something like this:

distance_type distance(Iterator first, Iterator last)
{
   distance_type n = 0;
   while (first != last) {
     ++first; ++n;
   }
   return n;
}

So, if my list has ten elements, and i4 is an iterator at the 4th node
and i2 is an iterator at the 2nd node, then std::distance(i4, i2) is
9. That's meaningless to me. What I want is distance(i4, i2) == -2.

So I implemented distance as a method of List<> like so:

int distance(Iterator i, Iterator j)
{
   Iterator tmp = i;
   int d = 0;
   while (tmp != j) {
       if (tmp == end()) {
           // oops, try the other way
           tmp = i; d = 0;
           while (tmp != j) {
               --tmp; --d;
           }
           return d;
       }
       ++tmp; ++d;
   }
   return d;
}

But this is butt-ugly and I don't like the constant checking for
tmp==end(). Can anyone think of a nicer way to implement this
functionality? Has it been done somewhere else that I can have a look
at?