On Sat, 3 May 2008 16:26:30 -0700 (PDT), MIG wrote:

I think I can answer that one. *Due to the awful positioning of the
readers at Heron Quays, I had marched well past them on leaving the
DLR before realising they were behind me, and assumed that as long as
I touched in at Canary Wharf Jubilee, it would tag me as going through
that interchange.

I was wrong. *It wouldn't let me through the gate. *It then wouldn't
let me rectify the situation by going back to the DLR, having touched
and been rejected at the Jubilee gate.

In fact, I couldn't proceed before queuing at the ticket office, where
it was still coct up and I ended up being charged two journeys instead
of a continuation.

Now that's really odd. Not letting you through the Jubilee gate I can
understand, but what (specifically) was it that made the DLR reader
reject the card? Maybe the card gets put into some sort of "get
rejected by everything until situation manually resolved" state.

It wasn't evident from the errors, but I am guessing that a penalty
had been charged, thus not leaving enough credit to let me through the
Jubilee gate.

It also would then not have enough credit to start a new DLR journey
(if by then that's what it thought I was doing).

Ah, good explanation. That makes perfect sense.

BUT it was well within the time limit, so it seemed to decide that
being at the Jubilee gate was reason to prematurely unresolve my
previous DLR journey about fifteen minutes after it had started.

This would have to have been done instantaneously in one touch at the
Jubilee gate, ie prematurely unresolve my DLR journey ... charge me
the penalty ... decide I haven't got enough credit to start a Jubilee
journey ... reject my passage.

Yes. (Well, strictly speaking, the penalty had already been deducted
when you started your journey, and would have been refunded had you
finished it cleanly.)

I think the logic works as follows. Your card was in the state of
being inside the fare-paid area. You touched it on a validator of type
"entry-part-of-interchange". The logic for this combination is to
carry out the following steps:
- Reset card state to being outside fare-paid area (without refunding
the £4)
- Then proceed as if originally presented with a card in that state.

If it's got the logic to figure that one out ... Well, you'd think it
could be programmed with the logic to work out the true situation as
well.

Indeed. All they'd need to do is remove the distinction between
validators of type entry-part-of-interchange and those of type
any-part-of-interchange (the DLR one at Heron Quays would be an
example of the latter).

The main purpose of the second experiment I proposed was to work out
if there actually was such a difference (I didn't think there would be
as on the face of it it seems a bit of a pointless over-complication).