This would have been a reply in the current thread, but i deleted the
relevant post - sorry!

Anyway, it was asserted that to do capping, the gate would have to read a
complete travel history from the card, work out if it should cap and write
the changes back, all in the time that the card was over the reader, and
that this was implausible.

Now, i reckon you actually don't need to work with the complete travel
history; you just need to track the total amount of money spent so far on
travel within each zone (with buses counting as a sort of 'zone 0', with a
bus-pass essentially being a travelcard for this zone). When the card
comes in, the gate first checks for an applicable travelcard, then, if it
doesn't find one, works out how much it's going to charge; it then works
out the new total for spending in that zone, then checks whether the
combined total spend in that zone and inward zones is enough to buy a
travelcard; if it is, it issues one, and only charges the amount needed to
do so (and clears the relevant totals).

Here's a worked example, where someone who lives in the inner suburbs goes
to work in town and goes out somewhere central for lunch. We track the
amount spent (in pence) in zone 6/5/4/3/2/1/buses after each trip. We
start with no spending and no travelcard. This all happens before off-peak
TCs are available (it's a very early lunch). I assume pre-pay prices for
everything.

Start: 0/0/0/0/0/0/0
Catch a bus to the tube station, in Z3: 0/0/0/0/0/0/70; charge 70
Get a train to Z1: 0/0/0/230/0/0/70; charge 230
Get a bus to work (in Z1): 0/0/0/230/0/0/140; charge 70
Get a tube in Z1: 0/0/0/230/0/160/140; charge 160
Get another tube in Z1: 0/0/0/0/0/0/0 + Z123 TC; charge 110
Get the tube home: 0/0/0/0/0/0/0 + Z123 TC; charge 0

The capping happens when charging for the second Z1-Z1 tube trip; it would
have come to 0/0/0/230/0/320/140, which is enough for the Z123 TC.

I haven't thought too hard about travelcards that don't include Z1 (or any
set of inner zones - hollow rather than solid travelcards, if you will) or
upgrading an existing travelcard to cover more zones, but i don't think
it'd be that hard.

Am i missing something, or is that still too complicated to do in one
swipe?

tom

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Remember when we said there was no future? Well, this is it.