"Graham J" wrote in message ...

However I do acknowledge you said "sufficient state" and "for example". A
practical solution would presumably require booleans indicating zones passed
through by rail, both in peak and off-peak, as well as running totals.


I've been having a think about this, trying to work out prepay rebate
just from the journey history already stored on the card is crazy -
even if it was me that suggested it It will never work without a
huge journey history which will take longer to read-process-write and
so add to the wait before the touch pad gives you the green light.

My best guess is that it would work by keeping count of each of the
possible journey types that can be made by prepay, which (for an
adult) I reckon is only 24 based on:

11 tube fares (both on or off peak)
1 bus fare
1 tramlink fare (needs to be seperate from bus to work out travalcard
validity)

You would also need to store the current 'capped' state of the card
which would be something like uncapped/Z12 peak travelcard/bus pass
etc. plus the amount refunded onto the card.

So based on the original example:

-------------------- Information Stored --------------------
Journey: Charge: Refund: Status: Z12Pk: Bus: Z12OPk: All others:
Z12Peak 2.00 0.00 Uncapped 1 0 0 0
Bus 2.70 0.00 Uncapped 1 1 0 0
Bus 3.40 0.00 Uncapped 1 2 0 0
Bus 4.10 0.00 Uncapped 1 3 0 0
Bus 4.80 0.30 Bus Pass 1 4 0 0
......
Bus 9.00 4.50 Bus Pass 1 10 0 0
Z12OffPeak 11.00 5.70 Z12Pk TCard 1 10 1 0

In addition you would need to store the last couple of validations for
spotting out of station transfers and passbacks.

Provided some kind of integer overflow does not occur then you can
make a couple of hundred journeys a day with the system still able to
keep track of you. Although full details of the last couple of
validations will still be required in order to spot continuations and
passbacks.


Does anyone actually know how it is being done, by the way. I haven't been
clear if other contributors have actually been speaking authoritatively or
just speculating as wildly as I am.

I'm making it up as I go along! However if someone from TfL/CTS does
want to know how my 'best guess' can be extended to work in a period
longer than 24 hours (I know it won't work for a longer capping
period) then I'm sure I can discuss it for a small consultation fee

--
Gareth Davis