Ugh - cylinderindex confusion

[Dirk Hohndel]

On December 30, 2012 1:14:02 PM Linus Torvalds 
<torvalds at linux-foundation.org> wrote:
> Looking at my the multi-cylinder dives with Uemis data, it shows how
> clearly broken the "tank switch in samples" model that the Uemis
> parser uses is.
>
> The uemis parts of the xml have all those idiotic repeated
> "cylinderindex='n'" things. Even with no pressure data.
>
> I really think that "cylinderindex" should only exist when there is
> pressure data, and it should be renamed to "sensor" so that we never
> make that mistake again. The cylinderindex naming was wrong (since
> sensor 0 is not necessariyl on cylinder 0), and came from
> libdivecomputer. And due to the naming, it then is easy to think that
> cylinderindex has anything to do with gas changes, which is obvious
> crap.
>
> So the gas switch stuff should always be events (or, if we want to,
> some totally new model, but right now we have it as events). Thinking
> that each event should have a "this is the cylinder you are breathing"
> is insane, and has nothing to do with the sensor anyway - even if some
> specific dive computers with multiple sensors *might* then decide to
> match the two up. It does not make sense as a generic model.
>
> Renaming our "cylinderindex" xml node would also allow us to change
> the semantics - instead of "always show it when it is non-zero", the
> new "sensor" field should be "show it only when there is a pressure,
> and the sensor index is different from the previous sample that had
> it".
>
> Renaming it wouldn't just avoid the whole current confusion with gas
> switch events, it would also mean that we could be compatible with old
> files.
>
> So for legacy cases, "cylinderindex" at parsing time could be turned
> into a gas change event if it changes (and when we parse it, a missing
> value is zero). But then we never write it out, and we only write out
> "sensor" data (with the new semantics, so at parse time, a missing
> sensor number would not be 0, but be "same as last sensor".
>
> What do you think?

I think it's a good step in the right direction.

But I'm not happy with the way we implement the gas change event today. 
The code that inserts additional plot info entries is utter crap... I 
really want to get rid of that.

But that is the next step after cleaning up the cylinderindex mess.

/D