understanding the sensor values in our samples

[Linus Torvalds]

On Thu, Apr 28, 2022 at 9:38 AM Dirk Hohndel <dirk at hohndel.org> wrote:
>
> except that's not what it does.
> If you have four cylinders, because that's what your dive computer provides
> in the download, then it sets index 0 to the cylinder that is in use, and index 1
> stays 1.
> Which then the code I mentioned interprets as "oh, cylinder 1 is in use"

Hmm. That "index[0] = 0" and "index[1] = 1" (if you have two or more
cylinders) is kind of very much intentional.

In fact, we should probably fill in all the other 'index[]' entries
too, but we never did, because dive computers that support more than
two sensors will also give the actual explicit sensor number when
downloading, so all this "implicit sensor number" is just garbage for
the legacy cases.

But that "index[1] = 1" shouldn't mean "cylinder 1 is in use" - unless
you then actually have pressure samples for cylinder 1 (in which case
it *is* in use, of course).

So I think *that* is the bug here, not the two lines you quoted.

cylinder_renumber() getting this case wrong - now THAT I can most
definitely believe.

I wonder if the issue is here (in dc_cylinder_renumber()):

        /* If the initial cylinder of a dive was remapped, add a gas
change event to that cylinder */
        if (mapping[0] > 0)
                add_initial_gaschange(dive, dc, 0, mapping[0]);

where we blindly add that initial gaschange simply because we go
"we've remapped the first cylinder, so now we need to add that gas
change" - but that is only valid if that first cylinder *was* our
initial gas to begin with.

Can you send me the test-case so that I can follow along?

                  Linus