[Subsurface-divelog/subsurface] Don't show cylinder pressure graph for wrong dc for merged dives (#815)

[Linus Torvalds]

On Thu, Nov 16, 2017 at 7:25 AM, Stefan Fuchs <sfuchs at gmx.de> wrote:
>
> Hi Linus, hi everybody,

Hmm.

That code is a bit too confusing, and I think the proposed change
isn't wrong, but it makes that "too confusing" part worse.

For example, you do this:

        if (!has_gaschange_event(dive, dc, i) && cyl_used_somewhere)
                continue;

and that's pretty pointless, because we already have that
"has_gaschange_event()" knowledge: it's that "seen[]" array.

So honestly, my reaction to this is that first we should de-confuse
that function, and then instead work on exactly that "seen[]" array.

Basically, right now, "seen[]" has two values: 0 or 1 depending on
whether this cylinder has been used by this particular dive computer.

But let's just extend on that existing knowledge instead, and make it
be "-1" if a cylinder is explicitly not interesting.

And I think it merits some cleanup first.

Something like the attached three patches?

NOTE! Untested. And it's not smaller than your patch. I just think
it's a bit more explicit about what it does, and thus maybe clearer.

But I don't _hate_ your patch either.

                 Linus
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