signed vs unsigned

[Robert Helling]

Dirk,

you committed

commit 59299f0ab34ca4d75e0b02d9a83f23ec6348e1df
Author: Dirk Hohndel <dirk at hohndel.org>
Date:   Wed Mar 9 18:42:39 2016 -0800

    Clean up signedness confusion in planner.c

in which you declared several ints to be unsigned. This breaks at least the ascents in recreational mode, see #1030.

I located the problem to be that we have

unsigned int deltad

in line 1097 but we do the comparison

if (deltad - depth > 0)

in line 1103. Now, depth happens to be signed (and it needs to be since we stop ascending when we step through the surface, i.e. reach negative depth which is of course not possible for unsigned values). This above comparison, however, gets casted to unsigned int and thus clearly makes no longer sense. (You can see the ascent problem goes away when deltad is made signed again.

I could now simply send a patch that does exactly that. But I wonder, what you intended to do with your above patch. It is not the first time I run into troubles when mixing signed and unsigned integers in expressions and in particular in comparisons where the implicit cast is to unsigned (which for me almost never makes sense) and which does not even cause a compiler warning (I think it should, at least it would save me a lot of debugging time, I see, there is -Wconversion).

Anyway, my conclusion from all this trouble in the past was to _always_ use the signed version since this prevents the trouble. As far as I can see, the potential cost is that the maximally representable number is only half the size but this should not be a problem for us since we stay safely away from those limits.

So, could you please explain to me your rationale for adding the unsigned statements in your above patch as I am clearly missing something.

Best
Robert


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Robert C. Helling     Elite Master Course Theoretical and Mathematical Physics
                      Scientific Coordinator
                      Ludwig Maximilians Universitaet Muenchen, Dept. Physik
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