[PATCH] Improved logic in cylinder management and gas change events

[Jan Mulder]

Attached a patch that may result in some discussion. I started looking 
at some code due to a question from Tomaz about a "ghost cylinder bug". 
Indeed, the code that is referenced by Tomaz seems straightforward, so 
where can a ghost-like bug come from?

First, I ran in to an obvious bug. Manually add a 2nd cylinder to a 
dive, and try to delete it right away. This should be possible, for 
example, in case somebody adds a cylinder by mistake and tries to 
correct it. The code to add a cylinder contains the intriguing remark 
"mark the cylinder as 'used' since it was manually added". I'm not sure 
what the original developers had in mind with the "used" status. 
However, I consider a cylinder used in case its being breathed during 
the dive. In other words; the set of used cylinders is a subset the 
cylinders carried by the diver. For example, bailout of safety cylinders 
(supposed only to be used in an emergency), or staging cylinders in a 
cave for later (next dive) use. So, the first part of this patch is not 
assuming that a manually added cylinder is used automatically.

Second part of the patch is that we now have to ensure that when a 
cylinders is getting used (due to a gas change event), it gets the 
proper used status, and when it becomes unused vise versa. However, we 
may not assume that a cylinder is used only once during a dive, so when 
removing a gas change event, we have to inspect the entire dive. A 
cylinder can only be removed when it is not used at all.

best,

--jan





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