On 2012-11-15 19:43, Dirk Hohndel wrote:
On Thu, 2012-11-15 at 12:16 +0100, Jef Driesen wrote:
C. Reference counting
An alternative option would be to make the new objects reference counted. This changes nothing to the fact that we have to decide whether the application has to take a reference explicitly (e.g. case A) or gains it automatically (e.g. case B). But with the reference count, we can detect whether the application still holds a reference or not, and avoid unnecessary memory allocations:
[...]
It's of course slightly more complex to implement, and we have to make sure the reference counting is thread-safe too using atomic operations or protected with a mutex), but I think the resulting api will be more intuitive. Reference counting is pretty standard today.
Yuck, yuck, yuck. It's standard and universally hated. It creates significantly more pointless code and boilerplate on both sides of the API for no real benefit.
And it's a wonderful way to create subtle, really hard to debug problems
- you forget to reference the object and keep using it - Valgrind
won't find that you are using 'freed' memory because it isn't freed - it just changes values under you. I absolutely HATE this idea.
Personally, I don't think reference counting is that bad. On the application side it's just a few extra dc_object_ref calls, and dc_object_unref instead of dc_object_free. And you can equally well shoot yourself in the foot with plain malloc. But okay, your preference is noted :-)
Please make the memory application owned and have the application be responsible for freeing memory. Unless you are planning to run libdivecomputer on a wristwatch the overhead for that is negligible with today's OSs (that all have a reasonably smart malloc implementation that will reuse the recently freed memory block on its own).
I certainly agree that this is probably the most intuitive and simple design.
But while I'm not too concerned about the memory allocations by themselves, I still find it a bit silly having to allocate resources where it's not strictly necessary. For the sample data I simply can't think of any use-case where you would want to hold on to the sample objects outside the callback function. Hence my search for possible alternatives.
D. Mixed ownership
The last option is to allow different ownership rules for the dive and sample objects. For example we could choose "applications owns" for the dives, and "library owns" for the samples. In that case, the dives can be used outside the callback function, while their is no unnecessary overhead for the samples. The obvious downside is less consistency.
I vote for consistency any time.
I usually prefer consistency too...
The idea originated from the fact that there exits two types of containers. In a traditional container (e.g. array, linked list, etc) the elements are already owned by the container. So when you iterate such a container, you just get a direct reference to the element in the container. There is no need to deal with ownership anywhere, because that's already taken care of.
But in our case, the containers act more like generators, where the elements are produced on-the-fly. There it makes sense to have the application own the generated items. However, for the samples, you can consider a dive as a traditional container for the samples. The samples are already present, they just need to be wrapped in a device independent structure.
- Querying available sample values?
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With the current design, the samples are pushed to the application, and there is no need to know which values are actually supported. You'll only get those values that are actually present. However, with the new design, the application has to call an accessor function to retrieve a piece of data, and thus it needs some way to query which values are presented. The only values that are guaranteed to be available for each sample are time and depth. Everything else is optional.
The level at which we can tell whether a particular sample value is present varies. It can be at the device level (for example a non-air integrated model will never store tank pressure data), at the dive level (for example a model with a wireless pressure sensor can have dives with and without tank pressure stored), or even at the sample level (for example a tank pressure every x sample).
Or you have a Suunto dive computer and the sample data is randomly missing... :-/
What do you mean?
To support all possible cases this querying should thus be done at the sample level.
I think the simplest solution is to use the accessor function itself to probe for the presence of the info. You just call the accessor function and if the info isn't available, a special error code like UNSUPPORTED is returned:
if (dc_sample_get_depth (sample, &depth) == DC_STATUS_SUCCESS) { /* Depth retrieved successfully. */ } else { /* No depth available (or an error occurred). */ }
For multivalued data, like the tank pressure, we could then use an extra index parameter in the accessor function, and then call it repeatedly until the UNSUPPORTED error is returned:
i = 0; while (dc_sample_get_pressure (sample, &pressure, i) == DC_STATUS_SUCCESS) { /* Tank pressure retrieved successfully. */
i++;}
An alternative solution for the multivalued data would be to retrieve all values at once using an array:
dc_sample_get_pressure (dc_sample_t *sample, double pressure[], unsigned int n);
But then the caller somehow has to know the maximum value for n, and the api also has to communicate back the actual number of values returned. The second part is easy with an extra output parameter, but I don't know a nice solution for the first problem (other than a second dc_sample_get_pressure_count function).
Alternatives for the probing approach outlined above, are an iterator like interface to iterator over the available sample values, or a function to retrieve a bitmap with the available sample values.
I was hoping for a somewhat different design.
int dc_sample_get_basic(sample, basic_sample);
returns DC_STATUS_SUCCESS if you got a sample back, various errors otherwise.
basic_sample_t { int time; // in seconds int depth; // in mm (or you can do a double in meters) uint64_t data_available; // bitmap on what else is available }
Then your header files define those bits and the corresponding data structures.
#define DC_PRESSURE_SAMPLE 1<<0 dc_pressure_sample_t { int nr_tanks; int *tankpressure; }
So this tells you how many pressure samples and returns a pointer to the array of pressures (or in the typical case to the single pressure) - an integer in mbar or a double in bar or whatever.
And for simple stuff just have an int (or you can go fancy and define types for all of them).
#define DC_TEMPERATURE_SAMPLE 1<<1 typedef dc_temperature_sample_t int; // temperature in mkelvin (or whatever you want)
This way ONE call gets you the basics that every dive computer supports and at the same time the information what else is available for THIS sample. And you can keep things API compatible by simply adding new bits whenever you add a new piece of information that you can get in a sample
- the app can then do
#ifdef DC_DECO_SAMPLE ... #endif
and work with older and newer versions of libdivecomputer...
I also considered using a bitmap (it was mentioned briefly somewhere in my mail I think), but I couldn't find a way to make it work with vendor extensions. A bitfield is limited to 8 to 64 different types, depending on the integer size we decide to use. For the normal samples, that is probably more than we'll ever need. But if every backend can support a few extension types, and each one needs another bit, then we'll quickly running out of bits. Do you have a solution for this?
While I'm definitely not opposed to the bitmap idea, I'm not that that happy with the basic_sample_t struct. I would prefer to implement this with separate functions. One for the bitmap itself, and one function for each value. Since there are only two values that are always available, I wouldn't treat them any different from the others.
For the multivalued data, I'm not sure how well such a struct (containing a length and pointer to an array) will work for language bindings (python, .net, etc). That's something we'll have to check.
I prefer enums over pre-processor constants too. I know this will loose the conditional compiling, but this is a trick that will work for C/C++ only.
- Supported dive and sample data
==================================
Which dive data and sample data should we support?
I already made a tentative list, but this is open for discussion of course. Items can be added or removed after discussion.
Dives:
- date/time
- divetime
length of dive as reported by DC?
Yes, and if such value isn't available, fallback to a value calculated from the samples. Exactly the same as we do today.
- maxdepth
- gasmixes
That's variable length, right?
- tank pressures (begin/end)
ditto
Yes, multiple gases and tanks will be fully supported.
- temperature
air temperature, I assume?
Depends. Different dive computers have different interpretations. It could be air/water temperature, the min/max/avg value, and so on. If that's even know to us. Not sure how to deal with this. We could just supply a temperature, and leaving the actual interpretation to the user, or we could have an extra flag with the exact interpretation (if known) as you mention below. But even then we're still left with devices that provide more than one temperature.
For example for the suunto vyper family, there is a temperature at the start/end of the dive, and at maximum depth. In this particular case I solved this by injecting artificial temperature values into the samples.
- salinity, altitude/atmospheric pressure
- dive mode (open/closed circuit, gauge, etc)
Samples:
- time
- depth
- temperature
- tank pressure
- heartrate
- airtime
That seems like a good list. Heartrate is the one that seems a bit out of place - are there many computers that support that? I would have seen CNS, NDL as more likely candidates for the default list.
Heartrate is only supported by the Uwatec Galileo as far as I know. It certainly not a must-have feature, but I can imagine that if you have such a device, this is an interesting value. Although the actual usefulness is questionable if you ask me, but it's cool gadget :-)
CNS/OTU can be calculated from the samples, but we can consider to support it too. I have no idea which devices record this in the data. I know the Suunto's have this weird OLF (Oxygen Limit Factor) value, which is either the CNS or OTU, whichever is higher. And I think the Oceanics have a scale from 1 to 5 or something. So that might be a bit more difficult to support in a uniform way.
I assume with NDL, you mean the remaining no-deco time? I don't think any device does support that. And then you are left with NDL equals the absense of decostop info.
Also, your tank pressure contains the tank index as well, right?
Of course.
Events:
- Gas changes
as we discussed in earlier email - I think having the tank index in the sample is a better solution...
Indeed.
- Deco stops (depth and time)
- Warnings, bookmarks, etc
For the events, I'm not sure yet how the querying (see previous item) should be done. The normal samples are continuous values, and will usually appear at least every X sample, or not at all. But the events are discrete in nature and will only appear occasionally. Should this be done differently in the api?
We could do this with the bitmap proposed above, right?
What I had in mind, is to have a single DC_SAMPLE_EVENT type. If this bit is set, there are events available. The actual event value would then be another bitmap containing all the events. With this approach we can support several events without needing additional sample types for each event type.
We'll also have to agree on a data structure and the exact semantics for each of those pieces of data. Some won't need much discussion, but others will. For example take the temperature. A device can record a single value which could be the minimum, maximum or average temperature. It can have all three, or a full temperature profile. How do we deal with that? Things are not always as easy as they may look.
I think the best way to deal with that is to have a bitfield flags that is part of every sample. Be generous, at least 64 bits. And simply have a clearly defined default semantic (whatever Suuntu uses, or some other common DC) and whenever there's a different semantic for such a value, add a flag bit that again has clear semantic.
So for example by default temperature is the minimum temperature. And then there can be a DC_DATA_TEMP_IS_AVERAGE bit that tells you that instead this is the average temperature.
So you have one of those flags bitfield in the dive data, and one in each sample. And you do your darnedest to make sure that there is a precise definition of the default meaning of each value, and a precise definition of how the bits modify that.
I fear that the use of extra flags will quickly lead to a mess again. We now have this for the temperature, tomorrow we may need extra flags for something else, and before we know we end up with a flags everywhere.
And it doesn't solve the case where there are multiple temperature values either.
Closely related to the data structures is the choice of the unit system. The choice is either metric or SI. SI units have the advantage all values are positive, but applications are likely to use metric or imperial. We can also consider to introduce a unit system as used in subsurface, where all values are scaled to fit into an integer (e.g. depths in millimeters instead of meters, and so on)? Or are we happy with floating point values?
I'm obviously biased here... but in the end, I'll be happy with floats or integers. But please, please, please, ONLY USE SI-related values. It's the applications job to convert this into whatever insane preferences the user has. Distance in ft, volume in liter, temperature in C, pressure in psi... whatever. But libdivecomputer should ALWAYS give us SI-related units (I say related because one can argue if C or K is the right temperature value - we went with mK because that gives us plenty of precision in an unsigned integer - but one might reasonably argue that float centigrade is an equally valid SI-related unit... just don't allow F or other random non SI units...). If the DC is silly enough to return data in the unit system that the user has set up, then libdivecomputer should convert that into sane units.
Sorry for the possible confusion, but imperial was never an option. It's either metric or SI. The only difference between the two is the temperature (Celsius vs Kelvin). For the other units it's just a difference in scaling factors (Pascal vs bar).
That's it for now. Comments are welcome! If you think I missed something important, don't hesitate to bring it up. It's not unlikely that I forgot about something :-)
I like the overall direction. And I'm glad that we can discuss this hear before it is set in stone... the current API has caused us some frustration here and there and this is a great opportunity to create something that can be maintained in a backwards compatible manner for a long time to come...
Great.
Jef