Handling of N/A values could be improved

[andyvan-trabus]

The current processing of values that need to be scaled or offset to yield "useful" values introduces some hardships for code that's trying to use the returned values.

Example AIS 8:1:21 message

Air Temperature is documented in the standard (IMO SN.1-Circ.289 - Guidance on ASMS.pdf) as follows:

Dry bulb temperature in degrees Celsius (as per 2's complement), in 0.1 degree steps.
-60.0 to +60.0 degrees
-1,024 = data not available = default
601 - 1,023 (reserved for future use)
-1,023 to -601 (reserved for future use)

The current code always divides the 11-bit value by 10.0, which results in a floating-point number. However, if the original value was one of the "special" values, such as -1,024, it will also be converted. So, the client code, when checking to see if the value is available, has to do a floating-point compare to -102.4 (actually "-102.4000015258789" in my testing).

This is not good.

An improvement would be the following:

• Have the bit-extraction routines, like in ais8.cpp, only extract signed and unsigned integers.
• Move all scaling and offsetting code to the ais_py.cpp routines.
• Have the ais_py.cpp routines check for the N/A values, and set the field to Py_None if so.

[andyvan-trabus]

Okay, my suggested solution won't work, in that the values presented to a C++ client won't be correct. I didn't realize that this library was able to be used directly from C code, I thought it was only for use with Python.

Alternate suggestion:
Add checking code for special values in the bit-extraction routines, and if the value is out of range (e.g. N/A value), then if it's a float, set it to a NAN value. If it's an int value, we have a bool that we set to indicate that it's valid/invalid.

Example for 8:367:24. Pressure field is defined as follows:

Air pressure, defined as pressure reduced to sea level, in 1
hPa increments.
0 = pressure <800 hPa; 1 - 401 = 800 – 1200 hPa;
402 = pressure of 1201 hPa or greater;
403 = data unavailable = default;
404-510 = reserved; 511 = not to be used.

If int value is in 0..402, add 799 to the value, resulting in a valid pressure, and set pressure_valid to true;
else leave value alone (don't add 799) and set new instance member pressure_valid to false;

This allows C++ code to tell whether a value is valid or not (pressure_valid field), and if not they actually have the value available in case that's useful.
The Python mapping code in ais_py.cpp would check the value, and if pressure_valid is false, it would set the pressure field to Py_None. Alternatively, you could duplicate the C++ functionality and add a pressure_valid field to the Python dictionary.