Handling of N/A values could be improved

[andyvan-trabus]

Handling of N/A values could be improved

[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.