Does encode throw away commas and just use them for parity? if so then..

[knormoyle]

I was trying to understand the encode function for a character and couldn't understand comma handling

Does a comma just get thrown away after xoring it into parity generation?

if so, how does a sentence like this get parsed correctly, if you have some empty fields in the middle, and good fields after that?

$GNGSA,A,3,30,14,09,04,,,,,,,,,1.37,0.77,1.14,10B
$GNGSA,A,3,68,78,67,69,,,,,,,,,1.37,0.77,1.14,20C
$GNGSA,A,3,05,09,06,25,03,,,,,,,,1.37,0.77,1.14,30C
$GNGSA,A,3,12,23,11,08,,,,,,,,,1.37,0.77,1.14,40F

seems like you need to retain commas to correctly identify fields in a finished sentence?
I thought fields aren't extracted until you get a finished sentence.

[robertlipe]

If your question is "does NMEA checksum include the commas", the answer is "yes". Nothing is special-cased. The leading dollar sign and the asterisk before the checksum are not included; they are the envelope and not the message.

There is no parity. There is only the XORed, weighted checksum technique. This was simple to compute with the electronics available at that time, and it's not there to protect against forged lines. It's there to detect common forms of serial corruption such as bursts of noise and loss of data.

Without counting commas in your example, I'm about 99% sure your examples are invalid. '10B' should never be a valid NMEA field for any value. Ditto for '20C', '30c', and your final line, '40F.' I can't think of any time a three-digit hex value is legal.

You may find this code easier to digest:
https://github.com/GPSBabel/gpsbabel/blob/850245fa4cab1dee836bc215f5af1b117881cb52/nmea.cc#L162

Though that function returns an 'int', computationally it's always a single 8-bit byte used. (the calling convention sign-extends things to a native register width). You can see that it's always printed as a two-character hexadecimal value.

Valid reference NMEA files, with checksums, can be found in the 'reference/track' directory of GPSBabel. Examples of GSA include:

$GPGSA,A,3,28,08,11,27,07,31,,,,,,,4.7,1.6,4.4*34

and

$GPGSA,A,2,,,,,,,,,,,,,4.9,1.6,4.5*38