If you asked me to write a JSON parser in 6502 assembly, I would put the fairly short and unambiguous JSON grammar into lex and yacc to get a parser in C, then compile that to 6502 assembly. Then maybe glance through the assembly code, although I doubt very much that I know anything about assembly that the compiler doesn't.
Of course the author is a highly skilled person who also did this for his enjoyment. But am I missing some way in which what he did would be more than incrementally better than what I would do? Aren't these tools pretty much as good as the best humans at solving this problem?
There are some assembly languages that are pretty straightforward to write code in, and if you're organized it is not too onerous a task.
That said, 6502 is probably one of the LEAST featureful assembly languages I've ever used. I remember finding out it didn't have an ADD instruction - you clear carry, then add with carry.
Processors like the 6502 aren't considered very C friendly for a variety of reasons, but mostly because the register size isn't big enough for the most common data types like int or char *. You often end up with terrible ASM.
Of course the author is a highly skilled person who also did this for his enjoyment. But am I missing some way in which what he did would be more than incrementally better than what I would do? Aren't these tools pretty much as good as the best humans at solving this problem?