I had the same problem (С code). Most likely the problem is garbage in array output. I solved it output[0] = '\0';

Why? The return function exits the function immediately.

*dst++ = ' ' means set the character that dst is pointing at to ' ' and then forward the address of dst one character.

What does it mean dst++=...?

There is more simple algoritm

It is the bad practice writes all code inline

flipping the 6th bit (0b100000, or by using the value 0x20) toggles a character between uppercase and lowercase
i.e.
printf("%c\n", 'a' ^ 0x20);

or to just set an uppercase letter to lowercase (also keeps it lowercase) use exclusive or
printf("%c\n", 'D' | 0x20);

No

Why do you need 31?