Capability Letter Choices
We assigned user capability characters using only lowercase ASCII letters at first, so those are the most important within Fossil: they control the functions most core to Fossil’s operation. Once we used up most of the lowercase letters, we started using uppercase, and then during the development of the forum feature we assigned most of the decimal numerals. Eventually, we might have to start using punctuation. We expect to run out of reasons to define new caps before we’re forced to switch to Unicode, though the possibilities for mnemonic assignments with emoji are intriguing. 😉
The existing caps are usually mnemonic, especially among the earliest and therefore most central assignments, made when we still had lots of letters to choose from. There is still hope for good future mnemonic assignments among the uppercase letters, which are mostly still unused.
Why Not Bitfields?
When Fossil is deciding whether a user has a given capability, it simply
searches the cap string for a given character. This is slower than
checking bits in a bitfield, but it’s fast enough in the context where
it runs: at the front end of an HTTP request handler, where the
nanosecond differences in such implementation details are completely
swamped by the millisecond scale ping time of that repo’s network
connection, followed by the requires I/O to satisfy the request. A
strchr() call is
plenty fast in that context.