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Overview
Comment: | Expanded the "Why Not Bitfields?" discussion in www/caps/impl.md. Also tweaked the "Capability Letter Choices" text a bit while in there. |
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Downloads: | Tarball | ZIP archive |
Timelines: | family | ancestors | descendants | both | caps-doc |
Files: | files | file ages | folders |
SHA3-256: |
3ac560a2d0b9c811ff42f9ba7c2e9853 |
User & Date: | wyoung 2019-08-28 22:09:02.339 |
Context
2019-08-28
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22:42 | Assorted small improvements to top-level caps doc ... (check-in: f21bfbee user: wyoung tags: caps-doc) | |
22:09 | Expanded the "Why Not Bitfields?" discussion in www/caps/impl.md. Also tweaked the "Capability Letter Choices" text a bit while in there. ... (check-in: 3ac560a2 user: wyoung tags: caps-doc) | |
2019-08-27
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20:35 | Moved www/capabilities.md to www/caps/index.md and www/admin-v-setup.md to www/caps/admin-v-setup.md. Then broke three chunks of the old capabilities.md doc out into new www/caps/* docs: login groups, implementation details of user capabilities, and the user capability reference material. The latter switched from Markdown to HTML format so we could make it a clearer-looking table, rather than its prior hard-to-read bullet list form. ... (check-in: c6cdf9ce user: wyoung tags: caps-doc) | |
Changes
Changes to www/caps/impl.md.
1 2 3 4 | # Implementation Details of User Capabilities ## <a name="choices"></a>Capability Letter Choices | | > | | > > > | > > > > > > | | | | | | < | > > > > > > > > | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 | # Implementation Details of User Capabilities ## <a name="choices"></a>Capability Letter Choices We [assigned][ref] 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][for] we assigned most of the decimal numerals. All of the lowercase ASCII letters are now assigned. Eventually, we might have to start using ASCII punctuation and symbols. We expect to run out of reasons to define new caps before we’re forced to switch to Unicode, though the possibilities for [mnemonic][mn] assignments with emoji are intriguing. <span style="vertical-align: bottom">😉</span> 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. ## <a name="bitfield"></a>Why Not Bitfields? Some may question the use of ASCII character strings for [capability sets][ucap] instead of bitfields, which are more efficient, both in terms of storage and processing time. Fossil handles these character strings in one of two ways. For most HTTP hits, Fossil [expands][sexp] the string into a [`struct` full of flags][sff] so that later code can just do simple Boolean tests. In a minority of cases, where Fossil only needs to check for the presence of a single flag, it just does a [`strchr()` call][sc] on the string instead. Both methods are slower than bit testing in a bitfield, but keep the execution context in mind: 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 required I/O to satisfy the request. Either method is plenty fast in that context. In exchange for this immeasurable cost per hit, we get human-readable capability sets. ----- *[Back to Administering User Capabilities](./)* [for]: ./forum.wiki [mn]: https://en.wikipedia.org/wiki/Mnemonic [ref]: ./ref.html [sexp]: http://fossil-scm.org/fossil/artifact?udc=1&ln=1223-1298&name=889d6724 [sff]: http://fossil-scm.org/fossil/artifact?udc=1&ln=80-117&name=52d2860f [sc]: https://en.cppreference.com/w/c/string/byte/strchr [ucap]: ./index.md#ucap |