Various use-cases require the possibility to reference a part of the message body. This was realized by providing the offset of the beginning and end of the referenced region as offset from the beginning of the message. XEPs doing so include References (XEP-0372)  (and thereof derived Stateless Inline Media Sharing (XEP-0385) ) and Message Markup (XEP-0394) .
For this method, it is highly relevant to decide how to count "characters" in a message body. While it at first sounds trivial, there are various ways of doing so in modern font systems. The purpose of this XEP is to define how characters shall be counted for the purpose of the aforementioned XEPs and any future XEP relying on a similar feature.
When counting characters in a body, they shall be counted by their number of Unicode code points. Message bodies must not be normalized when counting code points.
|String||Grapheme cluster||UTF-8 bytes||UTF-16 units (2 bytes)||Code points|
|🧛🏾 👨👨👦👦 🇺🇳||5 ||43||21||13|
As grapheme clusters may consist of multiple code points, a code point offset might be illegal if it points inside a grapheme cluster.
However, receiving entities SHOULD NOT consider illegal offsets invalid, as different Unicode versions may have different understanding of what a grapheme cluster is. Instead, receiving entities may choose one of the following behaviors:
Some programming languages include a string type that operates directly on Unicode code points. If these types are used, offset numbers can be used as-is in string operations. Popular examples of such programming languages are Python and Haskell.
C/C++ includes a wide character and string type. Those behave differently across platforms and as such should be used with care.
The most obvious way of counting characters is to count them how humans would. This sounds easy when only having western scripts in mind but becomes more complicated in other scripts and most importantly is not well-defined across Unicode versions. New unicode versions regularly added new possibilities to build grapheme clusters, including from existing code points. To be forward compatible, counting grapheme clusters, graphemes, glyphs or similar is thus not an option. This leaves basically the two options of using the number of code units of the encoded string or the number of code points.
The main advantage of using the code units would be that those are native to many programming languages, easing the task for developers. However programming languages do not share a common encoding for their string type (C/C++ use UTF-8, C#/Java use UTF-16, Python 3 hides the internal encoding from the developer and only presents it in code points), so there is no best pick here. If one was to choose an encoding, the best choice would be UTF-8, the native encoding of XMPP. However this makes counting bytes a more complex task for programming languages that use a different encoding like UTF-16, as strings would need to be transcoded first.
Counting code points has the advantage that offset counts cannot point inside a code point. This could happen when using code units of any encoding that may use more than one unit to represent a code point (such as UTF-8 and UTF-16). If an offset count points inside a code point, that would be an invalid offset, raising more uncertainty of the correct behavior in such cases. Most notably the opportunity of splitting (as it exists for grapheme cluster) is not an option in that case, because splitting a code point would not create any usable output. Counting code points is widely supported in programming languages and can easily be implemented for encoded strings when not.
Unicode terminology used across this document, can be looked up in the Unicode glossary at https://www.unicode.org/glossary/.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This document requires no interaction with XMPP Registrar .
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
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The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
4. There are spaces between the emojis. You may also perceive this as more than 5 glyphs if your font or display engine does not support the required Unicode version.
5. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
6. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/