Change character units from UTF-16 code unit to Unicode codepoint

[MaskRay]

Text document offsets are based on a UTF-16 string representation. This is strange enough in that text contents are transmitted in UTF-8.

Text Documents
......... The offsets are based on a UTF-16 string representation.

Here in TextDocumentContentChangeEvent, range is specified in UTF-16 column offsets while text is transmitted in UTF-8.

interface TextDocumentContentChangeEvent {
	range?: Range;
	rangeLength?: number;
	text: string;
}

Is it more reasonable to unify these, remove UTF-16 from the wording, and use UTF-8 as the solely used encoding? Line/character can be measured in units of Unicode codepoints, instead of UTF-16 code units.
A line cannot be too long and thus doing extra computing to get the N'th Unicode codepoint would not lay too much burden on editors and language servers.

jacobdufault/cquery#57

Survey: counting method of Position.character offsets supported by language servers/clients
https://docs.google.com/spreadsheets/d/168jSz68po0R09lO0xFK4OmDsQukLzSPCXqB6-728PXQ/edit#gid=0

[szatanjl]

I would suggest to go even one step further. Why editors and servers should know which bytes form a Unicode codepoint. Right now specification states it supports only utf-8 encoding, but with Content-Type header I guess there is an idea of supporting other encodings in the future too. I think it would be even better then to use number of bytes instead of UTF-16 code unit or Unicode codepoint.

[dbaeumer]

@MaskRay we need to distinguish between the encoding used to transfer the JSON-RPC message. We currently use utf-8 here but as the header indicates this can be change to any encoding assuming that the encoding is supported in all libraries (for example node per default as only a limited set on encodings).

The column offset in a document assumes that after the JSON-RPC message as been decoded when parsing the string document content needs to be stored in UTF-16 encoding. We choose UTF-16 encoding here since most language store strings in memory in UTF-16 not UTF-8. To save one encoding pass we could transfer the JSON-RPC message in UTF-16 instead which is easy to support.

If we want to support UTF-8 for internal text document representation and line offsets this would be a breaking change or needs to be a capability the client announces.

Regarding byte offsets: there was another discussion whether the protocol should be offset based. However the protocol was design to support tools and their UI a for example a reference match in a file could not be rendered using byte offsets in a list. So the client would need to read the content of the file and convert the offset in line / column. We decided to let the server do this since the server very likely has read the file before anyways.

[Marwes]

We choose UTF-16 encoding here since most language store strings in memory in UTF-16 not UTF-8.

Source? Isn't the only reason for this is that Java/Javascript/C# uses UTF-16 as their string representation? I'd say there is a good case to made that (in hindsight) UTF-16 was a poor choice for string type in those language as well which makes it dubious to optimize for that case. The source code itself is usually UTF-8 (or just ascii) and as has been said this is also the case when transferring over JSON-RPC so I'd say the case is pretty strong for assuming UTF-8 instead of UTF-16.

[puremourning]

We choose UTF-16 encoding here since most language store strings in memory in UTF-16 not UTF-8. To save one encoding pass we could transfer the JSON-RPC message in UTF-16 instead which is easy to support.

Citation needed? ;)

Of the 7 downstream language completers we support in ycmd:

• 1 uses byte offsets (libclang)
• 6 use unicode code points (gocode, tern, tsserver*, jedi, racer, omnisharp*)
• 0 use utf 16 code units

* full disclosure, I think these use code points, else we have a bug!

The last is a bit of a fib, because we're integrating Language Server API for java.

However, as we receive byte offsets from the client, and internally use unicode code points, we have to reencode the file as utf 16, do a bunch of hackery to count the code units, then send the file, encoded as utf8 over to the language server, with offsets in utf 16 code units.

Of the client implementations of ycmd (there are about 8 I think), all of them are able to provide line-byte offsets. I don't know for certain all of them, but certainly the main one (Vim) is not able to provide utf 16 code units; they would have to be calculated.

Anyway, the point is that it might not be as simple as originally thought :D Though I appreciate that a specification is such, and changing it would be breaking. Just my 2p

[puremourning]

Not that SO is particularly reliable, but it happens to support my point, so I'm shamelessly going to quote from: https://stackoverflow.com/questions/30775689/python-length-of-unicode-string-confusion

You have 5 codepoints. One of those codepoints is outside of the Basic Multilingual Plane which means the UTF-16 encoding for those codepoints has to use two code units for the character.

In other words, the client is relying on an implementation detail, and is doing something wrong. They should be counting codepoints, not codeunits. There are several platforms where this happens quite regularly; Python 2 UCS2 builds are one such, but Java developers often forget about the difference, as do Windows APIs.

[MaskRay]

Emacs uses some extended UTF-8 and its functions return numbers in units of Unicode codepoints.

https://github.com/emacs-lsp/lsp-mode/blob/master/lsp-methods.el#L657

@vibhavp for Emacs lsp-mode internal representation

[szatanjl]

I am sorry in advance if I am telling something stupid right now. I have a question to you guys.

My thought process is that if there is a file in different encoding than any utf, and we use other encoding than utf in JSON-RCP (which can happen in future) then why would there be any need for the client and server to know what Unicode is at all?

Of the client implementations of ycmd (there are about 8 I think), all of them are able to provide line-byte offsets.

That's it. It is easy to provide line-byte offset. So why would it be better to use Unicode codepoints instead of bytes?

Let's say for example we have file encoded in iso-8859-1 and we use the same encoding for JSON-RPC communication. There is a character ä (0xE4) that can be represented at least in two ways in Unicode: U+00C4 (ä) or U+0061 (a) U+0308 (¨ - combining diaeresis). Former is one unicode codepoint, latter is two, and both are equally good and correct. If client uses one and server another we have a problem. Simply using line-byte offset here we would avoid these problems.

@dbaeumer I think we misunderstood each other or at least I did. I didn't mean to use byte offset from beginning of the file which would require client to convert it but to still use {line, column} pair. But count column in bytes instead of utf-16 code units or unicode codepoints.

[eonil]

We choose UTF-16 encoding here since most language store strings in memory in UTF-16 not UTF-8.

If we want to support UTF-8 for internal text document representation and line offsets this would be a breaking change or needs to be a capability the client announces.

Are you serious? UTF-16 is one of worst choice of old days due to lack of alternative solutions. Now we have UTF-8, and to choose UTF-16, you need a real good reason rather than a very brave assumption on implementation details of every softwares in the world especially if we consider future softwares.

This assumption is very true on Microsoft platforms which will never consider UTF-8. I think some bias to Microsoft is unavoidable as leadership of this project is from Microsoft, but this is too much. This reminds me Embrace, extend, and extinguish strategy. If this is the case, this is an enough reason to boycott LSP for me. Because we gonna see this kind of Microsoft-ish nonsense decision making forever.

[kdvolder]

Just to be clear, I don't work for Microsoft, and generally haven't been a big fan of them (being a Linux user myself). But I feel compelled to defend the LSP / vscode team here. I really don't think there's a big conspiracy theory here. From where I stand, it looks to me like Vscode and LSP teams are doing their very best to be inclusive and open.

The UTF-8 vs UTF-16 choice may seem like a big and important point to some, but to others, including myself, the choice probably seems somewhat arbitrary. For decisions like these, it is natural to write into the spec something that confirms to your current prototype implementation for choices like these, and I think this is perfectly reasonable.

Some may think that is a mistake. As this is an open spec and subject to change / revision/ discussion, everyone is free to voice their opinion and argue what choice is right and whether it should be changed... but I think such discussions should stick to technical arguments there's no need to resort to insinuations of a Microsoft conspiracy theory (moreover, these insinuations are really unwarranted here, in my opinion).

[eonil]

I apology for involving my political view in my comment. I was over-sensitive due to traumatic memory from Microsoft in old days. Now I see this spec is in progress and subject to change.

I didn't mention technical reasons because these are mainly repetition of other people's opinion or well known. Anyway, I list my technical reasons here.

• IMO, UTF-8 is present and future, and UTF-16 is legacy to avoid. The reason is here.
• By requiring dependency to UTF-16, LSP effectively forces program implementation to involve the legacy.
• Simplicity is better than extra complexity and dependency. One encoding for everywhere is better.
• More complexity and dependency increases amount of work of implementation a lot.
• AFAIK, Converting indices between different Unicode encodings are very expensive.
• LSP is a new protocol. No reason to involve a bad legacy. The only benefit here is potential benefit to specific platforms with native UTF-16 native strings.
• For now, the only reason to require UTF-16 is to give such benefit to specific implementations.
• Other platforms wouldn't be very happy due to increased complexity and potential performance penalty in implementation.
• Such unfair benefit is likely going to break community.

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... or needs to be a capability the client announces

I think this is fine. An optional field which designates encoding mode of indices beside the index numbers. If the encoding mode is set to utf-8, interpret the numbers as UTF-8 code points, and if it is utf-16, interpret them as UTF-16 code point. If the field is missing, fallback to UTF-16 for legacy compatibility.