tendril
Warning: This library is at a very early stage of development, and it
contains a substantial amount of unsafe code. Use at your own risk!
Introduction
Tendril is a compact string/buffer type, optimized for zero-copy parsing.
Tendrils have the semantics of owned strings, but are sometimes views into
shared buffers. When you mutate a tendril, an owned copy is made if necessary.
Further mutations occur in-place until the string becomes shared, e.g. with
clone() or subtendril().
Buffer sharing is accomplished through thread-local (non-atomic) reference counting, which has very low overhead. The Rust type system will prevent you at compile time from sending a tendril between threads. (See below for thoughts on relaxing this restriction.)
Whereas String allocates in the heap for any non-empty string, Tendril can
store small strings (up to 8 bytes) in-line, without a heap allocation.
Tendril is also smaller than String on 64-bit platforms β 16 bytes versus
24. Option<Tendril> is the same size as Tendril, thanks to
NonZero.
The maximum length of a tendril is 4 GB. The library will panic if you attempt to go over the limit.
Formats and encoding
Tendril uses
phantom types
to track a buffer's format. This determines at compile time which
operations are available on a given tendril. For example, Tendril<UTF8> and
Tendril<Bytes> can be borrowed as &str and &[u8] respectively.
Tendril also integrates with
rust-encoding and has
preliminary support for WTF-8 buffers.
Plans for the future
Ropes
html5ever will use Tendril as a zero-copy text representation. It would
be good to preserve this all the way through to Servo's DOM. This would reduce
memory consumption, and possibly speed up text shaping and painting. However,
DOM text may conceivably be larger than 4 GB, and will anyway not be contiguous
in memory around e.g. a character entity reference.
Solution: Build a rope on top of these strings and use that as
Servo's representation of DOM text. We can perhaps do text shaping and/or
painting in parallel for different chunks of a rope. html5ever can additionally
use this rope type as a replacement for BufferQueue.
Because the underlying buffers are reference-counted, the bulk of this rope is already a persistent data structure. Consider what happens when appending two ropes to get a "new" rope. A vector-backed rope would copy a vector of small structs, one for each chunk, and would bump the corresponding refcounts. But it would not copy any of the string data.
If we want more sharing, then a 2-3 finger tree could be a good choice.
We would probably stick with VecDeque for ropes under a certain size.
UTF-16 compatibility
SpiderMonkey expects text to be in UCS-2 format for the most part. The
semantics of JavaScript strings are difficult to implement on UTF-8. This also
applies to HTML parsing via document.write. Also, passing SpiderMonkey a
string that isn't contiguous in memory will incur additional overhead and
complexity, if not a full copy.
Solution: Use WTF-8 in parsing and in the DOM. Servo will convert to contiguous UTF-16 when necessary. The conversion can easily be parallelized, if we find a practical need to convert huge chunks of text all at once.
Source span information
Some html5ever API consumers want to know the originating location in the HTML
source file(s) of each token or parse error. An example application would be a
command-line HTML validator with diagnostic output similar to rustc's.
Solution: Accept some metadata along with each input string. The type of
metadata is chosen by the API consumer; it defaults to (), which has size
zero. For any non-inline string, we can provide the associated metadata as well
as a byte offset.