Query model

[KaruroChori]

Queries should operate as containers, from which matching results should be provided as iterators for nodes or attributes.
Something like:

for(auto& element: document.root() / "base" / "resources" / "*" / "item"["attribute-a"]){
    //.... using element as `attribute`
}

Or in the string format I also use in vs.templ, like /base/resources/*/~attribute-a possibly constructed into a query via a constexpress builder.

[KaruroChori]

They will probably be implemented via generators as co-routines to make them lazy in evaluation.
The issue with co-routines in C++ is that they need dynamic memory, but it does not have to be heap.
In theory it is possible to use a custom allocator like they do for https://pigweed.dev/pw_async2/guides.html#coroutines

[KaruroChori]

The issues with co-routines, is that each call is a dynamic allocation, and it consumes around 72 bytes on my architecture, compared to the 24 of an equivalent tree iterator.
However, the simplicity of code is much higher, and dynamic allocations can be strongly diminished by implementing a proper allocator for this instance of generator.
I am willing to live with the increased memory footprint if code is going to be much simpler, at least for now.

Initial design of the query tokens @ https://github.com/lazy-eggplant/vs.xml/blob/master/docs/queries.md

[KaruroChori]

With 591e5b7 we got the first functioning version of queries 🥳 .
I consider the original task complete, but I am converting it to a tracked for what is still to add.

• Grouped attributes checks to avoid $k\cdot O(N)$ complexity.
• Enable custom allocator for the query co_routines to avoid those many memory allocations (already present in the test code)
• Clean up the interface to apply/perform the query.
• Enable missing text match (missing support in the rest of the library, code would be already there)
• Add a proper testsuite
• Update the docs, as they are now representative of an older revision of the queries proposal.

[KaruroChori]

This implementation for queries has been short lived, due to some annoying issues in the memory handling of temporary values, serialization and memory footprint.

Plans for revised implementation being worked on in specifications.

[KaruroChori]

Query rework unlocked by 96ed0e1