[Core] Provide information about source string to Clay_MeasureText

[Riquelme02]

Hello Nic,
I've been playing with Clay for a few weeks
now after a friend recommended it to me.

Unfortunately I've hit a brick wall when it comes to string handling when
I tried integrating it into an existing project. We handle text by shaping
our paragraphs with HarfBuzz and caching the result, which is an array of
glyph information - all further text operations work by specifying start
and end indices into that array. Meanwhile Clay can only give us raw strings
and no realistic method to extract the necessary indices from them.

To summarize: Clay provides a string slice, and what we
need is actually the start and end indices of that slice.
(we also need the entire original string, although this part I could easily
solve by passing around a handle to our cache through the element config).

Can you think of a feasible solution on Clay's side? I would love to
use this library without maintaing a local copy that does what I need.

[nicbarker]

Hello 👋
That sounds like the kind of thing we are likely to run into in future. We actually need to introduce a breaking change into the API for the text measurement callback anyway, so I'll get this one in too at the same time 🙂
Would it work to provide a pointer to the original entire string, so you can do pointer arithmetic on the string to measure to figure out the index?

[nicbarker]

I'm imagining something like

Clay_Dimensions Clay_MeasureText(Clay_String toMeasure, Clay_String originalString, Clay_TextElementConfig *config) {
    int32_t startIndex = toMeasure.chars - originalString.chars;
    int32_t endIndex = startIndex + toMeasure.length;
}

[Riquelme02]

Thank you, I think pointer arithmetic should work.
What about the text render command? Is there already
an easy way to access this information during that step
or will you propagate these changes there as well?

[nicbarker]

@Riquelme02 I will investigate providing that information in the render command as well 👍