[Fontconfig] Matching font by unicode coverage

[Sam Varshavchik]

Behdad Esfahbod writes:

> Thanks David,
>
> Comments below.
>
> On 15-01-19 12:36 PM, David Lattimore wrote:
> > Matching with an FC_CHARSET works for me. No idea if what I'm doing is  
> right,
> > but roughly what I do is:
> > - Identify script for each character
> > (using http://unicode.org/Public/UNIDATA/Scripts.txt)
> > - Split runs based on script (so that no run has two different scripts)
> > - Build an FC_CHARSET for the run.
> > - Match and use the resulting font.
>
> While this works, it will be extremely slow, and results in ransom-note
> effects whereas eg adding a diacritic mark to a run will change the font for
> the entire run.
>
> What most clients of Fontconfig do instead is to call FcFontSort() for all
> desired properties but NOT FC_CHARSET, and then walk down the returned list  
> of
> font and pick the first font that supports each desired character.
>
> Does that make sense?

Does matching by FC_CHARSET really add that much overhead? What about the  
following approach:

1. First, search for the font you want using the usual properties – font  
name, weight, etc…

2. Once you have your font, get its FcCharset, then start looking up unicode  
characters in your text in the charset.

3. When you find one that's not there, look up its unicode script property,  
collect all following, consecutive characters with the same property into an  
FcCharset, search for a replacement font, and continue with the unicode  
lookup using the replacement font. Some obvious edge conditions here, but  
that's the basic idea.

4. You could reset, and go back to the original font, after the last one of  
the characters from the same script property that used the replacement font,  
and then try again with it, for the remaining characters.

I would think this would be optimized for a typical scenario – the original  
font will be used for most of the text, with the same replacement font for  
the exceptions.
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