[XeTeX] Hyphenation of "--" with tex-text mapping on

[Jonathan Kew]

On 25 Nov 2005, at 12:34 am, Will Robertson wrote:

> On 05/11/2005, at 12pm, Jonathan Kew wrote:
>
>> The reason it can happen is that font mappings (unlike traditional  
>> TFM-based ligatures) are completely invisible to TeX's line- 
>> breaking process. As far as it is concerned, the text still  
>> contains two hyphens, and there's a legitimate discretionary break  
>> after each of them.
>
> Sorry to be dense, but if font mappings aren't involved with line  
> breaking, how on earth does justification still work correctly?  
> Won't the line break be determined based on the width of "--",  
> which will later change after conversion to "–"?

No, because the font mapping will be applied when the width of "--"  
is measured, just as it is applied when the "--" is rendered to the  
output.

But TeX looks for line-break positions in the underlying text  
(sequence of characters and glue, mainly), and the font mapping  
doesn't change that. (I considered implementing this differently, but  
that would lead to other issues....)

JK