Re: Line wrap reconsidered

[Yuan Fu]

> On May 30, 2020, at 2:14 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Yuan Fu <casouri@gmail.com>
>> Date: Fri, 29 May 2020 17:20:21 -0400
>> Cc: Lars Ingebrigtsen <larsi@gnus.org>,
>> emacs-devel@gnu.org
>> 
>>> Then you'll need to augment the test
>>> 
>>> else if (may_wrap)
>>> 
>>> with a test that the current character can be at BOL.  Maybe you
>>> should also make may_wrap a tristate instead of just a boolean YES/NO.
>>> 
>> 
>> I think I did jus that, i.e., if (may_wrap && char_can_wrap_before(it)).
> 
> Fundamentally, yes.  But having a complex condition
> 
>   if (FOO && BAR)
> 
> makes the code harder to read and understand, and thus makes logical
> errors easier, than a simple condition like
> 
>   if (FOOBAR == some_value)

In principle, yes, but I doubt the current logic can be simplified. Do you have 
some concrete example?

> 
>>>> Essentially I want to ask “if may_wrap == true, what does that mean? (In 
>>>> bidi context)” Did the character to the left of me (on glass) set it or 
>>>> the character to the right of me set it?
>>> 
>>> The one to the left.  But it is not necessarily the "previous"
>>> character in buffer position order.
>> 
>> I see, but then I don’t don’t understand how does the current code work with 
>> bidi display. In bidi context, space char can’t appear on the right of the 
>> line, which is the beginning of a logic line, right? That requires the logic 
>> to reverse. Is there something I’m missing?
> 
> I think the cause of the confusion is the "in bidi context" part.
> There are two such "contexts", and they behave differently:
> 
> . RTL characters when bidi-paragraph-direction is left-to-right
> . RTL characters when bidi-paragraph-direction is right-to-left
> 
> Which one were you talking about?  I was talking about the first one.
> 
>> Since may_wrap’s meaning is in terms of left and right, it need to be 
>> reversed in bidi text, no?
> 
> If bidi-paragraph-direction is right-to-left, then yes, they are
> reversed.  But not if the paragraph direction is left-to-right.

Then does bidi.c handle word wrapping when bidi-paragraph-direction is 
right-to-left? Paragraph 3.4 mentioned that “The accumulated widths of those 
glyphs (in logical order) are used to determine line breaks.”

> 
>> One more question. If I type 123456789... and set bidi-paragraph-direction 
>> to ‘right-to-left, it is still 123456789…, just aligned to the right. I 
>> expected to see …987654321, that’s what right-to-left mean in Chinese text. 
>> Why the order of each character not revered?
> 
> Digits and LTR characters (like English text) are rendered
> left-to-right even in RTL paragraphs, so what you see is correct.
> That's why this is called "bidirectional": the direction is not just
> universally right-to-left.

I see, so characters have their inherited order and that never changes, only 
paragraphs change orders. 

> 
>> UAX#9 didn’t say anything helpful.
> 
> It does, albeit in a convoluted and hard-to-grasp way.  See paragraph
> 3.3.6 there, and then rule L2 in paragraph 3.4, which describes the
> reordering procedure.

Thanks.

Yuan