Re: Is there an IT test for a tab-width spacer?

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 24 Oct 2017 17:59:47 -0700
> From: Keith David Bershatsky <esq@lawlist.com>
> 
> it.c lets us know whether we are on the initial tab that has been assigned a character by the user with the buffer-display-table, e.g., 187/».
> 
> it.c also lets us know whether we have moved beyond that location to a tab without a character; i.e., 9, aka (char-to-string 9).
> 
> I am having trouble calculating the X and HPOS for each and every location between the initial tab and the next textual character.  A buffer-display-table for a tab that is assigned a leading 187 followed by a 9 has a minimum tab-width of 2.  Even though the buffer-local value for tab-width may be 2, that combination of 187/9 might be a total of 3 characters wide depending upon the circumstances.  And as the user increases the buffer-local tab-width value, it seemingly becomes more difficult to predict the X and HPOS.

I'm sorry, but I don't understand your description: you use
terminology which I cannot decipher.  What is "the initial tab"?
"initial" in what sense?

Also, what do you mean by calculate position "for each and every
location between the initial tab and the next character"?  What are
those locations, and why do you need the coordinates of each one of
them?

Can you show an example of a buffer where this issue arises?

(I think you are making a mistake looking at it.c, you should look at
it.char_to_display instead.  The former is the character found in the
buffer or string, the latter is what actually needs to be displayed,
and it could be different, e.g., due to display-table in effect.)

> Inching along with a test similar to line 22682 in xdisp.c seems insufficient for this particular project:  while (it.current_x + it.pixel_width <= target_x).
> 
> To analyze the width of the spacer and calculate the X and HPOS for each location within that space, is it necessary to go to the next character and then mathematically determine what the previous tab-width was?

Sorry, I cannot answer these questions, because I don't understand
what you are trying to accomplish.