bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

This appears to a corner case where previous-single-property-change
breaks in a surprizing way.

1. ;;; Evaluate the below to get a string with properties.
(setq s

#("-L Trace line evolution (-L)

History simplification" 0 2 (face transient-blue button (t) category transient-button-button command magit-log:-L) 2 25 (button (t) category transient-button-button command magit-log:-L) 25 27 (face transient-inactive-value button (t) category transient-button-button command magit-log:-L) 27 28 (button (t) category transient-button-button command magit-log:-L) 30 52 (face transient-heading)))


2.;;; Insert into a new buffer
(switch-to-buffer "foo")
(insert s)

4. ;;; Place point on the 'H' of "history"

   5. ;;; eval
   (previous-single-property-change (point 'face)

   Instead of returning value of point before the 'H', this returns a
   value that is unexpected, it returns point  past the ')'

   --Raman


In GNU Emacs 28.0.50 (build 13, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0)
 of 2021-07-22 built on raman-glaptop
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-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Eli Zaretskii]

> Date: Mon, 26 Jul 2021 08:47:07 -0700
> From:  "T.V Raman" via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 
> This appears to a corner case where previous-single-property-change
> breaks in a surprizing way.
> 
> 1. ;;; Evaluate the below to get a string with properties.
> (setq s
> 
> #("-L Trace line evolution (-L)
> 
> History simplification" 0 2 (face transient-blue button (t) category transient-button-button command magit-log:-L) 2 25 (button (t) category transient-button-button command magit-log:-L) 25 27 (face transient-inactive-value button (t) category transient-button-button command magit-log:-L) 27 28 (button (t) category transient-button-button command magit-log:-L) 30 52 (face transient-heading)))
> 
> 
> 2.;;; Insert into a new buffer
> (switch-to-buffer "foo")
> (insert s)
> 
> 4. ;;; Place point on the 'H' of "history"
> 
>    5. ;;; eval
>    (previous-single-property-change (point 'face)
> 
>    Instead of returning value of point before the 'H', this returns a
>    value that is unexpected, it returns point  past the ')'

What does "M-x describe-text-properties RET" say at the position
returned by the above previous-single-property-change call?  Does that
value explain the result?  If not, why not?

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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Eli Zaretskii <eliz@gnu.org> writes:


See below for the results you ask. It does not explain itself because
property 'face is nil at positions  30 and 29.

Contents of help buffer after describe-text-properties below:
Text content at position 28:


Here is a ¡®nil¡¯ button labeled ¡®)¡¯.


There are text properties here:
  button               (t)
  category             transient-button-button
  command              magit-log:-L

[back]

>> Date: Mon, 26 Jul 2021 08:47:07 -0700
>> From:  "T.V Raman" via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>> 
>> 
>> This appears to a corner case where previous-single-property-change
>> breaks in a surprizing way.
>> 
>> 1. ;;; Evaluate the below to get a string with properties.
>> (setq s
>> 
>> #("-L Trace line evolution (-L)
>> 
>> History simplification" 0 2 (face transient-blue button (t) category transient-button-button command magit-log:-L) 2 25 (button (t) category transient-button-button command magit-log:-L) 25 27 (face transient-inactive-value button (t) category transient-button-button command magit-log:-L) 27 28 (button (t) category transient-button-button command magit-log:-L) 30 52 (face transient-heading)))
>> 
>> 
>> 2.;;; Insert into a new buffer
>> (switch-to-buffer "foo")
>> (insert s)
>> 
>> 4. ;;; Place point on the 'H' of "history"
>> 
>>    5. ;;; eval
>>    (previous-single-property-change (point 'face)
>> 
>>    Instead of returning value of point before the 'H', this returns a
>>    value that is unexpected, it returns point  past the ')'
>
> What does "M-x describe-text-properties RET" say at the position
> returned by the above previous-single-property-change call?  Does that
> value explain the result?  If not, why not?

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
7©4 Id: kg:/m/0285kf1  •0Ü8

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Eli Zaretskii]

> From: "T.V Raman" <raman@google.com>
> Cc: 49742@debbugs.gnu.org
> Date: Mon, 26 Jul 2021 10:19:13 -0700
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> 
> See below for the results you ask. It does not explain itself because
> property 'face is nil at positions  30 and 29.

Yes, it's nil.  So that is the last position before 'H' where the
value of the 'face' property changes -- from transient-inactive-value
to nil.  Which is why the function returns that position.

If this still doesn't explain the results, please tell what is missing
from the explanation.

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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Eli Zaretskii <eliz@gnu.org> writes:

It's not returning the position be 'H', it's returning a point 3
positions further back

>> From: "T.V Raman" <raman@google.com>
>> Cc: 49742@debbugs.gnu.org
>> Date: Mon, 26 Jul 2021 10:19:13 -0700
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> 
>> See below for the results you ask. It does not explain itself because
>> property 'face is nil at positions  30 and 29.
>
> Yes, it's nil.  So that is the last position before 'H' where the
> value of the 'face' property changes -- from transient-inactive-value
> to nil.  Which is why the function returns that position.
>
> If this still doesn't explain the results, please tell what is missing
> from the explanation.

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
7©4 Id: kg:/m/0285kf1  •0Ü8

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Eli Zaretskii]

> From: "T.V Raman" <raman@google.com>
> Cc: 49742@debbugs.gnu.org
> Date: Mon, 26 Jul 2021 19:02:03 -0700
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> It's not returning the position be 'H', it's returning a point 3
> positions further back

Yes, that's what I meant.  If you still think it is incorrect, could
you please explain why you think that result is incorrect?

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

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Eli Zaretskii <eliz@gnu.org> writes:

I believe  value returned should be point before the 'H', ie the newline
where property face is nil.
>> From: "T.V Raman" <raman@google.com>
>> Cc: 49742@debbugs.gnu.org
>> Date: Mon, 26 Jul 2021 19:02:03 -0700
>> 
>> Eli Zaretskii <eliz@gnu.org> writes:
>> 
>> It's not returning the position be 'H', it's returning a point 3
>> positions further back
>
> Yes, that's what I meant.  If you still think it is incorrect, could
> you please explain why you think that result is incorrect?

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
7©4 Id: kg:/m/0285kf1  •0Ü8

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Eli Zaretskii]

On July 27, 2021 6:06:31 AM GMT+03:00, "T.V Raman" <raman@google.com> wrote:
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> I believe  value returned should be point before the 'H', ie the
> newline
> where property face is nil.

No, because this function scans back from POSITION, and that is the position between the newline and 'H' in your case, because that's where point is.  So the function must return a position before that.

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

we're talking across one another,   point is on 'H' with face
'transient-heading. previous-single-property-change should return 1 <
(point) since the face property there is nil; instead it's returning a
value that is further back.

Eli Zaretskii writes:
 > On July 27, 2021 6:06:31 AM GMT+03:00, "T.V Raman" <raman@google.com> wrote:
 > > Eli Zaretskii <eliz@gnu.org> writes:
 > > 
 > > I believe  value returned should be point before the 'H', ie the
 > > newline
 > > where property face is nil.
 > 
 > No, because this function scans back from POSITION, and that is the position between the newline and 'H' in your case, because that's where point is.  So the function must return a position before that.

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

--

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Andreas Schwab]

previous-single-property-change starts with the property value on the
character preceding position, and returns a position so that the
property value on the preceding character is different.

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
GPG Key fingerprint = 7578 EB47 D4E5 4D69 2510  2552 DF73 E780 A9DA AEC1
"And now for something completely different."

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

That now all makes sense, Thanks Andreas for the explanation!

This also means that if you want to find the range of characters
having the same face, then it's a mess if you'r eon the first
character of that range

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

--

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[Lars Ingebrigtsen]

"T.V Raman" via "Bug reports for GNU Emacs, the Swiss army knife of text
editors" <bug-gnu-emacs@gnu.org> writes:

> This also means that if you want to find the range of characters
> having the same face, then it's a mess if you'r eon the first
> character of that range

Yup.  Consider using (text-property-search-forward 'face) instead.

But as there is no bug here, I'm closing this bug report.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

bug#49742: 28.0.50; previous-single-property-change sometimes wrong?

[T.V Raman via Bug reports for GNU Emacs, the Swiss army knife of text editors]

SG -- thanks Lars, and I only recently discovered
text-property-searc-* it's one of those hidden gems.

Lars Ingebrigtsen writes:
 > "T.V Raman" via "Bug reports for GNU Emacs, the Swiss army knife of text
 > editors" <bug-gnu-emacs@gnu.org> writes:
 > 
 > > This also means that if you want to find the range of characters
 > > having the same face, then it's a mess if you'r eon the first
 > > character of that range
 > 
 > Yup.  Consider using (text-property-search-forward 'face) instead.
 > 
 > But as there is no bug here, I'm closing this bug report.
 > 
 > -- 
 > (domestic pets only, the antidote for overdose, milk.)
 >    bloggy blog: http://lars.ingebrigtsen.no

-- 

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮

--

Thanks,

--Raman(I Search, I Find, I Misplace, I Research)
♉ Id: kg:/m/0285kf1  🦮