GNU bug report logs - #17181
24.4.50; previous-single-property-change: odd behavior

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: raman <at> google.com (T.V Raman)
To: bug-gnu-emacs <at> gnu.org
Subject: 24.4.50; previous-single-property-change: odd behavior 
Date: Thu, 03 Apr 2014 17:47:28 -0700

Function previous-single-property-change appears to have an odd bug, try
this in a new buffer:

M-x switch-to-buffer RET foo 
to create a  buffer in fundamental-mode.

Type 
foo RET
bar
Move to the beginning of the line with bar
Evaluate 
(put-text-property (point) (point-max) 'test t)
Move point to the front of "bar" ie  point is at the beginning of the
line.
(get-text-property (point) 'test) returns T as expected.
(point) evaluates to 6

Consequently, I'd  expect 
(previous-single-property-change (point) 'test) to return 5 
instead you get: nil

get-text-property does show that there is no text-property 'test set at
positions 5, 4 etc -- 
previous-single-property-change behaves as I had expected if I evaluate
the same form with point on "a" 
ie it returns where the property changes from t to nil

It looks  like previous-single-property-change might have an off-by-one
bug in that it is examining property on preceding-char?







In GNU Emacs 24.4.50.2 (x86_64-unknown-linux-gnu, GTK+ Version 2.24.10)
 of 2014-04-03 on lapdog
System Description:	Ubuntu 12.04.1 LTS

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-- 

Message #12 received at 17181-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: raman <at> google.com (T.V Raman)
Cc: 17181-done <at> debbugs.gnu.org
Subject: Re: bug#17181: 24.4.50; previous-single-property-change: odd behavior
Date: Thu, 03 Apr 2014 22:27:56 -0400

tags 17181 notabug
thanks

> (put-text-property (point) (point-max) 'test t)
> Move point to the front of "bar" ie  point is at the beginning of the
> line.
> (get-text-property (point) 'test) returns T as expected.
> (point) evaluates to 6
> Consequently, I'd  expect 
> (previous-single-property-change (point) 'test) to return 5 
> instead you get: nil

Buffer positions are *between* characters.  `get-text-property' gets you
the value of the property on "the character after point".
`previous-single-property-change' gets you the nearest change on
*previous* characters.  The property above is nil between point-min and
point, so previous-single-property-change finds no change at all.

The off-by-one error is in your expectations :-(


        Stefan

Message #15 received at 17181-done <at> debbugs.gnu.org (full text, mbox):

From: raman <at> google.com (T.V Raman)
To: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
Cc: 17181-done <at> debbugs.gnu.org
Subject: Re: bug#17181: 24.4.50; previous-single-property-change: odd behavior
Date: Fri, 04 Apr 2014 08:18:40 -0700

:-) Hi Stefan --
I understand how it works now:-)

Thanks,

-- 

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