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From: Eli Zaretskii <eliz@gnu.org>
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Subject: Re: line-start?
Date: Fri, 07 Feb 2020 17:56:01 +0200
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To: Bastien <bzg@gnu.org>
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> From: Bastien <bzg@gnu.org>
> Cc: emacs-devel@gnu.org
> Date: Fri, 07 Feb 2020 15:46:09 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > But that gives a buffer position, not column number.
> 
> (- (window-line-start) (line-beginning-position))
> 
> gives me the column number.

That's not the column number, that's the ordinal number of the
character on a line.  It doesn't take into account characters that
take more than one column, it doesn't take into account characters
that are composed to produce a single grapheme cluster (or even a
single glyph), and it doesn't take into account variable-size fonts.

Of course, if all these are unimportant in your use case, you can
disregard them.  But a general-purpose function cannot disregard them.