linux-kernel - Re: [PATCH 03/11] vt: properly support zero-width Unicode code points

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Message-ID: <5sq93805-27n3-76n1-ps76-n41o22sn914o@syhkavp.arg>
Date: Tue, 15 Apr 2025 15:06:01 -0400 (EDT)
From: Nicolas Pitre <nico@...xnic.net>
To: Jiri Slaby <jirislaby@...nel.org>
cc: Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
    linux-serial@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/11] vt: properly support zero-width Unicode code
 points

On Mon, 14 Apr 2025, Jiri Slaby wrote:

> On 10. 04. 25, 3:13, Nicolas Pitre wrote:
> > From: Nicolas Pitre <npitre@...libre.com>
> > 
> > Zero-width Unicode code points are causing misalignment in vertically
> > aligned content, disrupting the visual layout. Let's handle zero-width
> > code points more intelligently.
> ...
> > --- a/drivers/tty/vt/vt.c
> > +++ b/drivers/tty/vt/vt.c
> > @@ -443,6 +443,15 @@ static void vc_uniscr_scroll(struct vc_data *vc,
> > unsigned int top,
> >   	}
> >   }
> >   
> > +static u32 vc_uniscr_getc(struct vc_data *vc, int relative_pos)
> > +{
> > +	int pos = vc->state.x + vc->vc_need_wrap + relative_pos;
> > +
> > +	if (vc->vc_uni_lines && pos >= 0 && pos < vc->vc_cols)
> 
> So that is:
>   in_range(pos, 0, vc->vc_cols)
> right?

Good idea. Didn't know about that one.

> >   	if (vc->vc_utf && !vc->vc_disp_ctrl) {
> > -		if (ucs_is_double_width(c))
> > +		if (ucs_is_double_width(c)) {
> >   			width = 2;
> > +		} else if (ucs_is_zero_width(c)) {
> > +			prev_c = vc_uniscr_getc(vc, -1);
> > +			if (prev_c == ' ' &&
> > +			    ucs_is_double_width(vc_uniscr_getc(vc, -2))) {
> > +				/*
> > +				 * Let's merge this zero-width code point with
> > +				 * the preceding double-width code point by
> > +				 * replacing the existing whitespace padding.
> > +				 */
> > +				vc_con_rewind(vc);
> > +			} else if (c == 0xfe0f && prev_c != 0) {
> > +				/*
> > +				 * VS16 (U+FE0F) is special. Let it have a
> > +				 * width of 1 when preceded by a single-width
> > +				 * code point effectively making the later
> > +				 * double-width.
> > +				 */
> > +			} else {
> > +				/* Otherwise zero-width code points are
> > ignored */
> > +				goto out;
> > +			}
> > +		}
> 
> Please, extract this width evaluation to a separate function.

Done.