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

[Jiri Slaby <jirislaby@kernel.org>]

On 10. 04. 25, 3:13, Nicolas Pitre wrote:
> From: Nicolas Pitre <npitre@baylibre.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?

> +		return vc->vc_uni_lines[vc->state.y][pos];
> +	return 0;
> +}
> +
>   static void vc_uniscr_copy_area(u32 **dst_lines,
>   				unsigned int dst_cols,
>   				unsigned int dst_rows,
> @@ -2905,18 +2914,49 @@ static bool vc_is_control(struct vc_data *vc, int tc, int c)
>   	return false;
>   }
>   
> +static void vc_con_rewind(struct vc_data *vc)
> +{
> +	if (vc->state.x && !vc->vc_need_wrap) {
> +		vc->vc_pos -= 2;
> +		vc->state.x--;
> +	}
> +	vc->vc_need_wrap = 0;
> +}
> +
>   static int vc_con_write_normal(struct vc_data *vc, int tc, int c,
>   		struct vc_draw_region *draw)
>   {
> -	int next_c;
> +	int next_c, prev_c;
>   	unsigned char vc_attr = vc->vc_attr;
>   	u16 himask = vc->vc_hi_font_mask, charmask = himask ? 0x1ff : 0xff;
>   	u8 width = 1;
>   	bool inverse = false;
>   
>   	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.

...
> --- a/include/linux/consolemap.h
> +++ b/include/linux/consolemap.h
...
> @@ -63,6 +68,11 @@ static inline bool ucs_is_double_width(uint32_t cp)
>   {
>   	return false;
>   }
> +
> +static inline bool ucs_is_zero_width(uint32_t cp)
> +{
> +	return false;
> +}

Again, is this necessary?

thanks,
-- 
js
suse labs