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lib-vt: add unicode grapheme width API

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Embedders that render text outside the terminal grid need to predict
how many cells text will occupy once it is written to the terminal.
The existing codepoint width API exposes the table used by print, but
that is not enough for mode 2027 grapheme clustering: VS15/VS16, ZWJ
sequences, skin tone modifiers, and other continuation codepoints can
change the width of the whole cluster.

This exposes a single segment-and-measure API so callers use Ghostty
segmentation and width folding together:

    uint8_t width;
    size_t n = ghostty_unicode_grapheme_width(cps, len, &width);

From the Zig module:

    const vt = @import("ghostty-vt");
    const result = vt.unicode.graphemeWidth(u21, cps);

Callers loop until their string is consumed. The API is intentionally
not streaming: input must contain a complete first cluster or the
logical string end, so chunked readers should keep buffering when the
function consumes all available codepoints and more may arrive.

The terminal hot path now shares the width-decision func with the
API, the helper is inline and preserves the old branch structure. So
this doesn't change codegen at all.
This commit is contained in:
Mitchell Hashimoto
2026-07-04 14:02:16 -07:00
parent 61ce641fca
commit 65e61282a6
7 changed files with 381 additions and 38 deletions
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9
src/unicode/main.zig
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@@ -3,7 +3,11 @@ pub const lut = @import("lut.zig");
const grapheme = @import("grapheme.zig");
pub const table = @import("props_table.zig").table;
pub const Properties = @import("props.zig").Properties;
pub const GraphemeWidthEffect = grapheme.GraphemeWidthEffect;
pub const GraphemeWidth = grapheme.GraphemeWidth;
pub const graphemeBreak = grapheme.graphemeBreak;
pub const graphemeWidth = grapheme.graphemeWidth;
pub const graphemeWidthEffect = grapheme.graphemeWidthEffect;
/// Returns the terminal display width of a codepoint in terminal
/// grid cells: 0, 1, or 2.
@@ -16,8 +20,9 @@ pub const graphemeBreak = grapheme.graphemeBreak;
///
/// This operates on a single codepoint and cannot account for
/// grapheme-cluster-level width rules (VS16, combining sequences);
/// callers needing cluster-accurate widths must segment into
/// grapheme clusters and combine per-codepoint widths.
/// callers needing cluster-accurate widths should use graphemeWidth().
/// Summing per-codepoint widths is only correct when mode 2027 is
/// disabled.
pub fn codepointWidth(cp: u21) u2 {
return table.get(cp).width;
}