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.
Embedders that render text outside the terminal grid need to predict
how many cells a codepoint will occupy once it is written to the
terminal. The immediate motivation is IME preedit overlay rendering:
measuring preedit text with font APIs (e.g. CoreText advances) can
disagree with the terminal's unicode table on ambiguous-width CJK and
emoji, causing the overlay to visibly jump when the composed text
commits and reflows through the real grid layout.
This exposes the exact width table the terminal print path already
uses, so overlays are column-accurate by construction. From C:
uint8_t w = ghostty_unicode_codepoint_width(0x4E00); // 2
And from the Zig module:
const vt = @import("ghostty-vt");
const w = vt.unicode.codepointWidth(0x4E00); // 2
The function is total over its input: 0 for zero-width codepoints
(controls, combining marks, default-ignorables, surrogates), 2 for
wide codepoints (East Asian Wide/Fullwidth, regional indicators,
clamped at 2), and 1 for everything else, including invalid values
beyond U+10FFFF.
Perf: uses the LUT lookup we use for the main core terminal
Binary size: the width table was already linked into libghostty-vt
via the print path, so this adds only the exported wrapper.
This fixes the VS16 issues found in this test:
https://ucs-detect.readthedocs.io/sw_results/ghostty.html#ghostty
This is also a more robust way to handle VS15/16 in general.
This commit also changes our propeties to be a packed struct which
reduces its size from 4 bytes to 1 and likewise drops our unicode table
size 4x.
This makes it cleaner to add new sources of table generation and also
avoids inadvertently depending on different modules (despite Zig's lazy
analysis).
This also fixes up terminal to only use our look up tables which avoids
bringing ziglyph in for the terminal module.
