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perf(ui): memoize the input-field caret prefix width (#128)

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DrawInputField re-measured the cursor prefix via Font::GetLineWidth on
every frame of a focused field, even though only the blink (caretVisible)
changes frame-to-frame — value, cursorPos and fontSize are stable across
the vast majority of frames.

Cache the measured prefix WIDTH on the InputField, keyed on (prefix bytes,
fontSize). A cheap byte compare of the cached prefix guards the expensive
per-glyph UTF-8 decode + advance accumulation. The cache is mutable so the
const-ref draw fn can refresh it.

The absolute caretX is deliberately NOT cached: it adds the layout-
dependent textX (rect.x + paddingX) each frame, so caching it would give a
relocated field a stale caret. Caching the width keeps that correct.

Adds tests/InputFieldCaretCache pinning: the cached caret matches the
uncached GetLineWidth oracle across states, a redraw (cache hit) is
identical to the miss, the cache invalidates on value/cursorPos/fontSize
changes, the blink never moves the caret, and a relocated field shifts the
caret by exactly the rect delta.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
catbot 2026-06-18 13:29:55 +00:00
commit a45e793da4
5 changed files with 309 additions and 1 deletions
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16
implementations/Crafter.Graphics-InputField.cpp
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@ -150,7 +150,21 @@ void Crafter::DrawInputField(UIBuffer& buf, const InputField& f, Rect rect,
if (f.focused && caretVisible && *buf.quadCount < buf.quadCap) {
std::string_view sub(f.value.data(), std::min(f.cursorPos, f.value.size()));
float caretX = textX + (sub.empty() ? 0.0f : static_cast<float>(font.GetLineWidth(sub, fontSize)));
// Memoise the prefix width (issue #128): only the blink changes between
// frames, so re-walking the prefix every frame is wasted work. A byte
// compare of the cached prefix (plus fontSize) is far cheaper than the
// per-glyph UTF-8 decode + advance accumulation GetLineWidth does.
float width;
if (f.caretCacheFontSize_ == fontSize && f.caretCachePrefix_ == sub) {
width = f.caretCacheWidth_;
} else {
width = sub.empty() ? 0.0f
: static_cast<float>(font.GetLineWidth(sub, fontSize));
f.caretCachePrefix_.assign(sub);
f.caretCacheFontSize_ = fontSize;
f.caretCacheWidth_ = width;
}
float caretX = textX + width;
float caretH = fontSize * 1.1f;
float caretY = rect.y + (rect.h - caretH) * 0.5f;
float caretW = std::max(1.0f, fontSize / 16.0f);