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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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14
interfaces/Crafter.Graphics-InputField.cppm
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@ -58,6 +58,20 @@ export namespace Crafter {
InputFieldType type = InputFieldType::Text;
bool focused = false;
std::size_t cursorPos = 0; // byte offset into `value`
// Caret-x memo (issue #128). For a focused field DrawInputField measures
// the cursor prefix via Font::GetLineWidth every frame, but only the
// blink (caretVisible) changes frame-to-frame — value/cursorPos/fontSize
// are stable across the vast majority of frames. Cache the measured
// prefix WIDTH (the GetLineWidth result, in px) keyed on the inputs that
// determine it: the prefix bytes and fontSize. The final caretX adds the
// layout-dependent textX each frame, so it is intentionally NOT cached —
// a field that moves on screen with an unchanged value would otherwise
// get a stale caret. `mutable` so the `const InputField&` draw fn can
// refresh it; these fields are internal and need no user initialisation.
mutable std::string caretCachePrefix_;
mutable float caretCacheFontSize_ = -1.0f; // <0 = unset
mutable float caretCacheWidth_ = 0.0f;
};
// Returns true if `s` is a valid candidate for the given type, including