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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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32
project.cpp
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@ -863,6 +863,38 @@ extern "C" Configuration CrafterBuildProject(std::span<const std::string_view> a
hc.GetInterfacesAndImplementations(ifaces, hitImpls);
cfg.tests.push_back(std::move(hitTest));
// Issue #128: DrawInputField re-measured the cursor prefix via
// Font::GetLineWidth every frame of a focused field even though only the
// blink changes frame-to-frame. The prefix WIDTH is now memoised on the
// InputField keyed on (prefix bytes, fontSize); the absolute caretX is
// deliberately not cached so a relocated field can't get a stale caret.
// The memo is transparent presentation logic over a TrueType file —
// DrawText no-ops with a null atlas/renderer — so this test drives
// DrawInputField directly with no Vulkan device or window. The font is
// copied next to the binary; the test also probes the project root.
Test caretTest;
Configuration& crc = caretTest.config;
crc.path = cfg.path;
crc.name = "InputFieldCaretCache";
crc.outputName = "InputFieldCaretCache";
crc.type = ConfigurationType::Executable;
crc.target = cfg.target;
crc.march = cfg.march;
crc.mtune = cfg.mtune;
crc.debug = cfg.debug;
crc.sysroot = cfg.sysroot;
crc.dependencies = cfg.dependencies;
crc.externalDependencies = cfg.externalDependencies;
crc.compileFlags = cfg.compileFlags;
crc.linkFlags = cfg.linkFlags;
crc.defines = cfg.defines;
crc.cFiles = cfg.cFiles;
crc.files = { fs::path("tests/InputFieldCaretCache/font.ttf") };
std::vector<fs::path> caretImpls(impls.begin(), impls.end());
caretImpls.emplace_back("tests/InputFieldCaretCache/main");
crc.GetInterfacesAndImplementations(ifaces, caretImpls);
cfg.tests.push_back(std::move(caretTest));
// Issue #69: the engine feeds one shared Device::pipelineCache to every
// vkCreate*Pipelines call and persists it across runs, discarding an
// on-disk blob whose header doesn't match the current GPU. That gate —