Crafter.Graphics/tests/FontAdvanceCache/main.cpp

/*
Crafter®.Graphics
Copyright (C) 2026 Catcrafts®
catcrafts.net

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License version 3.0 as published by the Free Software Foundation;

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301 USA
*/

// Issue #57: Font::GetLineWidth used to call stbtt_GetCodepointHMetrics for
// every glyph on every call. Caret hit-testing (InputField_HitTestCursor)
// rescans the whole field once per character, so that is O(n²) HMetrics calls.
// Advances are now memoised per Font in *font units* (Font::AdvanceUnits):
// ASCII in a flat array, the rest in a map, rescaled per call.
//
// The cache must be in unscaled units, NOT pixels — the FontAtlas's
// Glyph::advance is baked at kBaseSize and is wrong at any other size, so it
// cannot be reused here. What is asserted:
//
//   - AdvanceUnits is size-independent: the value for a codepoint is identical
//     no matter which size was queried first (proves it stores units, not px).
//   - GetLineWidth equals the documented pipeline: sum of per-glyph
//     (int)(units * scaleForSize), with the truncation applied PER GLYPH.
//   - Repeated calls (cache hits) are byte-identical to the first (miss).
//   - Width scales with size — a 2× size is wider, ruling out a baked-at-one-
//     size advance that would make every size render the same width.
//   - The empty string is zero.
//   - Codepoints above ASCII (the map path) behave like the array path.
//
// Pure CPU: Font only touches stb_truetype, so no Vulkan device is needed.

#include <cstdlib>

import Crafter.Graphics;
import std;

using namespace Crafter;

namespace {

int failures = 0;

void Check(bool ok, std::string_view what) {
    std::println("{}  {}", ok ? "PASS" : "FAIL", what);
    if (!ok) ++failures;
}

std::filesystem::path FindFont() {
    for (const char* cand : {
            "font.ttf",
            "tests/FontAdvanceCache/font.ttf",
            "../../examples/HelloUI/font.ttf" }) {
        if (std::filesystem::exists(cand)) return cand;
    }
    return "font.ttf";
}

// The exact contract GetLineWidth must honour: truncate each glyph's scaled
// advance to int independently, then accumulate. Rescaling the cached unit
// advance per call is what keeps it correct at sizes other than kBaseSize.
std::uint32_t ExpectedWidth(Font& font, std::string_view text, float size) {
    float scale = font.ScaleForSize(size);
    std::uint32_t w = 0;
    std::size_t i = 0;
    while (i < text.size()) {
        std::uint32_t cp = DecodeUtf8(text, i);
        if (cp == 0) break;
        w += static_cast<int>(font.AdvanceUnits(cp) * scale);
    }
    return w;
}

} // namespace

int main() {
    Font font(FindFont());

    const std::string_view text = "Hello, World!";

    // ── 1. AdvanceUnits is size-independent (stored in font units). ─────────
    // Prime the cache via a large size, capture the unit advance, then prime
    // again via a tiny size. The unit value must not move — if the cache stored
    // pixels it would differ wildly between the two priming sizes.
    font.GetLineWidth(text, 96.0f);
    std::int32_t aBig = font.AdvanceUnits(static_cast<std::uint32_t>('A'));
    font.GetLineWidth(text, 6.0f);
    std::int32_t aSmall = font.AdvanceUnits(static_cast<std::uint32_t>('A'));
    Check(aBig == aSmall && aBig > 0,
          "AdvanceUnits('A') is a stable, positive font-unit value across sizes");

    // ── 2. GetLineWidth matches the per-glyph-truncated, rescaled pipeline. ─
    bool pipelineOk = true;
    for (float size : {6.0f, 11.0f, 18.0f, 32.0f, 64.0f, 100.0f}) {
        std::uint32_t got = font.GetLineWidth(text, size);
        std::uint32_t exp = ExpectedWidth(font, text, size);
        if (got != exp) {
            pipelineOk = false;
            std::println("    size {}: got {} expected {}", size, got, exp);
        }
    }
    Check(pipelineOk, "GetLineWidth == sum of per-glyph (int)(units * scale) at every size");

    // ── 3. Cache hits are byte-identical to the first (miss) call. ──────────
    std::uint32_t first = font.GetLineWidth(text, 18.0f);
    bool stable = true;
    for (int k = 0; k < 8; ++k) stable = stable && (font.GetLineWidth(text, 18.0f) == first);
    Check(stable && first > 0, "repeated GetLineWidth calls are identical (cache hit == miss)");

    // ── 4. Width scales with size — not baked at a single size. ─────────────
    std::uint32_t w18 = font.GetLineWidth(text, 18.0f);
    std::uint32_t w36 = font.GetLineWidth(text, 36.0f);
    Check(w36 > w18 && w36 > w18 + (w18 / 2),
          "doubling the size roughly doubles the width (advance is rescaled, not baked)");

    // ── 5. Empty string is zero. ────────────────────────────────────────────
    Check(font.GetLineWidth("", 18.0f) == 0, "empty string has zero width");

    // ── 6. Non-ASCII codepoints (the map path) behave like the array path. ──
    // U+00E9 'é' (UTF-8 0xC3 0xA9) is > 127 so it lands in advanceUnits_, not
    // the flat ASCII array. Its advance must be stable and a prefix narrower
    // than the whole.
    const std::string_view accented = "café";   // 'é' is two UTF-8 bytes
    std::uint32_t accWidth = font.GetLineWidth(accented, 24.0f);
    std::uint32_t accExp   = ExpectedWidth(font, accented, 24.0f);
    Check(accWidth == accExp && accWidth > font.GetLineWidth("caf", 24.0f),
          "non-ASCII codepoint goes through the map path and widens the line");
    std::uint32_t eFirst = static_cast<std::uint32_t>(font.AdvanceUnits(0x00E9));
    Check(static_cast<std::uint32_t>(font.AdvanceUnits(0x00E9)) == eFirst,
          "AdvanceUnits for a mapped codepoint is stable across calls");

    if (failures == 0) std::println("\nAll font advance-cache checks passed.");
    else               std::println("\n{} font advance-cache check(s) FAILED.", failures);
    return failures == 0 ? EXIT_SUCCESS : EXIT_FAILURE;
}