Using QGlyphRun for RTL text

qglyphrun.cpp

static void drawRTLTextWithDecorations(int xStart, QPainter *painter, QTextLayout *layout, const QVector<QTextLayout::FormatRange> &decorations)
{
    const auto glyphRuns = layout->glyphRuns();
    struct Glyph {
        quint32 glyphIndex;
        QPointF pos;
    };
    QVector<Glyph> glyphs;

    QRawFont f;
    // layout->glyphRuns() contains QGlyphRuns in arbitrary order so collect it into a list
    for (const auto &gr : glyphRuns) {
        const auto glyphIndexes = gr.glyphIndexes();
        const auto positions = gr.positions();
        for (int i = 0; i < glyphIndexes.size(); ++i) {
            glyphs.push_back({glyphIndexes[i], positions[i]});
        }
        if (!f.isValid()) {
            f = gr.rawFont();
        }
    }
    // sort them by descending order for rtl
    std::sort(glyphs.begin(), glyphs.end(), [](Glyph l, Glyph r) {
        return l.pos.x() > r.pos.x();
    });

    auto glyphRunForRange = [&glyphs, &f](int start, int length) -> std::optional<QGlyphRun> {
        QGlyphRun r;
        r.setRawFont(f);
        QVector<quint32> glyphIndices;
        QVector<QPointF> positions;
        for (int i = start; i < (start + length); ++i) {
            glyphIndices << glyphs[i].glyphIndex;
            positions << glyphs[i].pos;
        }
        r.setGlyphIndexes(glyphIndices);
        r.setPositions(positions);
        return r;
    };

    // glyphrun for decoration
    QVector<QGlyphRun> newGlyphRuns;
    for (const auto &fr : decorations) {
        const auto &fmt = fr.format;
        if (fmt.hasProperty(QTextCharFormat::ForegroundBrush) || fmt.hasProperty(QTextCharFormat::BackgroundBrush)) {
            auto run = glyphRunForRange(fr.start, fr.length);
            if (run.has_value()) {
                newGlyphRuns << run.value();
            }
        }
    }

    painter->setPen(Qt::red);
    for (const auto &r : newGlyphRuns) {
        painter->drawGlyphRun(QPointF(-xStart, 0), r);
        // painter->fillRect(r.boundingRect(), Qt::green);
    }
}