contourchain.h

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#ifndef OPENSWMMVIS_CONTOUR_CONTOURCHAIN_H
#define OPENSWMMVIS_CONTOUR_CONTOURCHAIN_H
 
#include <QLineF>
#include <QMultiHash>
#include <QPolygonF>
#include <QVector>
 
#include <cmath>
 
namespace OpenSWMM::Contour {
 
inline QVector<QPolygonF>
chainIsoSegments(const QVector<QLineF> &segs, double quantum)
{
    QVector<QPolygonF> chains;
    if (segs.isEmpty() || quantum <= 0.0) return chains;
 
    auto key = [quantum](const QPointF &pt) -> quint64 {
        const auto qx = qint32(std::lround(pt.x() / quantum));
        const auto qy = qint32(std::lround(pt.y() / quantum));
        return (quint64(quint32(qx)) << 32) | quint64(quint32(qy));
    };
 
    QMultiHash<quint64, int> byEndpoint;
    byEndpoint.reserve(segs.size() * 2);
    for (int i = 0; i < segs.size(); ++i) {
        byEndpoint.insert(key(segs[i].p1()), i);
        byEndpoint.insert(key(segs[i].p2()), i);
    }
 
    QVector<bool> used(segs.size(), false);
 
    auto takeNext = [&](const QPointF &tip, QPointF &nextPt) -> bool {
        const auto range = byEndpoint.equal_range(key(tip));
        for (auto it = range.first; it != range.second; ++it) {
            const int j = it.value();
            if (used[j]) continue;
            used[j] = true;
            // Continue from whichever endpoint matched the tip.
            const QLineF &s = segs[j];
            nextPt = (key(s.p1()) == key(tip)) ? s.p2() : s.p1();
            return true;
        }
        return false;
    };
 
    for (int i = 0; i < segs.size(); ++i) {
        if (used[i]) continue;
        used[i] = true;
        QPolygonF poly;
        poly << segs[i].p1() << segs[i].p2();
 
        QPointF next;
        while (takeNext(poly.last(), next))   poly.append(next);
        while (takeNext(poly.first(), next))  poly.prepend(next);
        chains.append(std::move(poly));
    }
    return chains;
}
 
} // namespace OpenSWMM::Contour
 
#endif // OPENSWMMVIS_CONTOUR_CONTOURCHAIN_H