void diff(const SubjectType &subject, const Slic3r::ExPolygons &clip, ResultType* retval, bool safety_offset_)
{
    Slic3r::Polygons pp;
    for (Slic3r::ExPolygons::const_iterator ex = clip.begin(); ex != clip.end(); ++ex) {
        Slic3r::Polygons ppp = *ex;
        pp.insert(pp.end(), ppp.begin(), ppp.end());
    }
    diff(subject, pp, retval, safety_offset_);
}

//-----------------------------------------------------------
// legacy code from Clipper documentation
void AddOuterPolyNodeToExPolygons(ClipperLib::PolyNode& polynode, Slic3r::ExPolygons& expolygons)
{  
  size_t cnt = expolygons.size();
  expolygons.resize(cnt + 1);
  ClipperPath_to_Slic3rMultiPoint(polynode.Contour, expolygons[cnt].contour);
  expolygons[cnt].holes.resize(polynode.ChildCount());
  for (int i = 0; i < polynode.ChildCount(); ++i)
  {
    ClipperPath_to_Slic3rMultiPoint(polynode.Childs[i]->Contour, expolygons[cnt].holes[i]);
    //Add outer polygons contained by (nested within) holes ...
    for (int j = 0; j < polynode.Childs[i]->ChildCount(); ++j)
      AddOuterPolyNodeToExPolygons(*polynode.Childs[i]->Childs[j], expolygons);
  }
}

Slic3r::Polygons
union_(const Slic3r::ExPolygons &subject1, const Slic3r::ExPolygons &subject2, bool safety_offset)
{
    Polygons pp;
    for (Slic3r::ExPolygons::const_iterator it = subject1.begin(); it != subject1.end(); ++it) {
        Polygons spp = *it;
        pp.insert(pp.end(), spp.begin(), spp.end());
    }
    for (Slic3r::ExPolygons::const_iterator it = subject2.begin(); it != subject2.end(); ++it) {
        Polygons spp = *it;
        pp.insert(pp.end(), spp.begin(), spp.end());
    }
    Polygons retval;
    union_(pp, &retval, safety_offset);
    return retval;
}

void
offset(const Slic3r::Surface &surface, Slic3r::Surfaces &retval, const float delta,
    double scale, ClipperLib::JoinType joinType, double miterLimit)
{
    // perform offset
    Slic3r::ExPolygons expp;
    offset_ex(surface.expolygon, expp, delta, scale, joinType, miterLimit);
    
    // clone the input surface for each expolygon we got
    retval.clear();
    retval.reserve(expp.size());
    for (ExPolygons::iterator it = expp.begin(); it != expp.end(); ++it) {
        Surface s = surface;  // clone
        s.expolygon = *it;
        retval.push_back(s);
    }
}

void
ClipperPaths_to_Slic3rExPolygons(const ClipperLib::Paths &input, Slic3r::ExPolygons &output)
{
    // init Clipper
    ClipperLib::Clipper clipper;
    clipper.Clear();
    
    // perform union
    clipper.AddPaths(input, ClipperLib::ptSubject, true);
    ClipperLib::PolyTree polytree;
    clipper.Execute(ClipperLib::ctUnion, polytree, ClipperLib::pftEvenOdd, ClipperLib::pftEvenOdd);  // offset results work with both EvenOdd and NonZero
    
    // write to ExPolygons object
    output.clear();
    PolyTreeToExPolygons(polytree, output);
}

void PolyTreeToExPolygons(ClipperLib::PolyTree& polytree, Slic3r::ExPolygons& expolygons)
{
  expolygons.clear();
  for (int i = 0; i < polytree.ChildCount(); ++i)
    AddOuterPolyNodeToExPolygons(*polytree.Childs[i], expolygons);
}

// This is a safe variant of the polygon offset, tailored for a single ExPolygon:
// a single polygon with multiple non-overlapping holes.
// Each contour and hole is offsetted separately, then the holes are subtracted from the outer contours.
void offset(const Slic3r::ExPolygons &expolygons, ClipperLib::Paths* retval, const float delta,
    ClipperLib::JoinType joinType, double miterLimit)
{
//    printf("new ExPolygon offset\n");
    const float delta_scaled = delta * float(CLIPPER_OFFSET_SCALE);
    ClipperLib::Paths contours;
    ClipperLib::Paths holes;
    contours.reserve(expolygons.size());
    {
        size_t n_holes = 0;
        for (size_t i = 0; i < expolygons.size(); ++ i)
            n_holes += expolygons[i].holes.size();
        holes.reserve(n_holes);
    }

    for (Slic3r::ExPolygons::const_iterator it_expoly = expolygons.begin(); it_expoly != expolygons.end(); ++ it_expoly) {
        // 1) Offset the outer contour.
        {
            ClipperLib::Path input;
            Slic3rMultiPoint_to_ClipperPath(it_expoly->contour, &input);
            scaleClipperPolygon(input);
            ClipperLib::ClipperOffset co;
            if (joinType == jtRound)
                co.ArcTolerance = miterLimit * double(CLIPPER_OFFSET_SCALE);
            else
                co.MiterLimit = miterLimit;
            co.AddPath(input, joinType, ClipperLib::etClosedPolygon);
            ClipperLib::Paths out;
            co.Execute(out, delta_scaled);
            contours.insert(contours.end(), out.begin(), out.end());
        }

        // 2) Offset the holes one by one, collect the results.
        {
            for (Polygons::const_iterator it_hole = it_expoly->holes.begin(); it_hole != it_expoly->holes.end(); ++ it_hole) {
                ClipperLib::Path input;
                Slic3rMultiPoint_to_ClipperPath_reversed(*it_hole, &input);
                scaleClipperPolygon(input);
                ClipperLib::ClipperOffset co;
                if (joinType == jtRound)
                    co.ArcTolerance = miterLimit * double(CLIPPER_OFFSET_SCALE);
                else
                    co.MiterLimit = miterLimit;
                co.AddPath(input, joinType, ClipperLib::etClosedPolygon);
                ClipperLib::Paths out;
                co.Execute(out, - delta_scaled);
                holes.insert(holes.end(), out.begin(), out.end());
            }
        }
    }

    // 3) Subtract holes from the contours.
    ClipperLib::Paths output;
    {
        ClipperLib::Clipper clipper;
        clipper.Clear();
        clipper.AddPaths(contours, ClipperLib::ptSubject, true);
        clipper.AddPaths(holes, ClipperLib::ptClip, true);
        clipper.Execute(ClipperLib::ctDifference, *retval, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
    }
    
    // 4) Unscale the output.
    unscaleClipperPolygons(*retval);
}

// This is a safe variant of the polygons offset, tailored for multiple ExPolygons.
// It is required, that the input expolygons do not overlap and that the holes of each ExPolygon don't intersect with their respective outer contours.
// Each ExPolygon is offsetted separately, then the offsetted ExPolygons are united.
ClipperLib::Paths _offset(const Slic3r::ExPolygons &expolygons, const float delta,
    ClipperLib::JoinType joinType, double miterLimit)
{
    const float delta_scaled = delta * float(CLIPPER_OFFSET_SCALE);
    // Offsetted ExPolygons before they are united.
    ClipperLib::Paths contours_cummulative;
    contours_cummulative.reserve(expolygons.size());
    // How many non-empty offsetted expolygons were actually collected into contours_cummulative?
    // If only one, then there is no need to do a final union.
    size_t expolygons_collected = 0;
    for (Slic3r::ExPolygons::const_iterator it_expoly = expolygons.begin(); it_expoly != expolygons.end(); ++ it_expoly) {
        // 1) Offset the outer contour.
        ClipperLib::Paths contours;
        {
            ClipperLib::Path input = Slic3rMultiPoint_to_ClipperPath(it_expoly->contour);
            scaleClipperPolygon(input);
            ClipperLib::ClipperOffset co;
            if (joinType == jtRound)
                co.ArcTolerance = miterLimit * double(CLIPPER_OFFSET_SCALE);
            else
                co.MiterLimit = miterLimit;
            co.ShortestEdgeLength = double(std::abs(delta_scaled * CLIPPER_OFFSET_SHORTEST_EDGE_FACTOR));
            co.AddPath(input, joinType, ClipperLib::etClosedPolygon);
            co.Execute(contours, delta_scaled);
        }
        if (contours.empty())
            // No need to try to offset the holes.
            continue;

        if (it_expoly->holes.empty()) {
            // No need to subtract holes from the offsetted expolygon, we are done.
            contours_cummulative.insert(contours_cummulative.end(), contours.begin(), contours.end());
            ++ expolygons_collected;
        } else {
            // 2) Offset the holes one by one, collect the offsetted holes.
            ClipperLib::Paths holes;
            {
                for (Polygons::const_iterator it_hole = it_expoly->holes.begin(); it_hole != it_expoly->holes.end(); ++ it_hole) {
                    ClipperLib::Path input = Slic3rMultiPoint_to_ClipperPath_reversed(*it_hole);
                    scaleClipperPolygon(input);
                    ClipperLib::ClipperOffset co;
                    if (joinType == jtRound)
                        co.ArcTolerance = miterLimit * double(CLIPPER_OFFSET_SCALE);
                    else
                        co.MiterLimit = miterLimit;
                    co.ShortestEdgeLength = double(std::abs(delta_scaled * CLIPPER_OFFSET_SHORTEST_EDGE_FACTOR));
                    co.AddPath(input, joinType, ClipperLib::etClosedPolygon);
                    ClipperLib::Paths out;
                    co.Execute(out, - delta_scaled);
                    holes.insert(holes.end(), out.begin(), out.end());
                }
            }

            // 3) Subtract holes from the contours.
            if (holes.empty()) {
                // No hole remaining after an offset. Just copy the outer contour.
                contours_cummulative.insert(contours_cummulative.end(), contours.begin(), contours.end());
                ++ expolygons_collected;
            } else if (delta < 0) {
                // Negative offset. There is a chance, that the offsetted hole intersects the outer contour. 
                // Subtract the offsetted holes from the offsetted contours.
                ClipperLib::Clipper clipper;
                clipper.Clear();
                clipper.AddPaths(contours, ClipperLib::ptSubject, true);
                clipper.AddPaths(holes, ClipperLib::ptClip, true);
                ClipperLib::Paths output;
                clipper.Execute(ClipperLib::ctDifference, output, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
                if (! output.empty()) {
                    contours_cummulative.insert(contours_cummulative.end(), output.begin(), output.end());
                    ++ expolygons_collected;
                } else {
                    // The offsetted holes have eaten up the offsetted outer contour.
                }
            } else {
                // Positive offset. As long as the Clipper offset does what one expects it to do, the offsetted hole will have a smaller
                // area than the original hole or even disappear, therefore there will be no new intersections.
                // Just collect the reversed holes.
                contours_cummulative.reserve(contours.size() + holes.size());
                contours_cummulative.insert(contours_cummulative.end(), contours.begin(), contours.end());
                // Reverse the holes in place.
                for (size_t i = 0; i < holes.size(); ++ i)
                    std::reverse(holes[i].begin(), holes[i].end());
                contours_cummulative.insert(contours_cummulative.end(), holes.begin(), holes.end());
                ++ expolygons_collected;
            }
        }
    }

    // 4) Unite the offsetted expolygons.
    ClipperLib::Paths output;
    if (expolygons_collected > 1 && delta > 0) {
        // There is a chance that the outwards offsetted expolygons may intersect. Perform a union.
        ClipperLib::Clipper clipper;
        clipper.Clear(); 
        clipper.AddPaths(contours_cummulative, ClipperLib::ptSubject, true);
        clipper.Execute(ClipperLib::ctUnion, output, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
    } else {
//.........这里部分代码省略.........