opencv FillEdgeCollection

static void  FillEdgeCollection( Mat& img, std::vector<PolyEdge>& edges, const void* color )
{
    PolyEdge tmp;
    int i, y, total = (int)edges.size();
    Size size = img.size();
    PolyEdge* e;
    int y_max = INT_MIN, y_min = INT_MAX;
    int64 x_max = 0xFFFFFFFFFFFFFFFF, x_min = 0x7FFFFFFFFFFFFFFF;
    int pix_size = (int)img.elemSize();

    if( total < 2 )
        return;

    for( i = 0; i < total; i++ )
    {
        PolyEdge& e1 = edges[i];
        assert( e1.y0 < e1.y1 );
        // Determine x-coordinate of the end of the edge.
        // (This is not necessary x-coordinate of any vertex in the array.)
        int64 x1 = e1.x + (e1.y1 - e1.y0) * e1.dx;
        y_min = std::min( y_min, e1.y0 );
        y_max = std::max( y_max, e1.y1 );
        x_min = std::min( x_min, e1.x );
        x_max = std::max( x_max, e1.x );
        x_min = std::min( x_min, x1 );
        x_max = std::max( x_max, x1 );
    }

    if( y_max < 0 || y_min >= size.height || x_max < 0 || x_min >= ((int64)size.width<<XY_SHIFT) )
        return;

    std::sort( edges.begin(), edges.end(), CmpEdges() );

    // start drawing
    tmp.y0 = INT_MAX;
    edges.push_back(tmp); // after this point we do not add
                          // any elements to edges, thus we can use pointers
    i = 0;
    tmp.next = 0;
    e = &edges[i];
    y_max = MIN( y_max, size.height );

    for( y = e->y0; y < y_max; y++ )
    {
        PolyEdge *last, *prelast, *keep_prelast;
        int sort_flag = 0;
        int draw = 0;
        int clipline = y < 0;

        prelast = &tmp;
        last = tmp.next;
        while( last || e->y0 == y )
        {
            if( last && last->y1 == y )
            {
                // exclude edge if y reaches its lower point
                prelast->next = last->next;
                last = last->next;
                continue;
            }
            keep_prelast = prelast;
            if( last && (e->y0 > y || last->x < e->x) )
            {
                // go to the next edge in active list
                prelast = last;
                last = last->next;
            }
            else if( i < total )
            {
                // insert new edge into active list if y reaches its upper point
                prelast->next = e;
                e->next = last;
                prelast = e;
                e = &edges[++i];
            }
            else
                break;

            if( draw )
            {
                if( !clipline )
                {
                    // convert x's from fixed-point to image coordinates
                    uchar *timg = img.ptr(y);
                    int x1, x2;

                    if (keep_prelast->x > prelast->x)
                    {
                        x1 = (int)((prelast->x + XY_ONE - 1) >> XY_SHIFT);
                        x2 = (int)(keep_prelast->x >> XY_SHIFT);
                    }
                    else
                    {
                        x1 = (int)((keep_prelast->x + XY_ONE - 1) >> XY_SHIFT);
                        x2 = (int)(prelast->x >> XY_SHIFT);
                    }

                    // clip and draw the line
                    if( x1 < size.width && x2 >= 0 )
                    {
                        if( x1 < 0 )
                            x1 = 0;
                        if( x2 >= size.width )
                            x2 = size.width - 1;
                        ICV_HLINE( timg, x1, x2, color, pix_size );
                    }
                }
                keep_prelast->x += keep_prelast->dx;
                prelast->x += prelast->dx;
            }
            draw ^= 1;
        }

        // sort edges (using bubble sort)
        keep_prelast = 0;

        do
        {
            prelast = &tmp;
            last = tmp.next;

            while( last != keep_prelast && last->next != 0 )
            {
                PolyEdge *te = last->next;

                // swap edges
                if( last->x > te->x )
                {
                    prelast->next = te;
                    last->next = te->next;
                    te->next = last;
                    prelast = te;
                    sort_flag = 1;
                }
                else
                {
                    prelast = last;
                    last = te;
                }
            }
            keep_prelast = prelast;
        }
        while( sort_flag && keep_prelast != tmp.next && keep_prelast != &tmp );
    }
}