Method get_corner_start_edge

source/matplot/util/contourc.cpp:1000–1088

    }

    Edge QuadContourGenerator::get_corner_start_edge(
        long quad, unsigned int level_index) const {
        assert(quad >= 0 && quad < _n && "Quad index out of bounds");
        assert((level_index == 1 || level_index == 2) &&
               "level index must be 1 or 2");
        assert(EXISTS_ANY_CORNER(quad) && "Quad is not a corner");

        // Diagram for NE corner.  Rotate for other corners.
        //
        //           edge12
        // point1 +---------+ point2
        //          \       |
        //            \     | edge23
        //       edge31 \   |
        //                \ |
        //                  + point3
        //
        long point1, point2, point3;
        Edge edge12, edge23, edge31;
        switch (_cache[quad] & MASK_EXISTS) {
        case MASK_EXISTS_SW_CORNER:
            point1 = POINT_SE;
            point2 = POINT_SW;
            point3 = POINT_NW;
            edge12 = Edge::Edge_S;
            edge23 = Edge::Edge_W;
            edge31 = Edge::Edge_NE;
            break;
        case MASK_EXISTS_SE_CORNER:
            point1 = POINT_NE;
            point2 = POINT_SE;
            point3 = POINT_SW;
            edge12 = Edge::Edge_E;
            edge23 = Edge::Edge_S;
            edge31 = Edge::Edge_NW;
            break;
        case MASK_EXISTS_NW_CORNER:
            point1 = POINT_SW;
            point2 = POINT_NW;
            point3 = POINT_NE;
            edge12 = Edge::Edge_W;
            edge23 = Edge::Edge_N;
            edge31 = Edge::Edge_SE;
            break;
        case MASK_EXISTS_NE_CORNER:
            point1 = POINT_NW;
            point2 = POINT_NE;
            point3 = POINT_SE;
            edge12 = Edge::Edge_N;
            edge23 = Edge::Edge_E;
            edge31 = Edge::Edge_SW;
            break;
        default:
            assert(0 && "Invalid EXISTS for quad");
            return Edge::Edge_None;
        }

        unsigned int config = (Z_LEVEL(point1) >= level_index) << 2 |