ring_flip_visitor.h

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#ifndef SQL_GIS_RING_FLIP_VISITOR_H_INCLUDED
#define SQL_GIS_RING_FLIP_VISITOR_H_INCLUDED
 
// Copyright (c) 2017, 2025, Oracle and/or its affiliates.
//
// This program is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License, version 2.0,
// as published by the Free Software Foundation.
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#include <boost/geometry.hpp>
#include <memory>  // std::unique_ptr
 
#include "sql/dd/types/spatial_reference_system.h"  // dd::Spatial_reference_system
#include "sql/gis/geometries_cs.h"
#include "sql/gis/geometries_traits.h"
#include "sql/gis/geometry_visitor.h"
 
namespace gis {
 
/// A visitor that flips polygon rings so that exterior rings are in a
/// counter-clockwise direction and interior rings in a clockwise direction.
///
/// Invalid polygon rings are not guaranteed to be flipped to the correct
/// direction.
class Ring_flip_visitor : public Nop_visitor {
 private:
  /// Strategy used for geographic SRSs.
  std::unique_ptr<boost::geometry::strategy::area::geographic<>>
      m_geographic_strategy;
  /// Whether the geometry is invalid. That happens either if the ellipsoid of
  /// a geographic SRS is invalid or if we encounter a ring with unknown
  /// direction.
  bool m_invalid;
 
 public:
  /// Construct a new ring flip visitor.
  ///
  /// @param[in] semi_major The semi-major axis of the ellipsoid.
  /// @param[in] semi_minor The semi-minor axis of the ellipsoid.
  Ring_flip_visitor(double semi_major, double semi_minor) {
    try {
      m_geographic_strategy.reset(
          new boost::geometry::strategy::area::geographic<>(
              boost::geometry::srs::spheroid<double>(semi_major, semi_minor)));
      m_invalid = false;
    } catch (...) {
      // Spheroid construction may fail if the axes are invalid.
      m_invalid = true;
    }
  }
 
  /// Check if anything wrong has been detected, either an invalid ellipsoid or
  /// a ring with an unknown direction.
  ///
  /// Polygon rings which direction can't be determined are invalid. This is the
  /// only way this visitor detects invalid rings. Other invalid rings, e.g.,
  /// rings crossing themselves, are not necessarily detected.
  ///
  /// @retval true Invalid SRS ellipsoid or invalid polygon ring.
  /// @retval false No invalid rings detected, but the geometry may still be
  /// invalid.
  bool invalid() const { return m_invalid; }
 
  using Nop_visitor::visit_enter;
  bool visit_enter(Polygon *py) override;
  bool visit_enter(Multipolygon *py) override;
 
  bool visit_enter(Multipoint *) override {
    return true;  // Don't descend into each point.
  }
 
  bool visit_enter(Multilinestring *) override {
    return true;  // Don't descend into each linestring.
  }
};
 
}  // namespace gis
 
#endif  // SQL_GIS_RING_FLIP_VISITOR_H_INCLUDED