buffering features with integer coordinates may result in numeric overflow in segment length calculation

[MaartenHilferink]

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in "boost/geometry/strategies/cartesian/buffer_side_straight.hpp"

lines 78..83:

    // Simulate a vector d (dx,dy)
    coordinate_type const dx = get<0>(input_p2) - get<0>(input_p1);
    coordinate_type const dy = get<1>(input_p2) - get<1>(input_p1);

   // For normalization [0,1] (=dot product d.d, sqrt)
    promoted_type const length = geometry::math::sqrt(dx * dx + dy * dy);

fail to calculate a correct length when coordinate_type is int and dx * dx + dy * dy should be bigger than INT_MAX, which it technically never is as the result is also an int, which becomes negative when for example dx = dy = 47000.

When the calculation of dx and dy are promoted directly to promoted_type, this problem is fixed:

    // Simulate a vector d (dx,dy)
    promoted_type const dx = get<0>(input_p2) - get<0>(input_p1);
    promoted_type const dy = get<1>(input_p2) - get<1>(input_p1);

   // For normalization [0,1] (=dot product d.d, sqrt)
    promoted_type const length = geometry::math::sqrt(dx * dx + dy * dy);

[MaartenHilferink]

Tested with: Boost 1.78, but seen in code of Boost 1.79 too.

[MaartenHilferink]

seen with:

boost::geometry::strategy::buffer::distance_symmetric<Float64> distStrategy(10.0);
boost::geometry::strategy::buffer::join_round          joinStrategy(8);
boost::geometry::strategy::buffer::end_round          endStrategy(8);
boost::geometry::strategy::buffer::point_circle         circleStrategy(8);
boost::geometry::strategy::buffer::side_straight       sideStrategy;

using PointType = Point<int>; // for example: rd-coordinates in millimeters
boost::geometry::model::ring<PointType> tmpRing;
boost::geometry::model::polygon<PointType> tmpPolygon;
boost::geometry::model::multi_polygon<decltype(tmpPolygon)> currMP, resMP;
// read currMP as pand from BAG containing line segments of at least 65m long (> 2^16 mm).
boost::geometry::buffer(currMP, resMP, distStrategy, sideStrategy, joinStrategy, endStrategy, circleStrategy);

where the following traits were defined for Point:

namespace boost::geometry::traits
{
	// define Point<T> as model for concept boost::geometry::point
	template <typename T> struct tag<Point<T>> { using type = point_tag; };
	template <typename T> struct dimension<Point<T>> : std::integral_constant<std::size_t, 2> {};
	template <typename T> struct coordinate_type<Point<T>> { using type = T; };
	template <typename T> struct coordinate_system<Point<T>> { using type = boost::geometry::cs::cartesian; };

	template <typename T, std::size_t Index>
	struct access<Point<T>, Index> {
		static_assert(Index < 2, "Out of range");
		using CoordinateType = T;
		static inline CoordinateType get(Point<T> const& p) { if constexpr (Index == 0) return p.X(); else return p.Y(); }
		static inline void set(Point<T>& p, CoordinateType const& value) { if constexpr (Index == 0) p.X() = value; else p.Y() = value; }
	};
} //  namespace boost::geometry::traits

but also seen with boost::geometry::buffer on boost::geometry::model::linestring<Point>

[barendgehrels]

Hi Maarten!

Nice to hear from you!

And thanks for your report! I cannot tackle it immediately but will come back to this soon.

Regards, Barend

[barendgehrels]

Hi Maarten, A bit delayed, but I could reproduce it, added a unit test and applied your fix. See #1028 Thanks again for reporting it, including the fix.