Unstable behaviour when plotting very small cells

[paleolimbot]

As reported by @mdsumner! I think he reported a crash...this just gives me a snap error. It is probably worth putting a limit on the level of simplification...I think this ends up boiling down to a radius of some kind and there's almost certainly a minimum useful value for that.

library(s2)

x <- s2_cell_polygon(s2_cell_parent(as_s2_cell(wk::xy(-67, 45)), 21))
plot(x, simplify = FALSE)

plot(x)
#> Error in eval(expr, envir, enclos): Snap function moved vertex (0.276288378258184, -0.650895382326694, 0.707106734026236) by 1.00136956986004e-09, which is more than the specified snap radius of 8.90734402968868e-10

^{Created on 2023-06-12 with reprex v2.0.2}

[edzer]

Indeed, I get a valgrind stacktrace:

plot(x)
==210380== Invalid read of size 4
==210380==    at 0x4BAEBF47: S2Builder::MaybeAddExtraSites(int, int, std::vector<int, std::allocator<int> > const&, MutableS2ShapeIndex const&, std::vector<int, std::allocator<int> >*) (s2builder.cc:804)
==210380==    by 0x4BAEC347: S2Builder::AddExtraSites(MutableS2ShapeIndex const&) (s2builder.cc:788)
==210380==    by 0x4BAED17A: S2Builder::ChooseSites() (s2builder.cc:512)
==210380==    by 0x4BAED29E: S2Builder::Build(S2Error*) (s2builder.cc:474)
==210380==    by 0x4BAD2EA7: S2BooleanOperation::Impl::Build(S2Error*) (s2boolean_operation.cc:2260)
==210380==    by 0x4BAD2FC7: S2BooleanOperation::Build(S2ShapeIndex const&, S2ShapeIndex const&, S2Error*) (s2boolean_operation.cc:2380)
==210380==    by 0x4BAB1FE9: s2geography::s2_boolean_operation(s2geography::ShapeIndexGeography const&, s2geography::ShapeIndexGeography const&, S2BooleanOperation::OpType, s2geography::GlobalOptions const&) (build.cc:121)
==210380==    by 0x4BA8978A: BooleanOperationOp::processFeature(Rcpp::XPtr<RGeography, Rcpp::PreserveStorage, &(void Rcpp::standard_delete_finalizer<RGeography>(RGeography*)), false>, Rcpp::XPtr<RGeography, Rcpp::PreserveStorage, &(void Rcpp::standard_delete_finalizer<RGeography>(RGeography*)), false>, long) (s2-transformers.cpp:24)
==210380==    by 0x4BA88067: BinaryGeographyOperator<Rcpp::Vector<19, Rcpp::PreserveStorage>, SEXPREC*>::processVector(Rcpp::Vector<19, Rcpp::PreserveStorage>, Rcpp::Vector<19, Rcpp::PreserveStorage>) (geography-operator.h:85)
==210380==    by 0x4BA85052: cpp_s2_intersection(Rcpp::Vector<19, Rcpp::PreserveStorage>, Rcpp::Vector<19, Rcpp::PreserveStorage>, Rcpp::Vector<19, Rcpp::PreserveStorage>) (s2-transformers.cpp:37)
==210380==    by 0x4BA8C1BB: _s2_cpp_s2_intersection (RcppExports.cpp:1122)
==210380==    by 0x495629D: ??? (in /usr/lib/R/lib/libR.so)
==210380==  Address 0x0 is not stack'd, malloc'd or (recently) free'd
==210380==