add C Callable earcut

[mdsumner]

My first C Callable function, thanks to @dcooley as discussed here

The tests include an example:

  library(Rcpp)
  cppFunction(
    code = '
    SEXP earcut( SEXP x , SEXP y, SEXP holes, SEXP numholes) {
      typedef SEXP R_DECIDO_FUN(SEXP, SEXP, SEXP, SEXP);
      R_DECIDO_FUN *decido_earcut_cpp = (R_DECIDO_FUN *) R_GetCCallable("decido","decido_earcut_cpp");
      return decido_earcut_cpp( x, y, holes, numholes );
    }
  '
  )


  ## single ring polygon
  x <- c(0, 0, 0.75, 1, 0.5, 0.8, 0.69)
  y <- c(0, 1, 1, 0.8, 0.7, 0.6, 0)
  (ind <- earcut(x, y, 0L, 0L))
  expect_equal(ind, c(1L, 0L, 6L, 6L, 5L, 4L, 4L, 3L, 2L, 1L, 6L, 4L, 4L, 2L, 1L))
 # plot_ears(cbind(x, y), ind + 1)  ## plus 1 because C++ context (the R-level translates)

[dcooley]

Nice one Michael.

Another approach you could consider is, since earcut is a header-only library itself, you could move earcut.h into /inst/include/, then other packages will be able to LinkingTo: decido in the DESCRIPTION and it should work as well.

[mdsumner]

Oh yes, right I see

[dcooley]

Will need an additional line in Makevars too if you do it that way

CXX_STD = CXX11

PKG_CXXFLAGS = -I../inst/include/

[dcooley]

I now have a use-case for this. If you don't have any strong opinions either way, I'd prefer to have the /Inst/include/ linkable header, rather than the Callable routine.

[mdsumner]

I don't have a strong opinion here, I also don't have capacity to look at it rn but will merge a PR if you do

[dcooley]

ok cool. I'm not in a rush yet; I first need to work out how to build the underlying Deckgl layer. But I'll work on a PR nonetheless.