Add the Dataptr* methods to ALTREP representation (#286)

* Add the Dataptr* methods to ALTREP representation (sometimes called inside `mclapply()`)

* Add tests for DATAPTR

* Throw an error on s2_altrep_Dataptr(writable = true), improve tests

Author: tzakharko

src/s2-altrep.cpp

@@ -22,6 +22,22 @@ static SEXP s2_altrep_Elt(SEXP obj, R_xlen_t i) {
   return VECTOR_ELT(data, i);
 }
 
+static void s2_altrep_SetElt(SEXP obj, R_xlen_t i, SEXP v) {
+  SEXP data = R_altrep_data1(obj);
+  SET_VECTOR_ELT(data, i, v);
+}
+
+static void* s2_altrep_Dataptr(SEXP obj, Rboolean writable) {
+  if (writable) Rf_error("unable to produce writable DATAPTR for list data");
+
+  SEXP data = R_altrep_data1(obj);
+  return (void*) DATAPTR_RO(data);
+}
+
+static const void* s2_altrep_Dataptr_or_null(SEXP obj) {
+  return s2_altrep_Dataptr(obj, FALSE);
+}
+
 static SEXP s2_altrep_Serialized_state(SEXP obj) {
   // fetch the pointer to s2::s2_geography_serialize()
   SEXP fn = Rf_findFun(Rf_install("s2_geography_serialize"), s2_ns_pkg);
@@ -61,6 +77,9 @@ void s2_init_altrep(DllInfo *dll) {
 
   R_set_altrep_Length_method(s2_geography_altrep_cls, s2_altrep_Length);
   R_set_altlist_Elt_method(s2_geography_altrep_cls, s2_altrep_Elt);
+  R_set_altlist_Set_elt_method(s2_geography_altrep_cls, s2_altrep_SetElt);
+  R_set_altvec_Dataptr_method(s2_geography_altrep_cls, s2_altrep_Dataptr);
+  R_set_altvec_Dataptr_or_null_method(s2_geography_altrep_cls, s2_altrep_Dataptr_or_null);
   R_set_altrep_Serialized_state_method(s2_geography_altrep_cls, s2_altrep_Serialized_state);
   R_set_altrep_Unserialize_method(s2_geography_altrep_cls, s2_altrep_Unserialize);
 #endif

tests/testthat/test-s2-geography.R

@@ -275,3 +275,18 @@ test_that("wk crs and geodesic methods are defined", {
 
   expect_error(wk::wk_set_geodesic(geog, FALSE), "Can't set geodesic")
 })
+
+test_that("s2_geography vectors support elementwise assignment", {
+  x <- as_s2_geography(c("POINT (0 0)", "POINT (90 0)"))
+  x[2] <- as_s2_geography(c("POINT (45 45)"))
+
+  expect_wkt_equal(x, c("POINT (0 0)", "POINT (45 45)"))
+})
+
+test_that("DATAPTR can be obtained for s2_geography", {
+  Rcpp::cppFunction("bool get_dataptr(SEXP obj) {
+    return DATAPTR_RO(obj) != NULL;
+  }")
+
+  expect_true(get_dataptr(as_s2_geography("POINT (0 0)")))
+})