Add s2_cell <-> integer64 support

DESCRIPTION

@@ -34,6 +34,7 @@ Imports:
     Rcpp,
     wk (>= 0.6.0)
 Suggests: 
+    bit64,
     testthat (>= 3.0.0),
     vctrs
 URL: https://r-spatial.github.io/s2/, https://github.com/r-spatial/s2, https://s2geometry.io/

NAMESPACE

@@ -12,6 +12,7 @@ S3method(as.character,s2_cell_union)
 S3method(as.character,s2_geography)
 S3method(as.list,s2_cell)
 S3method(as_s2_cell,character)
+S3method(as_s2_cell,integer64)
 S3method(as_s2_cell,s2_cell)
 S3method(as_s2_cell,s2_geography)
 S3method(as_s2_cell,wk_xy)

R/s2-cell.R

@@ -88,13 +88,31 @@ as_s2_cell.wk_xy <- function(x, ...) {
   cpp_s2_cell_from_lnglat(as_s2_lnglat(x))
 }
 
+#' @rdname s2_cell
+#' @export
+as_s2_cell.integer64 <- function(x, ...) {
+  storage <- unclass(x)
+  storage[is.na(x)] <- NA_real_
+  new_s2_cell(storage)
+}
+
 #' @rdname s2_cell
 #' @export
 new_s2_cell <- function(x) {
   stopifnot(is.double(x))
   structure(x, class = c("s2_cell", "wk_vctr"))
 }
 
+# registered in zzz.R
+as.integer64.s2_cell <- function(x, ...) {
+  # We store 64-bit integegers the same way bit64 does so we can just set the
+  # class attribute and propagate NA values in the way that bit64 expects them.
+  x_is_na <- is.na(x)
+  class(x) <- "integer64"
+  x[x_is_na] <- bit64::NA_integer64_
+  x
+}
+
 #' @export
 as.character.s2_cell <- function(x, ...) {
   cpp_s2_cell_to_string(x)

R/zzz.R

@@ -10,6 +10,8 @@
     s3_register("vctrs::vec_restore", cls)
     s3_register("vctrs::vec_ptype_abbr", cls)
   }
+
+  s3_register("bit64::as.integer64", "s2_cell")
 }
 
 s3_register <- function(generic, class, method = NULL) {

tests/testthat/test-s2-cell.R

@@ -10,6 +10,13 @@ test_that("s2_cell class works", {
   )
 })
 
+test_that("s2_cell bit64::integer64 support works", {
+  cells <- c(as_s2_cell(NA_character_), s2_cell_sentinel())
+  int64s <- bit64::as.integer64(cells)
+  expect_identical(int64s, bit64::as.integer64(c(NA, -1)))
+  expect_identical(as_s2_cell(int64s), cells)
+})
+
 test_that("invalid and sentinel values work as expected", {
   expect_false(s2_cell_is_valid(s2_cell_sentinel()))
   expect_false(s2_cell_is_valid(s2_cell_invalid()))