ARROW-13620: [R] Binding for n_distinct()

r/R/dplyr-functions.R

@@ -801,7 +801,6 @@ agg_funcs$all <- function(x, na.rm = FALSE) {
     options = list(na.rm = na.rm, na.min_count = 0L)
   )
 }
-
 agg_funcs$mean <- function(x, na.rm = FALSE) {
   list(
     fun = "mean",
@@ -825,6 +824,15 @@ agg_funcs$var <- function(x, na.rm = FALSE, ddof = 1) {
     options = list(ddof = ddof)
   )
 }
+
+agg_funcs$n_distinct <- function(x, na.rm = FALSE) {
+  list(
+    fun = "count_distinct",
+    data = x,
+    options = list(na.rm = na.rm)
+  )
+}
+
 agg_funcs$n <- function() {
   list(
     fun = "sum",

r/src/compute.cpp

@@ -194,6 +194,14 @@ std::shared_ptr<arrow::compute::FunctionOptions> make_compute_options(
     return out;
   }
 
+  if (func_name == "hash_count_distinct") {
+    using Options = arrow::compute::CountOptions;
+    auto out = std::make_shared<Options>(Options::Defaults());
+    out->mode =
+        cpp11::as_cpp<bool>(options["na.rm"]) ? Options::ONLY_VALID : Options::ALL;
+    return out;
+  }
+
   if (func_name == "min_element_wise" || func_name == "max_element_wise") {
     using Options = arrow::compute::ElementWiseAggregateOptions;
     bool skip_nulls = true;

r/tests/testthat/test-dplyr-aggregate.R

@@ -235,6 +235,23 @@ test_that("Group by any/all", {
   )
 })
 
+test_that("Group by n_distinct() on dataset", {
+  expect_dplyr_equal(
+    input %>%
+      group_by(some_grouping) %>%
+      summarize(distinct = n_distinct(lgl, na.rm = FALSE)) %>%
+      collect(),
+    tbl
+  )
+  expect_dplyr_equal(
+    input %>%
+      group_by(some_grouping) %>%
+      summarize(distinct = n_distinct(lgl, na.rm = TRUE)) %>%
+      collect(),
+    tbl
+  )
+})
+
 test_that("Filter and aggregate", {
   expect_dplyr_equal(
     input %>%