Fix cudf::strings::findall error with empty input (#16928)

Fixes `cudf::strings::findall` error when passed an empty input column.
Also adds a gtest for empty input and for all-rows do not match case.

Authors:
  - David Wendt (https://github.com/davidwendt)

Approvers:
  - Yunsong Wang (https://github.com/PointKernel)
  - Vyas Ramasubramani (https://github.com/vyasr)

URL: #16928

cpp/src/strings/search/findall.cu

@@ -23,6 +23,7 @@
 #include <cudf/detail/null_mask.hpp>
 #include <cudf/detail/nvtx/ranges.hpp>
 #include <cudf/detail/offsets_iterator_factory.cuh>
+#include <cudf/lists/detail/lists_column_factories.hpp>
 #include <cudf/strings/detail/strings_column_factories.cuh>
 #include <cudf/strings/findall.hpp>
 #include <cudf/strings/string_view.cuh>
@@ -97,8 +98,11 @@ std::unique_ptr<column> findall(strings_column_view const& input,
                                 rmm::cuda_stream_view stream,
                                 rmm::device_async_resource_ref mr)
 {
-  auto const strings_count = input.size();
-  auto const d_strings     = column_device_view::create(input.parent(), stream);
+  if (input.is_empty()) {
+    return cudf::lists::detail::make_empty_lists_column(input.parent().type(), stream, mr);
+  }
+
+  auto const d_strings = column_device_view::create(input.parent(), stream);
 
   // create device object from regex_program
   auto d_prog = regex_device_builder::create_prog_device(prog, stream);
@@ -113,7 +117,7 @@ std::unique_ptr<column> findall(strings_column_view const& input,
   auto strings_output = findall_util(*d_strings, *d_prog, total_matches, d_offsets, stream, mr);
 
   // Build the lists column from the offsets and the strings
-  return make_lists_column(strings_count,
+  return make_lists_column(input.size(),
                            std::move(offsets),
                            std::move(strings_output),
                            input.null_count(),

cpp/tests/strings/findall_tests.cpp

@@ -148,3 +148,31 @@ TEST_F(StringsFindallTests, LargeRegex)
   LCW expected({LCW{large_regex.c_str()}, LCW{}, LCW{}});
   CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(results->view(), expected);
 }
+
+TEST_F(StringsFindallTests, NoMatches)
+{
+  cudf::test::strings_column_wrapper input({"abc\nfff\nabc", "fff\nabc\nlll", "abc", "", "abc\n"});
+  auto sv = cudf::strings_column_view(input);
+
+  auto pattern = std::string("(^zzz$)");
+  using LCW    = cudf::test::lists_column_wrapper<cudf::string_view>;
+  LCW expected({LCW{}, LCW{}, LCW{}, LCW{}, LCW{}});
+  auto prog    = cudf::strings::regex_program::create(pattern);
+  auto results = cudf::strings::findall(sv, *prog);
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(results->view(), expected);
+}
+
+TEST_F(StringsFindallTests, EmptyTest)
+{
+  std::string pattern = R"(\w+)";
+
+  auto prog = cudf::strings::regex_program::create(pattern);
+
+  cudf::test::strings_column_wrapper input;
+  auto sv      = cudf::strings_column_view(input);
+  auto results = cudf::strings::findall(sv, *prog);
+
+  using LCW = cudf::test::lists_column_wrapper<cudf::string_view>;
+  LCW expected;
+  CUDF_TEST_EXPECT_COLUMNS_EQUIVALENT(results->view(), expected);
+}