Fix cudf::strings::replace_multiple hang on empty target (#16167)

Fixes logic in `cudf::strings::replace_multiple` to ignore empty targets correctly in the `replace_multi_fn` functor.
Also updated the doxygen and added a gtest for this case.

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

Approvers:
  - Nghia Truong (https://github.com/ttnghia)
  - Bradley Dice (https://github.com/bdice)

URL: #16167

cpp/include/cudf/strings/replace.hpp

@@ -122,7 +122,7 @@ std::unique_ptr<column> replace_slice(
  * If a target string is found, it is replaced by the corresponding entry in the repls column.
  * All occurrences found in each string are replaced.
  *
- * This does not use regex to match targets in the string.
+ * This does not use regex to match targets in the string. Empty string targets are ignored.
  *
  * Null string entries will return null output string entries.
  *

cpp/src/strings/replace/multi.cu

@@ -451,8 +451,8 @@ struct replace_multi_fn {
     while (spos < d_str.size_bytes()) {
       for (int tgt_idx = 0; tgt_idx < d_targets.size(); ++tgt_idx) {
         auto const d_tgt = d_targets.element<string_view>(tgt_idx);
-        if ((d_tgt.size_bytes() <= (d_str.size_bytes() - spos)) &&    // check fit
-            (d_tgt.compare(in_ptr + spos, d_tgt.size_bytes()) == 0))  // and match
+        if (!d_tgt.empty() && (d_tgt.size_bytes() <= (d_str.size_bytes() - spos)) &&  // check fit
+            (d_tgt.compare(in_ptr + spos, d_tgt.size_bytes()) == 0))                  // and match
         {
           auto const d_repl = (d_repls.size() == 1) ? d_repls.element<string_view>(0)
                                                     : d_repls.element<string_view>(tgt_idx);
@@ -468,9 +468,8 @@ struct replace_multi_fn {
       }
       ++spos;
     }
-    if (out_ptr)  // copy remainder
-    {
-      memcpy(out_ptr, in_ptr + lpos, d_str.size_bytes() - lpos);
+    if (out_ptr) {
+      memcpy(out_ptr, in_ptr + lpos, d_str.size_bytes() - lpos);  // copy remainder
     } else {
       d_sizes[idx] = bytes;
     }

cpp/tests/strings/replace_tests.cpp

@@ -532,6 +532,23 @@ TEST_F(StringsReplaceTest, ReplaceMultiLong)
   }
 }
 
+TEST_F(StringsReplaceTest, EmptyTarget)
+{
+  auto const input = cudf::test::strings_column_wrapper({"hello", "world", "", "accénted"});
+  auto const sv    = cudf::strings_column_view(input);
+
+  auto const targets = cudf::test::strings_column_wrapper({"e", "", "d"});
+  auto const tv      = cudf::strings_column_view(targets);
+
+  auto const repls = cudf::test::strings_column_wrapper({"E", "_", "D"});
+  auto const rv    = cudf::strings_column_view(repls);
+
+  // empty target should be ignored
+  auto results  = cudf::strings::replace_multiple(sv, tv, rv);
+  auto expected = cudf::test::strings_column_wrapper({"hEllo", "worlD", "", "accéntED"});
+  CUDF_TEST_EXPECT_COLUMNS_EQUAL(results->view(), expected);
+}
+
 TEST_F(StringsReplaceTest, EmptyStringsColumn)
 {
   auto const zero_size_strings_column = cudf::make_empty_column(cudf::type_id::STRING)->view();