From 09fd791c90b00403d04c283c65be9700023f47ca Mon Sep 17 00:00:00 2001
From: Liang Hu <lh3057@columbia.edu>
Date: Mon, 13 Jul 2026 16:35:21 -0400
Subject: [PATCH] GH-35815: [C++][Compute] Support dictionary inputs in UTF-8
 trim

---
 .../compute/kernels/scalar_string_internal.h   |  4 ++--
 .../compute/kernels/scalar_string_test.cc      | 11 +++++++++++
 .../compute/kernels/scalar_string_utf8.cc      | 18 +++++++++++++++---
 3 files changed, 28 insertions(+), 5 deletions(-)

diff --git a/cpp/src/arrow/compute/kernels/scalar_string_internal.h b/cpp/src/arrow/compute/kernels/scalar_string_internal.h
index 664f6b4c0b0f..f3d68468c7a3 100644
--- a/cpp/src/arrow/compute/kernels/scalar_string_internal.h
+++ b/cpp/src/arrow/compute/kernels/scalar_string_internal.h
@@ -159,11 +159,11 @@ void MakeUnaryStringBatchKernel(
   ARROW_DCHECK_OK(registry->AddFunction(std::move(func)));
 }
 
-template <template <typename> class ExecFunctor>
+template <template <typename> class ExecFunctor, typename FunctionType = ScalarFunction>
 void MakeUnaryStringBatchKernelWithState(
     std::string name, FunctionRegistry* registry, FunctionDoc doc,
     MemAllocation::type mem_allocation = MemAllocation::PREALLOCATE) {
-  auto func = std::make_shared<ScalarFunction>(name, Arity::Unary(), std::move(doc));
+  auto func = std::make_shared<FunctionType>(name, Arity::Unary(), std::move(doc));
   {
     using t32 = ExecFunctor<StringType>;
     ScalarKernel kernel{{utf8()}, utf8(), t32::Exec, t32::State::Init};
diff --git a/cpp/src/arrow/compute/kernels/scalar_string_test.cc b/cpp/src/arrow/compute/kernels/scalar_string_test.cc
index b279f991f6ea..67ce265bf832 100644
--- a/cpp/src/arrow/compute/kernels/scalar_string_test.cc
+++ b/cpp/src/arrow/compute/kernels/scalar_string_test.cc
@@ -2331,6 +2331,17 @@ TYPED_TEST(TestStringKernels, TrimUTF8) {
   EXPECT_RAISES_WITH_MESSAGE_THAT(Invalid, testing::HasSubstr("Invalid UTF8"),
                                   CallFunction("utf8_trim", {input}, &options_invalid));
 }
+
+TYPED_TEST(TestStringKernels, TrimUTF8Dictionary) {
+  auto input =
+      ArrayFromJSON(dictionary(int64(), this->type()), R"(["bcabc", "b", "a", null])");
+  auto options = TrimOptions{"bc"};
+  this->CheckUnary("utf8_trim", input, this->type(), R"(["a", "", "a", null])", &options);
+  this->CheckUnary("utf8_ltrim", input, this->type(), R"(["abc", "", "a", null])",
+                   &options);
+  this->CheckUnary("utf8_rtrim", input, this->type(), R"(["bca", "", "a", null])",
+                   &options);
+}
 #endif
 
 // produce test data with e.g.:
diff --git a/cpp/src/arrow/compute/kernels/scalar_string_utf8.cc b/cpp/src/arrow/compute/kernels/scalar_string_utf8.cc
index fd340bba6242..644f29c42b9c 100644
--- a/cpp/src/arrow/compute/kernels/scalar_string_utf8.cc
+++ b/cpp/src/arrow/compute/kernels/scalar_string_utf8.cc
@@ -726,6 +726,15 @@ void AddUtf8StringReverse(FunctionRegistry* registry) {
 // ----------------------------------------------------------------------
 // String trimming
 
+struct DictionaryDecodingScalarFunction : ScalarFunction {
+  using ScalarFunction::ScalarFunction;
+
+  Result<const Kernel*> DispatchBest(std::vector<TypeHolder>* types) const override {
+    EnsureDictionaryDecoded(types);
+    return DispatchExact(*types);
+  }
+};
+
 struct UTF8TrimState {
   TrimOptions options_;
   std::vector<bool> codepoints_;
@@ -882,9 +891,12 @@ const FunctionDoc utf8_rtrim_whitespace_doc(
 #endif  // ARROW_WITH_UTF8PROC
 
 void AddUtf8StringTrim(FunctionRegistry* registry) {
-  MakeUnaryStringBatchKernelWithState<UTF8Trim>("utf8_trim", registry, utf8_trim_doc);
-  MakeUnaryStringBatchKernelWithState<UTF8LTrim>("utf8_ltrim", registry, utf8_ltrim_doc);
-  MakeUnaryStringBatchKernelWithState<UTF8RTrim>("utf8_rtrim", registry, utf8_rtrim_doc);
+  MakeUnaryStringBatchKernelWithState<UTF8Trim, DictionaryDecodingScalarFunction>(
+      "utf8_trim", registry, utf8_trim_doc);
+  MakeUnaryStringBatchKernelWithState<UTF8LTrim, DictionaryDecodingScalarFunction>(
+      "utf8_ltrim", registry, utf8_ltrim_doc);
+  MakeUnaryStringBatchKernelWithState<UTF8RTrim, DictionaryDecodingScalarFunction>(
+      "utf8_rtrim", registry, utf8_rtrim_doc);
 #ifdef ARROW_WITH_UTF8PROC
   MakeUnaryStringBatchKernel<UTF8TrimWhitespace>("utf8_trim_whitespace", registry,
                                                  utf8_trim_whitespace_doc);