ARROW-9100: [C++] Add ascii_lower kernel

Closes #7357 from maartenbreddels/ARROW-555

Authored-by: Maarten A. Breddels <maartenbreddels@gmail.com>
Signed-off-by: Wes McKinney <wesm@apache.org>

Author: maartenbreddels

cpp/src/arrow/compute/kernels/scalar_string.cc

@@ -48,6 +48,16 @@ struct AsciiUpper {
   }
 };
 
+struct AsciiLower {
+  template <typename... Ignored>
+  static std::string Call(KernelContext*, const util::string_view& val) {
+    std::string result = val.to_string();
+    std::transform(result.begin(), result.end(), result.begin(),
+                   [](unsigned char c) { return std::tolower(c); });
+    return result;
+  }
+};
+
 void AddAsciiLength(FunctionRegistry* registry) {
   auto func = std::make_shared<ScalarFunction>("ascii_length", Arity::Unary());
   ArrayKernelExec exec_offset_32 =
@@ -108,6 +118,7 @@ void AddStrptime(FunctionRegistry* registry) {
 
 void RegisterScalarStringAscii(FunctionRegistry* registry) {
   MakeUnaryStringToString<AsciiUpper>("ascii_upper", registry);
+  MakeUnaryStringToString<AsciiLower>("ascii_lower", registry);
   AddAsciiLength(registry);
   AddStrptime(registry);
 }

cpp/src/arrow/compute/kernels/scalar_string_test.cc

@@ -61,6 +61,11 @@ TYPED_TEST(TestStringKernels, AsciiUpper) {
                    "[\"AAA&\", null, \"\", \"B\"]");
 }
 
+TYPED_TEST(TestStringKernels, AsciiLower) {
+  this->CheckUnary("ascii_lower", "[\"aAa&\", null, \"\", \"b\"]", this->string_type(),
+                   "[\"aaa&\", null, \"\", \"b\"]");
+}
+
 TYPED_TEST(TestStringKernels, Strptime) {
   std::string input1 = R"(["5/1/2020", null, "12/11/1900"])";
   std::string output1 = R"(["2020-05-01", null, "1900-12-11"])";

python/pyarrow/compute.py

@@ -93,6 +93,7 @@ def func(arg):
 
 ascii_length = _simple_unary_function('ascii_length')
 ascii_upper = _simple_unary_function('ascii_upper')
+ascii_lower = _simple_unary_function('ascii_lower')
 
 
 def sum(array):