GH-45946: [C++][Parquet] Variant decoding

Author: qzyu999

cpp/src/arrow/CMakeLists.txt

@@ -391,6 +391,7 @@ set(ARROW_SRCS
     extension/bool8.cc
     extension/json.cc
     extension/parquet_variant.cc
+    extension/variant_internal.cc
     extension/uuid.cc
     pretty_print.cc
     record_batch.cc

cpp/src/arrow/extension/CMakeLists.txt

@@ -15,7 +15,8 @@
 # specific language governing permissions and limitations
 # under the License.
 
-set(CANONICAL_EXTENSION_TESTS bool8_test.cc json_test.cc uuid_test.cc)
+set(CANONICAL_EXTENSION_TESTS bool8_test.cc json_test.cc uuid_test.cc
+                              variant_internal_test.cc)
 
 if(ARROW_JSON)
   list(APPEND CANONICAL_EXTENSION_TESTS tensor_extension_array_test.cc opaque_test.cc)

cpp/src/arrow/extension/meson.build

@@ -15,7 +15,7 @@
 # specific language governing permissions and limitations
 # under the License.
 
-canonical_extension_tests = ['bool8_test.cc', 'json_test.cc', 'uuid_test.cc']
+canonical_extension_tests = ['bool8_test.cc', 'json_test.cc', 'uuid_test.cc', 'variant_internal_test.cc']
 
 if needs_json
     canonical_extension_tests += [
@@ -40,5 +40,8 @@ install_headers(
         'parquet_variant.h',
         'uuid.h',
         'variable_shape_tensor.h',
+        # variant_internal.h: public API for variant binary encoding/decoding.
+        # "internal" refers to the binary encoding internals, not visibility.
+        'variant_internal.h',
     ],
 )