GH-45948: [C++][Parquet] Variant shredding

Author: qzyu999

cpp/src/arrow/CMakeLists.txt

@@ -393,6 +393,7 @@ set(ARROW_SRCS
     extension/parquet_variant.cc
     extension/variant.cc
     extension/variant_builder.cc
+    extension/variant_shredding.cc
     extension/uuid.cc
     pretty_print.cc
     record_batch.cc

cpp/src/arrow/extension/CMakeLists.txt

@@ -16,7 +16,8 @@
 # under the License.
 
 set(CANONICAL_EXTENSION_TESTS bool8_test.cc json_test.cc uuid_test.cc
-                              variant_test.cc variant_builder_test.cc)
+                              variant_test.cc variant_builder_test.cc
+                              variant_shredding_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', 'variant_test.cc', 'variant_builder_test.cc']
+canonical_extension_tests = ['bool8_test.cc', 'json_test.cc', 'uuid_test.cc', 'variant_test.cc', 'variant_builder_test.cc', 'variant_shredding_test.cc']
 
 if needs_json
     canonical_extension_tests += [
@@ -41,5 +41,6 @@ install_headers(
         'uuid.h',
         'variable_shape_tensor.h',
         'variant.h',
+        'variant_shredding.h',
     ],
 )

cpp/src/arrow/extension/parquet_variant.h

@@ -70,7 +70,10 @@ class ARROW_EXPORT VariantExtensionType : public ExtensionType {
   std::shared_ptr<Field> value() const { return value_; }
 
  private:
-  // TODO GH-45948 added shredded_value
+  // TODO: Track shredded_value field when integrating with Parquet reader.
+  // The shredding implementation (variant_shredding.h) operates on raw arrays
+  // externally; a future PR may extend VariantExtensionType to understand
+  // shredded storage layouts for seamless Parquet I/O integration.
   std::shared_ptr<Field> metadata_;
   std::shared_ptr<Field> value_;
 };

cpp/src/arrow/extension/variant.h

@@ -678,8 +678,15 @@ class ARROW_EXPORT VariantBuilder {
   void Reset();
   /// @}
 
+  /// @name Shredding support (GH-45948)
+  /// @{
+  Result<std::vector<uint8_t>> BuildWithoutMeta();
+  void UnsafeAppendEncoded(const uint8_t* data, int64_t size);
+  void SetAllowDuplicates(bool allow);
+  /// @}
+
   /// @name Internal (used by scopes and shredding)
-  /// @name Internal (used by scopes)
+  /// @{
   void Truncate(int64_t offset);
   /// @}
 

cpp/src/arrow/extension/variant_builder.cc

@@ -632,4 +632,22 @@ Status ListScope::Finish() {
   return Status::OK();
 }
 
+Result<std::vector<uint8_t>> VariantBuilder::BuildWithoutMeta() {
+  if (buffer_.empty()) {
+    return Status::Invalid("VariantBuilder::BuildWithoutMeta: no value written");
+  }
+  std::vector<uint8_t> result = std::move(buffer_);
+  buffer_.clear();
+  return result;
+}
+
+void VariantBuilder::UnsafeAppendEncoded(const uint8_t* data, int64_t size) {
+  DCHECK_NE(data, nullptr);
+  DCHECK_GT(size, 0);
+  if (size <= 0) return;
+  buffer_.insert(buffer_.end(), data, data + size);
+}
+
+void VariantBuilder::SetAllowDuplicates(bool allow) { allow_duplicates_ = allow; }
+
 }  // namespace arrow::extension::variant