apacheGH-39336: [C++][Parquet] Minor: Style enhancement for parquet::…

…FileMetaData (apache#39337)

### Rationale for this change

Enhance the style of `parquet::FileMetaData::Subset`.

### Are these changes tested?

Already tested

### Are there any user-facing changes?

no

* Closes: apache#39336

Lead-authored-by: mwish <maplewish117@gmail.com>
Co-authored-by: mwish <1506118561@qq.com>
Co-authored-by: Antoine Pitrou <pitrou@free.fr>
Signed-off-by: mwish <maplewish117@gmail.com>

cpp/src/arrow/util/bit_stream_utils.h

@@ -183,7 +183,7 @@ class BitReader {
 
   /// Returns the number of bytes left in the stream, not including the current
   /// byte (i.e., there may be an additional fraction of a byte).
-  int bytes_left() {
+  int bytes_left() const {
     return max_bytes_ -
            (byte_offset_ + static_cast<int>(bit_util::BytesForBits(bit_offset_)));
   }

cpp/src/parquet/metadata.cc

@@ -761,7 +761,7 @@ class FileMetaData::FileMetaDataImpl {
     return metadata_->row_groups[i];
   }
 
-  void AppendRowGroups(const std::unique_ptr<FileMetaDataImpl>& other) {
+  void AppendRowGroups(FileMetaDataImpl* other) {
     std::ostringstream diff_output;
     if (!schema()->Equals(*other->schema(), &diff_output)) {
       auto msg = "AppendRowGroups requires equal schemas.\n" + diff_output.str();
@@ -800,6 +800,7 @@ class FileMetaData::FileMetaDataImpl {
     metadata->schema = metadata_->schema;
 
     metadata->row_groups.resize(row_groups.size());
+
     int i = 0;
     for (int selected_index : row_groups) {
       metadata->num_rows += row_group(selected_index).num_rows;
@@ -822,7 +823,7 @@ class FileMetaData::FileMetaDataImpl {
   }
 
   void set_file_decryptor(std::shared_ptr<InternalFileDecryptor> file_decryptor) {
-    file_decryptor_ = file_decryptor;
+    file_decryptor_ = std::move(file_decryptor);
   }
 
  private:
@@ -886,13 +887,14 @@ std::shared_ptr<FileMetaData> FileMetaData::Make(
     const void* metadata, uint32_t* metadata_len,
     std::shared_ptr<InternalFileDecryptor> file_decryptor) {
   return std::shared_ptr<FileMetaData>(new FileMetaData(
-      metadata, metadata_len, default_reader_properties(), file_decryptor));
+      metadata, metadata_len, default_reader_properties(), std::move(file_decryptor)));
 }
 
 FileMetaData::FileMetaData(const void* metadata, uint32_t* metadata_len,
                            const ReaderProperties& properties,
                            std::shared_ptr<InternalFileDecryptor> file_decryptor)
-    : impl_(new FileMetaDataImpl(metadata, metadata_len, properties, file_decryptor)) {}
+    : impl_(new FileMetaDataImpl(metadata, metadata_len, properties,
+                                 std::move(file_decryptor))) {}
 
 FileMetaData::FileMetaData() : impl_(new FileMetaDataImpl()) {}
 
@@ -942,7 +944,7 @@ const std::string& FileMetaData::footer_signing_key_metadata() const {
 
 void FileMetaData::set_file_decryptor(
     std::shared_ptr<InternalFileDecryptor> file_decryptor) {
-  impl_->set_file_decryptor(file_decryptor);
+  impl_->set_file_decryptor(std::move(file_decryptor));
 }
 
 ParquetVersion::type FileMetaData::version() const {
@@ -975,7 +977,7 @@ const std::shared_ptr<const KeyValueMetadata>& FileMetaData::key_value_metadata(
 void FileMetaData::set_file_path(const std::string& path) { impl_->set_file_path(path); }
 
 void FileMetaData::AppendRowGroups(const FileMetaData& other) {
-  impl_->AppendRowGroups(other.impl_);
+  impl_->AppendRowGroups(other.impl_.get());
 }
 
 std::shared_ptr<FileMetaData> FileMetaData::Subset(
@@ -1839,7 +1841,7 @@ class FileMetaDataBuilder::FileMetaDataBuilderImpl {
   std::unique_ptr<FileMetaData> Finish(
       const std::shared_ptr<const KeyValueMetadata>& key_value_metadata) {
     int64_t total_rows = 0;
-    for (auto row_group : row_groups_) {
+    for (const auto& row_group : row_groups_) {
       total_rows += row_group.num_rows;
     }
     metadata_->__set_num_rows(total_rows);
@@ -1858,7 +1860,7 @@ class FileMetaDataBuilder::FileMetaDataBuilderImpl {
         format::KeyValue kv_pair;
         kv_pair.__set_key(key_value_metadata_->key(i));
         kv_pair.__set_value(key_value_metadata_->value(i));
-        metadata_->key_value_metadata.push_back(kv_pair);
+        metadata_->key_value_metadata.push_back(std::move(kv_pair));
       }
       metadata_->__isset.key_value_metadata = true;
     }

cpp/src/parquet/metadata.h

@@ -306,9 +306,15 @@ class PARQUET_EXPORT FileMetaData {
   int num_schema_elements() const;
 
   /// \brief The total number of rows.
+  ///
+  /// If the FileMetaData was obtained by calling `SubSet()`, this is the total
+  /// number of rows in the selected row groups.
   int64_t num_rows() const;
 
   /// \brief The number of row groups in the file.
+  ///
+  /// If the FileMetaData was obtained by calling `SubSet()`, this is the number
+  /// of selected row groups.
   int num_row_groups() const;
 
   /// \brief Return the RowGroupMetaData of the corresponding row group ordinal.
@@ -338,7 +344,7 @@ class PARQUET_EXPORT FileMetaData {
   /// \brief Size of the original thrift encoded metadata footer.
   uint32_t size() const;
 
-  /// \brief Indicate if all of the FileMetadata's RowGroups can be decompressed.
+  /// \brief Indicate if all of the FileMetaData's RowGroups can be decompressed.
   ///
   /// This will return false if any of the RowGroup's page is compressed with a
   /// compression format which is not compiled in the current parquet library.