add comment for BufferedStats

Author: wecharyu

cpp/src/parquet/file_writer.cc

@@ -68,7 +68,7 @@ int64_t RowGroupWriter::total_compressed_bytes_written() const {
   return contents_->total_compressed_bytes_written();
 }
 
-BufferedStats RowGroupWriter::estimated_buffered_stats() const {
+RowGroupWriter::BufferedStats RowGroupWriter::estimated_buffered_stats() const {
   return contents_->EstimatedBufferedStats();
 }
 
@@ -199,8 +199,8 @@ class RowGroupSerializer : public RowGroupWriter::Contents {
     return total_compressed_bytes_written;
   }
 
-  BufferedStats EstimatedBufferedStats() const override {
-    BufferedStats stats;
+  RowGroupWriter::BufferedStats EstimatedBufferedStats() const override {
+    RowGroupWriter::BufferedStats stats;
     if (closed_) {
       return stats;
     }

cpp/src/parquet/file_writer.h

@@ -34,15 +34,17 @@ class ColumnWriter;
 static constexpr uint8_t kParquetMagic[4] = {'P', 'A', 'R', '1'};
 static constexpr uint8_t kParquetEMagic[4] = {'P', 'A', 'R', 'E'};
 
-struct PARQUET_EXPORT BufferedStats {
-  int64_t def_level_bytes = 0;
-  int64_t rep_level_bytes = 0;
-  int64_t value_bytes = 0;
-  int64_t dict_bytes = 0;
-};
-
 class PARQUET_EXPORT RowGroupWriter {
  public:
+  // Estimated uncompressed byte sizes of data buffered by column writers
+  // that have not yet been serialized into data pages.
+  struct BufferedStats {
+    int64_t def_level_bytes = 0;
+    int64_t rep_level_bytes = 0;
+    int64_t value_bytes = 0;
+    int64_t dict_bytes = 0;
+  };
+
   // Forward declare a virtual class 'Contents' to aid dependency injection and more
   // easily create test fixtures
   // An implementation of the Contents class is defined in the .cc file