[GitHub] [arrow] westonpace commented on a diff in pull request #34720: GH-34633: [C++][Parquet] Fix StreamReader to read decimals

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #34720:
URL: https://github.com/apache/arrow/pull/34720#discussion_r1147827525


##########
cpp/src/parquet/stream_reader.cc:
##########
@@ -173,6 +174,60 @@ StreamReader& StreamReader::operator>>(std::string& v) {
   return *this;
 }
 
+StreamReader& StreamReader::operator>>(::arrow::Decimal128& v) {

Review Comment:
   Is there anyway to avoid repeating the implementation here?  For example, would something like this work?
   
   ```
   StreamReader& StreamReader::operator>>(::arrow::Decimal128& v) {
     std::optional<::arrow::Decimal128> maybe_v;
     this >> maybe_v;
     if (!maybe_v) {
       // I'm not sure what happens here, maybe throw an exception?
     }
     v = *maybe_v;
     return this;
   }
   ```



##########
cpp/src/parquet/stream_reader_test.cc:
##########
@@ -912,5 +913,39 @@ TEST_F(TestReadingDataFiles, Int64Decimal) {
   EXPECT_EQ(i, 25);
 }
 
+TEST_F(TestReadingDataFiles, FLBADecimal) {
+  PARQUET_ASSIGN_OR_THROW(auto infile, ::arrow::io::ReadableFile::Open(
+                                           GetDataFile("fixed_length_decimal.parquet")));
+
+  auto file_reader = ParquetFileReader::Open(infile);
+  auto reader = StreamReader{std::move(file_reader)};
+
+  ::arrow::Decimal128 x;
+  int i = 0;
+
+  for (i = 1; !reader.eof(); ++i) {
+    reader >> x >> EndRow;
+    EXPECT_EQ(x.ToString(2), ::arrow::Decimal128(i * 100).ToString(2));

Review Comment:
   Why `ToString`.  Can't we compare the decimals directly?



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