[GitHub] [arrow-rs] tustvold commented on a diff in pull request #1964: Fix the behavior of `from_fixed_size_list` when offset > 0

[GitBox <gi...@apache.org>]

tustvold commented on code in PR #1964:
URL: https://github.com/apache/arrow-rs/pull/1964#discussion_r909524690


##########
arrow/src/array/array_binary.rs:
##########
@@ -831,22 +831,26 @@ impl DecimalArray {
         precision: usize,
         scale: usize,
     ) -> Self {
+        let child_data = &v.data_ref().child_data()[0];
         assert_eq!(
-            v.data_ref().child_data()[0].child_data().len(),
+            child_data.child_data().len(),
             0,
             "DecimalArray can only be created from list array of u8 values \
              (i.e. FixedSizeList<PrimitiveArray<u8>>)."
         );
         assert_eq!(
-            v.data_ref().child_data()[0].data_type(),
+            child_data.data_type(),
             &DataType::UInt8,
             "DecimalArray can only be created from FixedSizeList<u8> arrays, mismatched data types."
         );
 
+        let list_offset = v.offset();
+        let child_offset = child_data.offset();
         let builder = ArrayData::builder(DataType::Decimal(precision, scale))
             .len(v.len())
-            .add_buffer(v.data_ref().child_data()[0].buffers()[0].clone())
-            .null_bit_buffer(v.data_ref().null_buffer().cloned());
+            .add_buffer(v.data_ref().child_data()[0].buffers()[0].slice(child_offset))

Review Comment:
   ```suggestion
               .add_buffer(child_data.buffers()[0].slice(child_offset))
   ```



##########
arrow/src/array/array_binary.rs:
##########
@@ -831,22 +831,26 @@ impl DecimalArray {
         precision: usize,
         scale: usize,
     ) -> Self {
+        let child_data = &v.data_ref().child_data()[0];

Review Comment:
   Somewhat tangential to this PR, but what happens to the child data's null buffer? Perhaps worth a docstring saying it is ignored?



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