[GitHub] [arrow-nanoarrow] paleolimbot commented on a diff in pull request #174: feat: Implement `ArrowArrayViewValidateFull()`

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

paleolimbot commented on code in PR #174:
URL: https://github.com/apache/arrow-nanoarrow/pull/174#discussion_r1151298323


##########
src/nanoarrow/nanoarrow_types.h:
##########
@@ -503,7 +503,8 @@ struct ArrowArrayView {
   ///
   /// If storage_type is a union type, a 256-byte ArrowMalloc()ed buffer
   /// such that child_index == union_type_id_map[type_id] and
-  /// type_id == union_type_id_map[128 + child_index]
+  /// type_id == union_type_id_map[128 + child_index]. This value may be
+  /// NULL in the case where child_id == type_id.

Review Comment:
   This is an implementation detail of the `ArrowArrayView`...in the spec the type ids are always there. Arguably they should always be here, too, but there's at least one test that exploits that behaviour that I discovered so I figured I should probably document it. I think the reason it ended up that way was because you can `ArrowArrayViewInitFromType()` and `ArrowArrayViewAllocateChildren()` and letting the type id map stay NULL saves some special casing of unions there.



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