ritchie46 commented on a change in pull request #9402:
URL: https://github.com/apache/arrow/pull/9402#discussion_r570767432
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File path: rust/arrow/src/compute/kernels/cast.rs
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@@ -1175,6 +1254,76 @@ where
Ok(Arc::new(b.finish()))
}
+/// Cast the container type of List/Largelist array but not the inner types.
+/// This function can leave the value data intact and only has to cast the
offset dtypes.
+fn cast_list_container<OffsetSizeFrom, OffsetSizeTo>(array: &ArrayRef) ->
Result<ArrayRef>
+where
+ OffsetSizeFrom: OffsetSizeTrait + ToPrimitive,
+ OffsetSizeTo: OffsetSizeTrait + NumCast,
+{
+ let data = array.data_ref();
+ // the value data stored by the list
+ let value_data = data.child_data()[0].clone();
+
+ let out_dtype = match array.data_type() {
+ DataType::List(value_type) => {
+ assert_eq!(
+ std::mem::size_of::<OffsetSizeFrom>(),
+ std::mem::size_of::<i32>()
+ );
+ assert_eq!(
+ std::mem::size_of::<OffsetSizeTo>(),
+ std::mem::size_of::<i64>()
+ );
+ DataType::LargeList(value_type.clone())
+ }
+ DataType::LargeList(value_type) => {
+ assert_eq!(
+ std::mem::size_of::<OffsetSizeFrom>(),
+ std::mem::size_of::<i64>()
+ );
+ assert_eq!(
+ std::mem::size_of::<OffsetSizeTo>(),
+ std::mem::size_of::<i32>()
+ );
+ if value_data.len() > i32::MAX as usize {
+ return Err(ArrowError::ComputeError(
+ "LargeList too large to cast to List".into(),
+ ));
+ }
+ DataType::List(value_type.clone())
+ }
+ // implementation error
+ _ => unreachable!(),
+ };
+
+ // the offsets
+ // SAFETY
+ // We asserted the correct size of OffsetSizeFrom above.
+ let offsets = unsafe { data.buffers()[0].typed_data::<OffsetSizeFrom>() };
Review comment:
Ah, I was not aware of that function, nice! I had to change the
visibility from `pub (super)` to pub (crate)`. May also be a nice method for
the public API.
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