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Add downcast_array (#2901) #3117

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merged 1 commit into from Nov 15, 2022
Merged

Add downcast_array (#2901) #3117

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33 changes: 33 additions & 0 deletions arrow-array/src/cast.rs
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Expand Up @@ -19,6 +19,7 @@

use crate::array::*;
use crate::types::*;
use arrow_data::ArrayData;

/// Repeats the provided pattern based on the number of comma separated identifiers
#[doc(hidden)]
Expand Down Expand Up @@ -550,6 +551,38 @@ array_downcast_fn!(as_union_array, UnionArray);
array_downcast_fn!(as_map_array, MapArray);
array_downcast_fn!(as_decimal_array, Decimal128Array);

/// Downcasts a `dyn Array` to a concrete type
///
/// ```
/// # use arrow_array::{BooleanArray, Int32Array, RecordBatch, StringArray};
/// # use arrow_array::cast::downcast_array;
/// struct ConcreteBatch {
/// col1: Int32Array,
/// col2: BooleanArray,
/// col3: StringArray,
/// }
///
/// impl ConcreteBatch {
/// fn new(batch: &RecordBatch) -> Self {
/// Self {
/// col1: downcast_array(batch.column(0).as_ref()),
/// col2: downcast_array(batch.column(1).as_ref()),
/// col3: downcast_array(batch.column(2).as_ref()),
/// }
/// }
/// }
/// ```
///
/// # Panics
///
/// Panics if array is not of the correct data type
pub fn downcast_array<T>(array: &dyn Array) -> T
where
T: From<ArrayData>,
{
T::from(array.data().clone())
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There may be some mechanism to avoid this clone, if/when we work this out this can be optimised. In practice, this clone should be irrelevant, as it all the underlying buffers are reference counted

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Will this clone make us unable to Arc::try_unwrap later?

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@tustvold tustvold Nov 15, 2022 •
edited

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Provided the source Array is dropped first, and there are no other references to the underlying Buffer, e.g. as a result of slicing the array, it will work.

Edit: There isn't really a way to avoid this, even approaches using AsDynAny, etc... will have the same complication

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Okay. Let's merge this so I can play with it in #3115.

}

#[cfg(test)]
mod tests {
use arrow_buffer::i256;
Expand Down