scovich commented on code in PR #8360:
URL: https://github.com/apache/arrow-rs/pull/8360#discussion_r2357541537
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parquet-variant-compute/src/variant_array_builder.rs:
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@@ -205,6 +208,156 @@ impl VariantBuilderExt for VariantArrayBuilder {
}
}
+/// A builder for creating only the value column of a [`VariantArray`]
+///
+/// This builder is used when you have existing metadata and only need to build
+/// the value column. It's useful for scenarios like variant unshredding, data
+/// transformation, or filtering where you want to reuse existing metadata.
+///
+/// The builder produces a [`BinaryViewArray`] that can be combined with
existing
+/// metadata to create a complete [`VariantArray`].
+///
+/// # Example:
+/// ```
+/// # use arrow::array::Array;
+/// # use parquet_variant::{Variant};
+/// # use parquet_variant_compute::VariantValueArrayBuilder;
+/// // Create a variant value builder for 10 rows
+/// let mut builder = VariantValueArrayBuilder::new(10);
+///
+/// // Append some values with their corresponding metadata. In practice, some
of the variant
+/// // values would be objects with internal references to pre-existing
metadata, which the
+/// // builder takes advantage of to avoid creating new metadata.
+/// builder.append_value(Variant::from(42));
+/// builder.append_null();
+/// builder.append_value(Variant::from("hello"));
+///
+/// // Build the final value array
+/// let value_array = builder.build().unwrap();
+/// assert_eq!(value_array.len(), 3);
+/// ```
+#[derive(Debug)]
+pub struct VariantValueArrayBuilder {
+ value_builder: ValueBuilder,
+ value_offsets: Vec<usize>,
+ nulls: NullBufferBuilder,
+}
+
+impl VariantValueArrayBuilder {
+ /// Create a new `VariantValueArrayBuilder` with the specified row capacity
+ pub fn new(row_capacity: usize) -> Self {
+ Self {
+ value_builder: ValueBuilder::new(),
+ value_offsets: Vec::with_capacity(row_capacity),
+ nulls: NullBufferBuilder::new(row_capacity),
+ }
+ }
+
+ /// Build the final value array
+ ///
+ /// Returns a [`BinaryViewArray`] containing the serialized variant values.
+ /// This can be combined with existing metadata to create a complete
[`VariantArray`].
+ pub fn build(mut self) -> Result<BinaryViewArray, ArrowError> {
+ let value_buffer = self.value_builder.into_inner();
+ let mut array = binary_view_array_from_buffers(value_buffer,
self.value_offsets);
+ if let Some(nulls) = self.nulls.finish() {
+ let (views, buffers, _) = array.into_parts();
+ array = BinaryViewArray::try_new(views, buffers, Some(nulls))?;
+ }
+ Ok(array)
+ }
+
+ /// Append a null row to the builder
+ ///
+ /// WARNING: It is only valid to call this method when building the
`value` field of a shredded
+ /// variant column (which is nullable). The `value` field of a binary
(unshredded) variant
+ /// column is non-nullable, and callers should instead invoke
[`Self::append_value`] with
+ /// `Variant::Null`, passing the appropriate metadata value.
+ pub fn append_null(&mut self) {
+ self.value_offsets.push(self.value_builder.offset());
+ self.nulls.append_null();
+ }
+
+ /// Append a variant value with its corresponding metadata
+ ///
+ /// # Arguments
+ /// * `value` - The variant value to append
+ /// * `metadata` - The metadata dictionary for this variant (used for
field name resolution)
+ ///
+ /// # Returns
+ /// * `Ok(())` if the value was successfully appended
+ /// * `Err(ArrowError)` if the variant contains field names not found in
the metadata
+ ///
+ /// # Example
+ /// ```
+ /// # use parquet_variant::Variant;
+ /// # use parquet_variant_compute::VariantValueArrayBuilder;
+ /// let mut builder = VariantValueArrayBuilder::new(10);
+ /// builder.append_value(Variant::from(42));
+ /// ```
+ pub fn append_value(&mut self, value: Variant<'_, '_>) {
+ let metadata =
value.metadata().cloned().unwrap_or(EMPTY_VARIANT_METADATA);
Review Comment:
That's a great question! Until now, that method was only used in tests, so
its semantics weren't clear. I like the idea a lot tho -- it would eliminate
the `unwrap_or` (which my next PR also has to use).
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