scovich commented on code in PR #8208:
URL: https://github.com/apache/arrow-rs/pull/8208#discussion_r2296036836
##########
parquet-variant/src/builder.rs:
##########
@@ -431,13 +433,97 @@ impl ValueBuilder {
}
}
+/// A trait for building variant metadata dictionaries, to be used in
conjunction with a
+/// [`ValueBuilder`]. The trait provides methods for managing field names and
their IDs, as well as
+/// rolling back a failed builder operation that might have created new field
ids.
+pub trait MetadataBuilder: std::fmt::Debug {
+ /// Attempts to register a field name, returning the corresponding
(possibly newly-created)
+ /// field id on success. Attempting to register the same field name twice
will _generally_
+ /// produce the same field id both times, but the variant spec does not
actually require it.
+ fn try_upsert_field_name(&mut self, field_name: &str) -> Result<u32,
ArrowError>;
+
+ /// Retrieves the field name for a given field id, which must be less than
+ /// [`Self::num_field_names`]. Panics if the field id is out of bounds.
+ fn field_name(&self, field_id: usize) -> &str;
+
+ /// Returns the number of field names stored in this metadata builder. Any
number less than this
+ /// is a valid field id. The builder can be reverted back to this size
later on (discarding any
+ /// newer/higher field ids) by calling [`Self::truncate_field_names`].
+ fn num_field_names(&self) -> usize;
+
+ /// Reverts the field names to a previous size, discarding any newly out
of bounds field ids.
+ fn truncate_field_names(&mut self, new_size: usize);
+}
+
+impl MetadataBuilder for BasicMetadataBuilder {
+ fn try_upsert_field_name(&mut self, field_name: &str) -> Result<u32,
ArrowError> {
+ Ok(self.upsert_field_name(field_name))
+ }
+ fn field_name(&self, field_id: usize) -> &str {
+ self.field_name(field_id)
+ }
+ fn num_field_names(&self) -> usize {
+ self.num_field_names()
+ }
+ fn truncate_field_names(&mut self, new_size: usize) {
+ self.field_names.truncate(new_size)
+ }
+}
+
+/// A metadata builder that cannot register new field names, and merely
returns the field id
+/// associated with a known field name. This is useful for variant unshredding
operations, where the
+/// metadata column is fixed and -- per variant shredding spec -- already
contains all field names
+/// from the typed_value column. It is also useful when projecting a subset of
fields from a variant
+/// object value, since the bytes can be copied across directly without
re-encoding their field ids.
+#[derive(Debug)]
+pub struct ReadOnlyMetadataBuilder<'m> {
+ metadata: VariantMetadata<'m>,
+ known_field_names: HashMap<&'m str, u32>,
+}
+
+impl<'m> ReadOnlyMetadataBuilder<'m> {
+ /// Creates a new read-only metadata builder from the given metadata
dictionary.
+ pub fn new(metadata: VariantMetadata<'m>) -> Self {
+ Self {
+ metadata,
+ known_field_names: HashMap::new(),
+ }
+ }
+}
+
+impl MetadataBuilder for ReadOnlyMetadataBuilder<'_> {
+ fn try_upsert_field_name(&mut self, field_name: &str) -> Result<u32,
ArrowError> {
+ if let Some(field_id) = self.known_field_names.get(field_name) {
+ return Ok(*field_id);
+ }
+
+ let Some((field_id, field_name)) = self.metadata.get_entry(field_name)
else {
+ return Err(ArrowError::InvalidArgumentError(format!(
+ "Field name '{field_name}' not found in metadata dictionary"
+ )));
+ };
+
+ self.known_field_names.insert(field_name, field_id);
+ Ok(field_id)
+ }
+ fn field_name(&self, field_id: usize) -> &str {
+ &self.metadata[field_id]
+ }
+ fn num_field_names(&self) -> usize {
+ self.metadata.len()
+ }
+ fn truncate_field_names(&mut self, new_size: usize) {
+ debug_assert_eq!(self.metadata.len(), new_size);
+ }
+}
+
/// Builder for constructing metadata for [`Variant`] values.
///
/// This is used internally by the [`VariantBuilder`] to construct the metadata
///
/// You can use an existing `Vec<u8>` as the metadata buffer by using the
`from` impl.
#[derive(Default, Debug)]
-struct MetadataBuilder {
+struct BasicMetadataBuilder {
Review Comment:
Went with `WritableMetadataBuilder`.
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