friendlymatthew commented on code in PR #8891:
URL: https://github.com/apache/arrow-rs/pull/8891#discussion_r2593588072
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arrow-schema/src/fields.rs:
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@@ -339,19 +339,212 @@ impl UnionFields {
///
/// See <https://arrow.apache.org/docs/format/Columnar.html#union-layout>
///
+ /// # Errors
+ ///
+ /// This function returns an error if:
+ /// - Any type_id appears more than once (duplicate type ids)
+ /// - The number of type_ids doesn't match the number of fields
+ ///
+ /// # Examples
+ ///
+ /// ```
+ /// use arrow_schema::{DataType, Field, UnionFields};
+ /// // Create a new UnionFields with type id mapping
+ /// // 1 -> DataType::UInt8
+ /// // 3 -> DataType::Utf8
+ /// let result = UnionFields::try_new(
+ /// vec![1, 3],
+ /// vec![
+ /// Field::new("field1", DataType::UInt8, false),
+ /// Field::new("field3", DataType::Utf8, false),
+ /// ],
+ /// );
+ /// assert!(result.is_ok());
+ ///
+ /// // This will fail due to duplicate type ids
+ /// let result = UnionFields::try_new(
+ /// vec![1, 1],
+ /// vec![
+ /// Field::new("field1", DataType::UInt8, false),
+ /// Field::new("field2", DataType::Utf8, false),
+ /// ],
+ /// );
+ /// assert!(result.is_err());
+ /// ```
+ pub fn try_new<F, T>(type_ids: T, fields: F) -> Result<Self, ArrowError>
+ where
+ F: IntoIterator,
+ F::Item: Into<FieldRef>,
+ T: IntoIterator<Item = i8>,
+ {
+ let mut type_ids_iter = type_ids.into_iter();
+ let mut fields_iter = fields.into_iter().map(Into::into);
+
+ let mut seen_type_ids = 0u128;
+
+ let mut out = Vec::new();
+
+ loop {
+ match (type_ids_iter.next(), fields_iter.next()) {
+ (None, None) => return Ok(Self(out.into())),
+ (Some(type_id), Some(field)) => {
+ // check type id is non-negative
+ if type_id < 0 {
+ return Err(ArrowError::InvalidArgumentError(format!(
+ "type ids must be non-negative: {type_id}"
+ )));
+ }
+
+ // check type id uniqueness
+ let mask = 1_u128 << type_id;
+ if (seen_type_ids & mask) != 0 {
+ return Err(ArrowError::InvalidArgumentError(format!(
+ "duplicate type id: {type_id}"
+ )));
+ }
+
+ seen_type_ids |= mask;
+
+ out.push((type_id, field));
+ }
+ (None, Some(_)) => {
+ return Err(ArrowError::InvalidArgumentError(
+ "fields iterator has more elements than type_ids
iterator".to_string(),
+ ));
+ }
+ (Some(_), None) => {
+ return Err(ArrowError::InvalidArgumentError(
+ "type_ids iterator has more elements than fields
iterator".to_string(),
+ ));
+ }
+ }
+ }
+ }
+
+ /// Create a new [`UnionFields`] from a collection of fields with
automatically
+ /// assigned type IDs starting from 0.
+ ///
+ /// The type IDs are assigned in increasing order: 0, 1, 2, 3, etc.
+ ///
+ /// See <https://arrow.apache.org/docs/format/Columnar.html#union-layout>
+ ///
+ /// # Panics
+ ///
+ /// Panics if the number of fields exceeds 127 (the maximum value for i8
type IDs).
+ ///
+ /// If you want to avoid panics, use [`UnionFields::try_from_fields`]
instead, which
+ /// returns a `Result`.
Review Comment:
here's a note about the fallible/infallible ways to construct `UnionFields`
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