Function hash_union_array_default

datafusion/common/src/hash_utils.rs:744–775

(
    array: &UnionArray,
    union_fields: &UnionFields,
    random_state: &RandomState,
    hashes_buffer: &mut [u64],
)

/// this simpler approach is preferred.
#[cfg(not(feature = "force_hash_collisions"))]
fn hash_union_array_default(
    array: &UnionArray,
    union_fields: &UnionFields,
    random_state: &RandomState,
    hashes_buffer: &mut [u64],
) -> Result<()> {
    let mut child_hashes: HashMap<i8, Vec<u64>> =
        HashMap::with_capacity(union_fields.len());

    // Hash each child array fully
    for (type_id, _field) in union_fields.iter() {
        let child = array.child(type_id);
        let mut child_hash_buffer = vec![0; child.len()];
        create_hashes([child], random_state, &mut child_hash_buffer)?;

        child_hashes.insert(type_id, child_hash_buffer);
    }

    // Combine hashes for each row using the appropriate child offset
    // For dense unions: value_offset points to the actual position in the child
    // For sparse unions: value_offset equals the row index
    #[expect(clippy::needless_range_loop)]
    for i in 0..array.len() {
        let type_id = array.type_id(i);
        let child_offset = array.value_offset(i);

        let child_hash = child_hashes.get(&type_id).expect("invalid type_id");
        hashes_buffer[i] = combine_hashes(hashes_buffer[i], child_hash[child_offset]);
    }

    Ok(())
}

/// Hash a sparse union array.
/// Sparse unions have child arrays with the same length as the union array.