Struct ReadOnlyMetadataBuilder 

pub struct ReadOnlyMetadataBuilder<'m> {
    metadata: VariantMetadata<'m>,
    known_field_names: HashMap<&'m str, u32>,
}

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.

NOTE: Self::finish is a no-op. If the intent is to make a copy of the underlying bytes each time finish is called, a different trait impl will be needed.

Fields

metadata: VariantMetadata<'m>

known_field_names: HashMap<&'m str, u32>

Implementations

impl<'m> ReadOnlyMetadataBuilder<'m>

pub fn new(metadata: VariantMetadata<'m>) -> Self

Creates a new read-only metadata builder from the given metadata dictionary.

Trait Implementations

impl<'m> Debug for ReadOnlyMetadataBuilder<'m>

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl MetadataBuilder for ReadOnlyMetadataBuilder<'_>

fn try_upsert_field_name(&mut self, field_name: &str) -> Result<u32, ArrowError>

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 field_name(&self, field_id: usize) -> &str

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 num_field_names(&self) -> usize

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 truncate_field_names(&mut self, new_size: usize)

Reverts the field names to a previous size, discarding any newly out of bounds field ids.

fn finish(&mut self) -> usize

Finishes the current metadata dictionary, returning the new size of the underlying buffer.