Module dictionary 

Dictionary encoding for low-cardinality string field values.

When a field’s unique value count stays below a configurable threshold, storing the same string bytes in every packed document wastes space. ValueDictionary assigns a compact u32 dictionary ID to each unique byte sequence and returns a StoredValue::DictRef instead of inlining the full payload. High-cardinality fields (or values shorter than min_len_for_dictionary) fall back to StoredValue::Inline.

Cardinality Estimation

Per-field cardinality is tracked with a HashSet<u64> of value hashes, giving exact counts up to the threshold and constant-space tracking thereafter. Once the cardinality of a field exceeds low_cardinality_threshold, all subsequent values for that field are stored inline.

Encoding / Decoding

• Encode: encode(field_id, value_bytes) returns DictRef(id) or Inline(bytes).
• Decode: decode(stored) maps DictRef(id) back to owned bytes, or clones Inline(bytes) directly.

Structs

ValueDictionary

Per-collection value dictionary.

ValueDictionaryConfig

Value dictionary tuning parameters.

Enums

DictionaryError

Errors returned by dictionary operations.

StoredValue

Stored field value representation after dictionary encoding.