fix(frequencies): write the full empty preamble long in serialize#143
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An empty FrequentItemsSketch serialized to 6 bytes: the preamble declares one 8-byte preamble long, but serialization stopped after the flags byte. Deserialization (in Rust as well as the Java and C++ readers) consumes the full 8-byte preamble, so a serialized empty sketch failed to deserialize with an insufficient-data error. Emptiness is reachable from non-trivial streams: a map saturated with count-1 items purges every counter, so pipelines that persist sketches can hit the failure on real data. Write the two unused padding bytes so the empty encoding is one full preamble long, matching the Java and C++ implementations. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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What
FrequentItemsSketch::serializeemits only 6 bytes for an empty sketch: the preamble declares one 8-byte preamble long, but serialization stops after the flags byte.deserializeconsumes the full 8-byte preamble before checking the empty flag, so a serialized empty sketch always fails to deserialize withinsufficient data: <unused>. The truncated form also diverges from the Java and C++ implementations, which both write the full preamble long for empty sketches.Empty sketches are reachable from non-trivial streams: a map saturated with count-1 items (e.g. unique identifiers) purges every counter, leaving an empty sketch that then cannot be persisted and restored.
Changes
serialize_inner, making the empty encoding one full preamble long and byte-compatible with the Java/C++ readers.