Method distance_prepared

nodedb-vector/src/rerank/codecs/bbq.rs:184–214

Compute asymmetric L2 distance from a prepared query to a BBQ-encoded candidate. The query is the exact centered FP32 vector. The stored candidate is reconstructed from its sign bits and `residual_norm` (each dim ≈ ±norm/√dim). Returns L2 distance between them. Expects `PreparedQuery::Bytes` produced by `prepare_query`.

(
        &self,
        prepared: &PreparedQuery,
        encoded: &[u8],
    )

    ///
    /// Expects `PreparedQuery::Bytes` produced by `prepare_query`.
    fn distance_prepared(
        &self,
        prepared: &PreparedQuery,
        encoded: &[u8],
    ) -> Result<f32, RerankError> {
        let payload = match prepared {
            PreparedQuery::Bytes(b) => b.as_slice(),
            _ => {
                return Err(RerankError::BadInput(
                    "bbq distance: prepared query is not Bytes".to_string(),
                ));
            }
        };

        let (_query_norm, centered) = decode_payload(payload, self.dim)?;

        let packed_len = self.dim.div_ceil(8);
        let uqv_ref = UnifiedQuantizedVectorRef::from_bytes(encoded, packed_len).map_err(|e| {
            RerankError::BadInput(format!("bbq distance: failed to parse encoded bytes: {e}"))
        })?;

        let header = uqv_ref.header();
        let recon = bbq_dequantize(uqv_ref.packed_bits(), header.residual_norm, self.dim);
        let dist = centered
            .iter()
            .zip(recon.iter())
            .map(|(&a, &b)| (a - b) * (a - b))
            .sum::<f32>()
            .sqrt();
        Ok(dist)
    }

    fn name(&self) -> CodecName {
        CodecName::Bbq