Method apply_rotation

nodedb-codec/src/vector_quant/rabitq.rs:185–205

Apply the randomised WHT rotation to a residual vector. Steps: 1. Apply signed-diagonal `D` (deterministic from `rotation_seed`). 2. Zero-pad to the next power of two if `dim` is not pow2. 3. WHT in-place. 4. Truncate back to `dim`.

(&self, v: &[f32])

    /// 3. WHT in-place.
    /// 4. Truncate back to `dim`.
    pub fn apply_rotation(&self, v: &[f32]) -> Vec<f32> {
        let dim = self.dim;
        let pow2 = next_pow2(dim);

        // Signed-diagonal multiply: generate ±1 per dim from seed.
        let mut seed = self.rotation_seed;
        let mut buf = vec![0.0f32; pow2];
        for (i, &vi) in v.iter().take(dim).enumerate() {
            let flip = if xorshift64(&mut seed) & 1 == 0 {
                1.0f32
            } else {
                -1.0f32
            };
            buf[i] = vi * flip;
        }
        // Trailing elements remain zero (pad).

        wht_inplace(&mut buf);
        buf.truncate(dim);
        buf
    }

    /// Encode a single vector into a [`UnifiedQuantizedVector`] with
    /// `QuantMode::RaBitQ`.