Method outlier_at

nodedb-codec/src/vector_quant/layout.rs:244–262

Return the outlier `(dim_index, value)` for the dimension at position `slot` in the bitmask. `slot` is the dimension index (0–63). Returns `None` if the bit for `slot` is not set in `outlier_bitmask`, or if `slot ≥ 64`. Uses a branchless popcnt to find the dense offset into the outlier payload.

(&self, slot: u32)

    /// Uses a branchless popcnt to find the dense offset into the outlier
    /// payload.
    pub fn outlier_at(&self, slot: u32) -> Option<(u32, f32)> {
        if slot >= 64 {
            return None;
        }
        let bitmask = self.header().outlier_bitmask;
        if bitmask & (1u64 << slot) == 0 {
            return None;
        }
        // Number of set bits below `slot` gives the dense array index.
        let mask = bitmask & ((1u64 << slot).wrapping_sub(1));
        let offset = mask.count_ones() as usize;

        let header_bytes = core::mem::size_of::<QuantHeader>();
        let base = header_bytes + self.packed_bits_len + offset * OUTLIER_ENTRY_BYTES;

        let dim_idx = u32::from_le_bytes(self.buf[base..base + 4].try_into().ok()?);
        let value = f32::from_le_bytes(self.buf[base + 4..base + 8].try_into().ok()?);
        Some((dim_idx, value))
    }

    /// Full backing buffer suitable for direct I/O.
    #[inline]