fn optional_multiscalar_mul()

in src/backend/serial/scalar_mul/straus.rs [157:194]

• 31 lines of code
• 5 McCabe index (conditional complexity)

    fn optional_multiscalar_mul<I, J>(scalars: I, points: J) -> Option<EdwardsPoint>
    where
        I: IntoIterator,
        I::Item: Borrow<Scalar>,
        J: IntoIterator<Item = Option<EdwardsPoint>>,
    {
        use backend::serial::curve_models::{CompletedPoint, ProjectiveNielsPoint, ProjectivePoint};
        use window::NafLookupTable5;
        use traits::Identity;

        let nafs: Vec<_> = scalars
            .into_iter()
            .map(|c| c.borrow().non_adjacent_form(5))
            .collect();

        let lookup_tables = points
            .into_iter()
            .map(|P_opt| P_opt.map(|P| NafLookupTable5::<ProjectiveNielsPoint>::from(&P)))
            .collect::<Option<Vec<_>>>()?;

        let mut r = ProjectivePoint::identity();

        for i in (0..256).rev() {
            let mut t: CompletedPoint = r.double();

            for (naf, lookup_table) in nafs.iter().zip(lookup_tables.iter()) {
                if naf[i] > 0 {
                    t = &t.to_extended() + &lookup_table.select(naf[i] as usize);
                } else if naf[i] < 0 {
                    t = &t.to_extended() - &lookup_table.select(-naf[i] as usize);
                }
            }

            r = t.to_projective();
        }

        Some(r.to_extended())
    }