fn reduce()

in src/backend/serial/u64/field.rs [283:316]

• 19 lines of code
• 1 McCabe index (conditional complexity)

    fn reduce(mut limbs: [u64; 5]) -> FieldElement51 {
        const LOW_51_BIT_MASK: u64 = (1u64 << 51) - 1;

        // Since the input limbs are bounded by 2^64, the biggest
        // carry-out is bounded by 2^13.
        //
        // The biggest carry-in is c4 * 19, resulting in
        //
        // 2^51 + 19*2^13 < 2^51.0000000001
        //
        // Because we don't need to canonicalize, only to reduce the
        // limb sizes, it's OK to do a "weak reduction", where we
        // compute the carry-outs in parallel.

        let c0 = limbs[0] >> 51;
        let c1 = limbs[1] >> 51;
        let c2 = limbs[2] >> 51;
        let c3 = limbs[3] >> 51;
        let c4 = limbs[4] >> 51;

        limbs[0] &= LOW_51_BIT_MASK;
        limbs[1] &= LOW_51_BIT_MASK;
        limbs[2] &= LOW_51_BIT_MASK;
        limbs[3] &= LOW_51_BIT_MASK;
        limbs[4] &= LOW_51_BIT_MASK;

        limbs[0] += c4 * 19;
        limbs[1] += c0;
        limbs[2] += c1;
        limbs[3] += c2;
        limbs[4] += c3;

        FieldElement51(limbs)
    }