lldiv0p5

fn lldiv0p5(quo: []Limb, rem: *Limb, a: []const Limb, b: HalfLimb) void {
    assert(a.len > 1 or a[0] >= b);
    assert(quo.len >= a.len);

    rem.* = 0;
    for (a, 0..) |_, ri| {
        const i = a.len - ri - 1;
        const ai_high = a[i] >> half_limb_bits;
        const ai_low = a[i] & ((1 << half_limb_bits) - 1);

        // Split the division into two divisions acting on half a limb each. Carry remainder.
        const ai_high_with_carry = (rem.* << half_limb_bits) | ai_high;
        const ai_high_quo = ai_high_with_carry / b;
        rem.* = ai_high_with_carry % b;

        const ai_low_with_carry = (rem.* << half_limb_bits) | ai_low;
        const ai_low_quo = ai_low_with_carry / b;
        rem.* = ai_low_with_carry % b;

        quo[i] = (ai_high_quo << half_limb_bits) | ai_low_quo;
    }
}