popCount

    pub fn popCount(self: Const, bit_count: usize) usize {
        var sum: usize = 0;
        if (self.positive) {
            for (self.limbs) |limb| {
                sum += @popCount(limb);
            }
        } else {
            assert(self.fitsInTwosComp(.signed, bit_count));
            assert(self.limbs[self.limbs.len - 1] != 0);

            var remaining_bits = bit_count;
            var carry: u1 = 1;
            var add_res: Limb = undefined;

            // All but the most significant limb.
            for (self.limbs[0 .. self.limbs.len - 1]) |limb| {
                const ov = @addWithOverflow(~limb, carry);
                add_res = ov[0];
                carry = ov[1];
                sum += @popCount(add_res);
                remaining_bits -= limb_bits; // Asserted not to underflow by fitsInTwosComp
            }

            // The most significant limb may have fewer than @bitSizeOf(Limb) meaningful bits,
            // which we can detect with @clz().
            // There may also be fewer limbs than needed to fill bit_count.
            const limb = self.limbs[self.limbs.len - 1];
            const leading_zeroes = @clz(limb);
            // The most significant limb is asserted not to be all 0s (above),
            // so ~limb cannot be all 1s, and ~limb + 1 cannot overflow.
            sum += @popCount(~limb + carry);
            sum -= leading_zeroes; // All leading zeroes were flipped and added to sum, so undo those
            const remaining_ones = remaining_bits - (limb_bits - leading_zeroes); // All bits not covered by limbs
            sum += remaining_ones;
        }
        return sum;
    }