next

        pub fn next(self: *Self) ?usize {
            while (self.bits_remain == 0) {
                if (self.words_remain.len == 0) return null;
                self.nextWord(false);
                switch (direction) {
                    .forward => self.bit_offset += @bitSizeOf(MaskInt),
                    .reverse => self.bit_offset -= @bitSizeOf(MaskInt),
                }
            }

            switch (direction) {
                .forward => {
                    const next_index = @ctz(self.bits_remain) + self.bit_offset;
                    self.bits_remain &= self.bits_remain - 1;
                    return next_index;
                },
                .reverse => {
                    const leading_zeroes = @clz(self.bits_remain);
                    const top_bit = (@bitSizeOf(MaskInt) - 1) - leading_zeroes;
                    const no_top_bit_mask = (@as(MaskInt, 1) << @as(ShiftInt, @intCast(top_bit))) - 1;
                    self.bits_remain &= no_top_bit_mask;
                    return top_bit + self.bit_offset;
                },
            }
        }