SingleWordIterator

        fn SingleWordIterator(comptime direction: IteratorOptions.Direction) type {
            return struct {
                const IterSelf = @This();
                // all bits which have not yet been iterated over
                bits_remain: MaskInt,

                /// Returns the index of the next unvisited set bit
                /// in the bit set, in ascending order.
                pub fn next(self: *IterSelf) ?usize {
                    if (self.bits_remain == 0) return null;

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