Putnam 2015 A4

For each real number $x$, let $$f(x) = \sum_{n\in S_x} \frac{1}{2^n},$$ where $S_x$ is the set of positive integers $n$ for which $\lfloor nx \rfloor$ is even. What is the largest real number $L$ such that $f(x) \geq L$ for all $x \in [0,1)$? (As usual, $\lfloor z \rfloor$ denotes the greatest integer less than or equal to $z$.)