A Quasi-Polynomial-Time Algorithm for Sampling Words from a Context-Free Language

A quasi-polynomial-time algorithm is presented for sampling almost uniformly at random from the n-slice of the language L(G) generated by an arbitrary context-free grammar G. (The n-slice of a language L over an alphabet is the subset L\ n of words of length exactly n.) The time complexity of the algorithm is " ?2 (n jGj) O(log n) , where the parameter " bounds the variation of the output distribution from uniform , and jGj is a natural measure of the size of grammar G. The algorithm applies to a class of language sampling problems that includes slices of context-free languages as a proper subclass.