Sampling Lovász Local Lemma For General Constraint Satisfaction Solutions In Near-Linear Time (2204.01520v3)

Abstract: We give a fast algorithm for sampling uniform solutions of general constraint satisfaction problems (CSPs) in a local lemma regime. Suppose that the CSP has $n$ variables with domain size at most q, each constraint contains at most k variables, shares variables with at most $\Delta$ constraints, and is violated with probability at most $p$ by a uniform random assignment. The algorithm returns an almost uniform satisfying assignment in expected $\mathrm{poly}(q,k,\Delta)\cdot\tilde{O}(n)$ time, as long as a local lemma condition is satisfied: [ k\cdot p\cdot q^2\cdot \Delta^5\le C_0\quad\text{for a suitably small absolute constant }C_0. ] Previously, under similar local lemma conditions, sampling algorithms with running time polynomial in both $n$ and $\Delta$ were only known for the almost atomic case, where each constraint is violated by a small number of forbidden local configurations. The key term $\Delta^5$ in our local lemma condition also improves the previously best known $\Delta^7$ for general CSPs [JPV21b] and $\Delta^{5.714}$ for atomic CSPs, including the special case of $k$-CNF [JPV21a, HSW21]. Our sampling approach departs from previous fast algorithms for sampling LLL, which were based on Markov chains. A crucial step of our algorithm is a recursive marginal sampler that is of independent interests. Within a local lemma regime, this marginal sampler can draw a random value for a variable according to its marginal distribution, at a cost independent of the size of the CSP.