Package varopt contains an implementation of VarOpt, an unbiased weighted sampling algorithm described in the paper "Stream sampling for variance-optimal estimation of subset sums" https://arxiv.org/pdf/0803.0473.pdf (2008), by Edith Cohen, Nick Duffield, Haim Kaplan, Carsten Lund, and Mikkel Thorup. VarOpt is a reservoir-type sampler that maintains a fixed-size sample and provides a mechanism for merging unequal-weight samples. This package also includes a simple reservoir sampling algorithm, often useful in conjunction with weighed reservoir sampling, using Algorithm R from "Random sampling with a reservoir", https://en.wikipedia.org/wiki/Reservoir_sampling#Algorithm_R (1985), by Jeffrey Vitter. See https://github.com/lightstep/varopt/blob/master/README.md for more detail.