  The extraction of $k$-mers is a fundamental component in many complex
analyses of large next-generation sequencing datasets, including reads
classification in genomics and the characterization of RNA-seq datasets. The
extraction of all $k$-mers and their frequencies is extremely demanding in
terms of running time and memory, owing to the size of the data and to the
exponential number of $k$-mers to be considered. However, in several
applications, only frequent $k$-mers, which are $k$-mers appearing in a
relatively high proportion of the data, are required by the analysis. In this
work we present SPRISS, a new efficient algorithm to approximate frequent
$k$-mers and their frequencies in next-generation sequencing data. SPRISS
employs a simple yet powerful reads sampling scheme, which allows to extract a
representative subset of the dataset that can be used, in combination with any
$k$-mer counting algorithm, to perform downstream analyses in a fraction of the
time required by the analysis of the whole data, while obtaining comparable
answers. Our extensive experimental evaluation demonstrates the efficiency and
accuracy of SPRISS in approximating frequent $k$-mers, and shows that it can be
used in various scenarios, such as the comparison of metagenomic datasets and
the identification of discriminative $k$-mers, to extract insights in a
fraction of the time required by the analysis of the whole dataset.
