1. A method comprising: generating, by a computing device, a feature vector for a digital document, the generated feature vector comprising a plurality of features of the digital document; partitioning, by the computing device, the digital document's feature vector into a plurality of feature vector segments, each feature vector segment corresponding to a subspace of a plurality of subspaces of a feature space, each subspace comprising a plurality of cells; generating, by the computing device, a code vector for the digital document, each code in the digital document's code vector corresponding to a respective subspace of the plurality subspaces of the feature space and identifying, using a corresponding feature vector segment, a cell of the respective subspace's plurality of cells with which the digital document is most similar relative to other cells of the plurality in the respective subspace; and making a determination, by the computing device, whether to select the digital document, in response to a digital document request, the determination comprising determining a score for the digital document by comparing the digital document's code vector and a code vector associated with the digital document request to determine a number of matches and making the determination using the determined score.