49. A system comprising: one or more computers; and a computer-readable storage device storing instructions that, when executed by the one or more computers, cause the one or more computers to perform operations comprising: determining a partial similarity score for a vector x in a set of vectors and each other vector in the set of vectors, each partial similarity score representing a degree of similarity between features of the vector x and corresponding features of other vectors in the set of vectors; determining an upper bound, the upper bound being an estimate of the maximum similarity between non-processed features of the vector x and non-processed features of the other vectors, the non-processed features being features that have not been used to calculate the partial similarity scores; as long as the upper bound is greater than or equal to the similarity threshold, adding vectors to a candidate set of vectors and repeating the operations of determining a partial similarity score and determining an upper bound; when the upper bound is lower than the similarity threshold, determining partial similarity scores only for vectors in the candidate set of vectors; and identifying x and a vector y in the candidate set of vectors as similar vectors using the partial similarity score between x and y.
49. A system comprising: one or more computers; and a computer-readable storage device storing instructions that, when executed by the one or more computers, cause the one or more computers to perform operations comprising: determining a partial similarity score for a vector x in a set of vectors and each other vector in the set of vectors, each partial similarity score representing a degree of similarity between features of the vector x and corresponding features of other vectors in the set of vectors; determining an upper bound, the upper bound being an estimate of the maximum similarity between non-processed features of the vector x and non-processed features of the other vectors, the non-processed features being features that have not been used to calculate the partial similarity scores; as long as the upper bound is greater than or equal to the similarity threshold, adding vectors to a candidate set of vectors and repeating the operations of determining a partial similarity score and determining an upper bound; when the upper bound is lower than the similarity threshold, determining partial similarity scores only for vectors in the candidate set of vectors; and identifying x and a vector y in the candidate set of vectors as similar vectors using the partial similarity score between x and y. 51. The system of claim 49 , the operations further comprising identifying a vector y in the set of candidate vectors such that a similarity between the vector y and the comparison vector x meets the similarity threshold.