Searching with Autocompletion: An Indexing Scheme with Provably Fast Response Time Holger Bast 1 , Christian W. Mortensen 2 , and Ingmar Weber 1 1 Max-Planck-Institut f¨ ur Informatik, Saarbr¨ ucken, Germany 2 IT University of Copenhagen, Denmark Abstract. We study the following autocompletion problem, which is at the core of a new full-text search technology that we have developed over the last year. The problem is, for a given document collection, to precompute a data structure using as little space as possible such that queries of the following kind can be processed as quickly as possible: given a range of words and an arbitrary set of documents, compute the set of those words from the given range, which occur in at least one of the given documents, as well as the subset of the given documents, which contain at least one of these words. With a standard inverted index, one inverted list has to be processed for each word from the given range. We propose a new indexing scheme that without using more space than an inverted index has a guaranteed query processing time that is independent of the size of the given word range. Experiments on real-world data confirm our theoretical analysis and show the practicability of our new scheme.