CRAM: compressed random access memory. (English) Zbl 1272.68116

Jansson, Jesper; Sadakane, Kunihiko; Sung, Wing-Kin

Czumaj, Artur (ed.) et al., Automata, languages, and programming. 39th international colloquium, ICALP 2012, Warwick, UK, July 9–13, 2012. Proceedings, Part I. Berlin: Springer (ISBN 978-3-642-31593-0/pbk). Lecture Notes in Computer Science 7391, 510-521 (2012).

Summary: We present a new data structure called the compressed random access memory (CRAM) that can store a dynamic string \(T\) of characters, e.g., representing the memory of a computer, in compressed form while achieving asymptotically almost-optimal bounds (in terms of empirical entropy) on the compression ratio. It allows short substrings of \(T\) to be decompressed and retrieved efficiently and, significantly, characters at arbitrary positions of \(T\) to be modified quickly during execution without decompressing the entire string. This can be regarded as a new type of data compression that can update a compressed file directly. Moreover, at the cost of slightly increasing the time spent per operation, the CRAM can be extended to also support insertions and deletions. Our key observation that the empirical entropy of a string does not change much after a small change to the string, as well as our simple yet efficient method for maintaining an array of variable-length blocks under length modifications, may be useful for many other applications as well.
For the entire collection see [Zbl 1268.68011].

MSC:

68P05	Data structures
68P30	Coding and information theory (compaction, compression, models of communication, encoding schemes, etc.) (aspects in computer science)