Lempel-Ziv Decoding in External Memory

TL;DR

This paper introduces the first external memory algorithms for LZ77 decoding, achieving optimal I/O complexity and practical speed close to in-memory decoding, addressing a key challenge in external memory text decompression.

Contribution

It presents novel external memory algorithms for LZ77 decoding, proving their I/O optimality and demonstrating practical efficiency.

Findings

I/O complexity of the algorithms is proven to be optimal.

Decoding speed is only about three times slower than in-memory decoding.

Algorithms are practical and effective for external memory environments.

Abstract

Simple and fast decoding is one of the main advantages of LZ77-type text encoding used in many popular file compressors such as gzip and 7zip. With the recent introduction of external memory algorithms for Lempel-Ziv factorization there is a need for external memory LZ77 decoding but the standard algorithm makes random accesses to the text and cannot be trivially modified for external memory computation. We describe the first external memory algorithms for LZ77 decoding, prove that their I/O complexity is optimal, and demonstrate that they are very fast in practice, only about three times slower than in-memory decoding (when reading input and writing output is included in the time).