Using T-codes as locally decodable source codes

Abstract

A locally decodable source code (LDSC) allows the recovery of arbitrary parts of an unencoded message from its encoded version, using only a part of the encoded message as input, a challenge that arises when searching within compressed data sets. Simple source codes such as Huffman codes or Lempel-Ziv compression are not well suited to this task: A decoder starting at an arbitrary point within the compressed sequence generally cannot determine its position with respect to the boundaries between encoded symbols, or requires information found before the starting point in order to be able to decode. In this paper, we propose the use of subsets of self-synchronising variable-length T-codes as source codes and show that local decoding is feasible and practical using subsets of T-codes with bounded synchronisation delay (BSD).