Efficient and Compact Representations of Some Non-canonical Prefix-Free Codes

Abstract

For many kinds of prefix-free codes there are efficient and compact alternatives to the traditional tree-based representation. Since these put the codes into canonical form, however, they can only be used when we can choose the order in which codewords are assigned to characters. In this paper we first show how, given a probability distribution over an alphabet of $\sigma$ characters, we can store a nearly optimal alphabetic prefix-free code in $o (\sigma )$ bits such that we can encode and decode any character in constant time. We then consider a kind of code introduced recently to reduce the space usage of wavelet matrices (Claude, Navarro, and Ordóñez, Information Systems, 2015). They showed how to build an optimal prefix-free code such that the codewords’ lengths are non-decreasing when they are arranged such that their reverses are in lexicographic order. We show how to store such a code in $\mathcal {O}\!\left( {\sigma \log L + 2^{\epsilon L}}\right)$ bits, where L is the maximum codeword length and $\epsilon$ is any positive constant, such that we can encode and decode any character in constant time under reasonable assumptions. Otherwise, we can always encode and decode a codeword of $\ell$ bits in time $\mathcal {O}\!\left( {\ell }\right)$ using $\mathcal {O}\!\left( {\sigma \log L}\right)$ bits of space.

Funded in part by European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 690941 (project BIRDS). The first author was supported by: MINECO (PGE and FEDER) grants TIN2013-47090-C3-3-P and TIN2015-69951-R; MINECO and CDTI grant ITC-20151305; ICT COST Action IC1302; and Xunta de Galicia (co-founded with FEDER) grant GRC2013/053. The second author was supported by Academy of Finland grants 268324 and 250345 (CoECGR). The fourth author was supported by Millennium Nucleus Information and Coordination in Networks ICM/FIC P10-024F, Chile.