Christian Steinruecken


I am a researcher in machine learning / artificial intelligence at the University of Cambridge. I work in the CBL Laboratory at the Department of Engineering. I am interested in Bayesian inference, information theory, data compression, probabilistic programming, sampling methods, language design, automated model construction, and other topics.

I have worked with Prof David J.C. MacKay and with Prof Zoubin Ghahramani, and I am currently working on the Automatic Statistician Project.

My Erdős number is 3, and I am a member of King's College.

Contact

I can best be reached by email:
tcs27 [at] cam . ac . uk

Some recent work

Making compression algorithms for Unicode text

ArXiv · IEEE
The majority of online content is written in languages other than English, and is most commonly encoded in UTF-8, the world's dominant Unicode character encoding. Traditional compression algorithms typically operate on individual bytes. While this approach works well for the single-byte ASCII encoding, it works poorly for UTF-8, where characters often span multiple bytes. Previous research has focused on developing Unicode compressors from scratch, which often failed to outperform established algorithms such as bzip2. We develop a technique to modify byte-based compressors to operate directly on Unicode characters, and implement variants of LZW and PPM that apply this technique. We find that our method substantially improves compression effectiveness on a UTF-8 corpus, with our PPM variant outperforming the state-of-the-art PPMII compressor. On ASCII and binary files, our variants perform similarly to the original unmodified compressors.
Proceedings of the Data Compression Conference, DCC 2017. [ CRH, BibTeX, RIS ]

Compressing combinatorial objects

Pre-print · ArXiv · IEEE
Most of the world's digital data is currently encoded in a sequential form, and compression methods for sequences have been studied extensively. However, there are many types of non-sequential data for which good compression techniques are still largely unexplored. This paper contributes insights and concrete techniques for compressing various kinds of non-sequential data via arithmetic coding, and derives re-usable probabilistic data models from fairly generic structural assumptions. Near-optimal compression methods are described for certain types of permutations, combinations and multisets; and the conditions for optimality are made explicit for each method.
Proceedings of the Data Compression Conference, DCC 2016. [ CRH, BibTeX, RIS ]

Improving PPM with dynamic parameter updates

Pre-print · IEEE · slide transcript
This article makes several improvements to the classic PPM algorithm, resulting in a new algorithm with superior compression effectiveness on human text. The key differences of our algorithm to classic PPM are that: Ⓐ rather than the original escape mechanism, we use a generalised blending method with explicit hyper-parameters that control the way symbol counts are combined to form predictions; Ⓑ different hyper-parameters are used for classes of different contexts; and Ⓒ these hyper-parameters are updated dynamically using gradient information. The resulting algorithm (PPM-DP) compresses human text better than all currently published variants of PPM, CTW, DMC, LZ, CSE and BWT, with runtime only slightly slower than classic PPM.
Proceedings of the Data Compression Conference, DCC 2015. [ CRH, BibTeX, RIS ]

Compressing Sets and Multisets of Sequences

ArXiv · IEEE
This article describes lossless compression algorithms for multisets of sequences, taking advantage of the multiset's unordered structure. Multisets are a generalisation of sets where members are allowed to occur multiple times. A multiset can be encoded naïvely by simply storing its elements in some sequential order, but then information is wasted on the ordering. We propose a technique that transforms the multiset into an order-invariant tree representation, and derive an arithmetic code that optimally compresses the tree. Our method achieves compression even if the sequences in the multiset are individually incompressible (such as cryptographic hash sums). The algorithm is demonstrated practically by compressing collections of SHA-1 hash sums, and multisets of arbitrary, individually encodable objects.
IEEE Transactions on Information Theory, Vol 61, No. 3, March 2015. [ CRH, BibTeX, RIS ]

Lossless Data Compression

PhD thesis
This thesis makes several contributions to the field of data compression. Compression algorithms are derived for a variety of applications, employing probabilistic modelling, Bayesian inference, and arithmetic coding; and making the underlying probability distributions explicit throughout. A general compression toolbox is described, consisting of practical algorithms for compressing data distributed by various fundamental probability distributions, and mechanisms for combining these algorithms in a principled way. New mathematical theory is introduced for compressing objects with an underlying combinatorial structure, such as permutations, combinations, and multisets. For text compression, a novel unifying construction is developed for a family of context-sensitive compression algorithms, special cases of which include the PPM algorithm and the Sequence Memoizer. The work concludes with experimental results, example applications, and a brief discussion on cost-sensitive compression and adversarial sequences.
University of Cambridge, 2014. [ CRH, BibTeX, RIS ]

SAT-solving: Performance analysis of Survey Propagation and DPLL

Technical report
The Boolean Satisfiability Problem (SAT) belongs to the class of NP-complete problems, meaning that there is no known deterministic algorithm that can solve an arbitrary problem instance in less than exponential time (parametrized on the length of the input). There is great industrial demand for solving SAT, motivating the need for algorithms which perform well. I present a comparison of two approaches for solving SAT instances: DPLL (an exact algorithm from classical computer science) and Survey Propagation (a probabilistic algorithm from statistical physics). The two algorithms were compared on randomly generated 3-SAT problems with varying clause to variable ratios.
Cavendish Laboratory, University of Cambridge. [ CRH, BibTeX, RIS ]

Quantum computation on non-quantum computers

BA dissertation
This dissertation outlines the the design of a statically typed programming language for quantum computers, and describes a working implementation of it for classical computer systems.
Computer Laboratory, University of Cambridge. [ CRH, BibTeX, RIS ]

Teaching

I have taught various undergraduate courses at Cambridge University, including: Probability Theory, Machine Learning, Functional Programming, Type Theory, Computation Theory, Quantum Computation, various programming languages (ML, Prolog, Java), and Computer Science Foundations.

I also supervised several B.A. dissertations and MPhil dissertations, on topics ranging from machine learning to computation theory.



Miscellaneous


Computer Science Miscellania