# Søren Vind

## About

I am a PhD student in the Algorithms, Logic and Graphs Section in the Department of Applied Mathematics and Computer Science at the Technical University of Denmark, from where I also received my BSc and MSc.

I am associated with the Managed Video as a Service Advanced Technology Foundation project. The goal is to research and develop compressed data structures for indexing massive amounts of meta data that supports efficient queries.

My supervisors are Philip Bille and Inge Li Gørtz, and I work closely together with fellow PhD students Patrick Hagge Cording, Frederik Rye Skjoldjensen and Hjalte Wedel Vildhøj.

### Contact

DTU Compute

Building 322, Office 008

DK-2800 Kongens Lyngby

Denmark

Email: sovi@dtu.dk

## Conferences

- ISM 2014

Taichung, Taiwan

December 10-12 2014 - ICALP 2014

IT University of Copenhagen, Denmark

July 7-11 2014 - SWAT 2014

Presented*Colored Range Searching in Linear Space*

Technical University of Denmark, Denmark

July 2-4 2014 - ARCO Workshop

Malmö University, Sweden

April 25 2014 - WADS 2013

Presented*Fingerprints in Compressed Strings*

University of Western Ontario, Canada

August 12-14 2013 - CPM 2013

Bad Herrenalb, Germany

June 17-19 2013 - ARCO Workshop

Gave a talk on*String Matching with Fingerprints*

SDU Odense, Denmark

April 5 2013 - Storage & Indexing of Massive Data

The Royal Society at Chicheley Hall, Buckinghamshire, UK

February 7-8 2013 - ARCO Workshop

IT University, Denmark

November 15 2012 - CPM 2012 / SWAT 2012

University of Helsinki, Finland

July 3-6 2012

## Summer Schools

- EADS Summer School on Hashing: Theory and Applications

University of Copenhagen, Denmark

July 14-17 2013 - MADALGO Summer School on Data Structures

Aarhus University, Denmark

August 18-22 2013 - ADFOCS 2013

Saarbrücken, Germany

August 5-9 2013 - MADALGO Summer School on Algorithms for Modern Parallel and Distributed Models

Aarhus University, Denmark

August 20-23 2012 - MADALGO Summer School on Geometric Data Structures

Aarhus University, Denmark

August 16-19 2010

## Research

My research interests are within fundamental algorithms and data structure problems, stringology, compression, computational geometry and distributed computing.

### Publications

#### Output-Sensitive Pattern Extraction in Sequences

with Roberto Grossi, Giulia Menconi, Nadia Pisanti and Roberto Trani. To appear at *FSTTCS 2014*.
Abstract.

Several notions of patterns exist, and many share the common idea of strictly specifying some parts of the pattern and to

*don't care*about the remaining parts. Since the number of patterns can be exponential in the length of the sequences,

*pattern extraction*focuses on statistically relevant patterns, where any attempt to further refine or extend them causes a loss of significant information (number of occurrences).

We address the problem of extracting maximal patterns with at most $k$ don't care symbols and at least $q$ occurrences. We give a simple algorithm with the first known output-sensitive bounds for pattern extraction.

#### Indexing Motion Detection Data for Surveillance Video

with Philip Bille and Inge Li Gørtz. To appear at *ISM 2014*. The prototype source code and test data set is available on github.
Abstract.

*motion detection*queries. A query specifies an area $A$ in the video, a time interval $T$ and two thresholds: a minimum pixel value change $v$ and minimal percentage $p$ of $A$ affected by said change. The answer to a query is the list of timestamps in $T$ where at least $p\%$ of the pixels in $A$ has changed by at least $v$ values.

Our results show that by building a small index, we can support queries with a speedup of two or three orders of magnitude compared to motion detection without an index. For high resolution video, the index size is about $20\%$ of the compressed video size.

#### Colored Range Searching in Linear Space

with Roberto Grossi. Conference version at *SWAT 2014*.
Abstract.

*colored range searching*, we are given a set of $n$ colored points in $d \geq 2$ dimensions to store, and want to support orthogonal range queries taking colors into account. In the

*colored range counting*problem, a query must report the number of distinct colors found in the query range, while an answer to the

*colored range reporting*problem must report the distinct colors in the query range.

We give the first linear space data structure for both problems in two dimensions ($d=2$) with $o(n)$ worst case query time. We also give the first data structure obtaining almost-linear space usage and $o(n)$ worst case query time for points in $d > 2$ dimensions. Finally, we present the first dynamic solution to both counting and reporting with $o(n)$ query time for $d \geq 2$ and $d \geq 3$ dimensions, respectively.

#### Fingerprints in Compressed Strings

with Philip Bille, Patrick Hagge Cording, Inge Li Gørtz, Benjamin Sach, and Hjalte Wedel Vildhøj. Conference version at *WADS 2013*. Full version (draft),
Abstract.

#### String Indexing for Patterns with Wildcards

with Philip Bille, Inge Li Gørtz, and Hjalte Wedel Vildhøj. Conference version at *SWAT 2012*, Journal version in *Theory of Computing Systems*, Arxiv version,
Abstract.

- A linear space index with query time $O(m+\sigma^j \log \log n + occ)$. This significantly improves the previously best known linear space index by Lam et al. [ISAAC 2007], which requires query time $\Theta(jn)$ in the worst case.
- An index with query time $O(m+j+occ)$ using space $O(\sigma^{k^2} n \log^k \log n)$, where $k$ is the maximum number of wildcards allowed in the pattern. This is the first non-trivial bound with this query time.
- A time-space trade-off, generalizing the index by Cole et al. [STOC 2004].

#### Master's Thesis: String Indexing for Patterns with Wildcards

with Hjalte Wedel Vildhøj. Supervised by Philip Bille and Inge Li Gørtz.

Technical University of Denmark, August 8, 2011. Full version.

## Stays Abroad

- External research visit with Benjamin Sach

Gave a talk on*Algorithms in Practice*

University of Bristol, UK

August 31 - October 3 2014 - External research visit with Roberto Grossi

Gave a talk on*Fingerprints in Compressed Strings*

University of Pisa, Italy

January 7 - April 10 2014 - Invited visit hosted by Raphael Clifford

University of Bristol, UK

November 15-22 2013 - Invited visit hosted by Benjamin Sach

University of Warwick, UK

February 4-6 2013 - Erasmus Intensive Programme: DOSSEE 2011

Universidad de Alcalá, Spain

March 2011 - Exchange student: BSc/MSc

California Institute of Technology, Pasadena, USA

September - December 2009

## Teaching

- 02122 Software Technology Project

Supervisor for two groups, Spring 2013

Search Engine Project - 02282 Algorithms for Massive Data Sets

Teaching Assistant, Spring 2013

Course webpage - 02105 Algorithms and Data Structures 1

Teaching Assistant, Spring 2013 - 02102 Introductory Programming

Teaching Assistant, Spring 2010 - 02326 Algorithms and Data Structures

Teaching Assistant, Spring 2009

## Miscellaneous

- I was a co-organiser for the SWAT 2014 conference.
- I maintain the Algolog section website.
- Here is my public LinkedIn profile.