ISSN 1822-7732 INTERNATIONAL OLYMPIAD IN INFORMATICS INSTITUTE OF MATHEMATICS AND INFORMATICS INTERNATIONAL FEDERATION FOR INFORMATION PROCESSING OLYMPIADS IN INFORMATICS Tasks and Training Volume 2 2008 Selected papers of the International Conference joint with the XX International Olympiad in Informatics Cairo, Egypt, August 16–23, 2008 OLYMPIADS IN INFORMATICS Country Experiences and Developments ISSN 1822-7732 Editor Valentina Dagiene˙ Institute of Mathematics and Informatics, Lithuania, [email protected] Co-editors Arturo Cepeda, the Mexican Committee for Informatics, Mexico, [email protected] Richard Forster, British Informatics Olympiad, UK, [email protected] Krassimir Manev, Sofia University, Bulgaria, [email protected]fia.bg Troy Vasiga, University of Waterloo, Canada, [email protected] http://www.mii.lt/olympiads_in_informatics c Institute of Mathematics and Informatics, 2008 Olympiads in Informatics, Volume 2, 2008. 2008.06.25. 13 leidyb. apsk. l. Tirazˇas 160 egz. Uzˇsakymas Nr. 1715. Printed by Printing house “Mokslo aidai”, Gosˇtauto 12, 01108 Vilnius, Lithuania Cover design: Multimedia Center, Arab Academy for Science and Technology, Alexandria, Egypt Olympiads in Informatics, 2008, Vol. 2, 3–4 3 © 2008 Institute of Mathematics and Informatics, Vilnius Foreword OLYMPIADS IN INFORMATICS is a refereed scholarly journal that provides an international forum for presenting research and development in the specific area of teach- ing and learning informatics through competition. The journal is focused on the re- search and practice of professionals who are working in this field. OLYMPIADS IN INFORMATICS is published annually (in summer). Only original high quality research papers are accepted. All submitted papers are peer reviewed. The journal is substantially connected with the conference organized during Interna- tional Olympiads in Informatics (IOI). Papers are requested to be presented during the conference. The main goals of the conference (and journal) are: – to improve the quality of olympiads, – to improve the skills of contestants and educators, and – to discuss developing tasks and topics for olympiads. The first OLYMPIADS IN INFORMATICS conference was organized in Zagreb (2007) and put attention on organizing olympiads at the national level. The papers in this volume are being presented during the IOI in Cairo (2008). The national olympiads do not exist in isolation, and the papers in the inaugural con- ference showed how similar problems arise in different environments, and illustrated some of the solutions that both identify and distinguish the community. This volume concentrates on training and task types, and many of the ideas and experiences are drawn from the national olympiads. There are common trends here and, we hope, useful discus- sions for both the national and international level. Tasks are perennial issue for contests, their most visible aspect and, for many contes- tants, the primary reason for participation. We strive for quality, variety and suitability. We endeavour to make tasks interesting, understandable and accessible. They are used to test contestants and to train them, and perhaps even to capture the imagination of those outside the contest, be they family, sponsors or the media. If tasks seem the main purpose of an Olympiad to the contestants often, from an educator’s perspective, there is equal interest in training the contestants. This is not only a question of how we choose the best, or enable the to show their true ability. We seek to enthuse them with a passion for the subject. Some of the best students in the world participate in our olympiads, nationally and internationally; their interest in the subject is win for everyone. That the national olympiads exist in a wider community is also true of the international olympiads. We are delighted, for this second conference, to have an invited paper by Prof. M. Revilla discussing an international topic outside of the IOI. This is intended to be the start of a trend, both for invited papers and for a widening of the contributors and audience for the conference. 4 Foreword Thanks are due to everyone who has contributed to the IOI conference. In particu- larly, we would like to thank the organizers of this year’s IOI in Egypt – the Ministry of Communications and Information Technology, the Ministry of Education, and the Arab Academy for Science, Technology and Maritime Transport. Many thanks to Prof. Dr. Ekram F. Abdel Gawad, Prof. Dr. Mohamed Aly Youssef, Dr. Salah A. Elewa – without their assistance it would not have been possible to hold this event. Editorial Board Valentina Dagiene˙ Arturo Cepeda Richard Forster Krassimir Manev Troy Vasiga Olympiads in Informatics, 2008, Vol. 2, 5–15 5 © 2008 Institute of Mathematics and Informatics, Vilnius Breaking the Routine: Events to Complement Informatics Olympiad Training Benjamin A. BURTON Department of Mathematics, SMGS, RMIT University GPO Box 2476V, Melbourne, VIC 3001, Australia e-mail: [email protected] Abstract. Like many other countries, Australia holds live-in schools to train and select students for the International Olympiad in Informatics. Here we discuss some of the more interesting events at these schools that complement the usual routine of lectures, problems and exams. In particular we focus on three events: (i) codebreakers, aimed at designing tests and finding counterexamples; (ii) “B-sessions”, aimed at implementation and rigour; and (iii) team events, which use challenges in cryptography and information security to encourage teamwork in a competitive setting. Practical issues are also discussed. Key words: programming contests, training, testing, implementation, teamwork. 1. Introduction Since 1999, Australia has hosted live-in schools to train and select secondary school students for the International Olympiad in Informatics (IOI). Many countries follow a similar pattern; examples are discussed by Anido and Menderico (2007) and Forišek (2007), amongst others. The Australian training schools run for approximately ten days. A typical day consists of lectures and laboratories in the morning (or possibly a programming contest instead), more lectures and laboratories in the afternoon, and then an evening session at a white- board where students present and analyse their solutions to problems. In order to keep the students interested and to help them learn a broader range of skills, a variety of different events are slotted into the programme over the ten days. Some of these events focus on particular skills that are useful in the IOI, and others extend students beyond traditional IOI material. The aim of this paper is to describe some of these additional events that complement the usual routine of lectures, laboratories, contests and problem sessions. In particular, we focus on: • codebreakers, in which students create input files to break incorrect solutions to contest problems; • B-sessions, in which students focus on the implementation of a difficult problem that they already understand in theory; 6 B.A. Burton • team events, in which students work in groups to solve puzzles in cryptography and information security. These three events are described in Sections 2, 3 and 4 respectively. Section 5 dis- cusses practical requirements for running such events, including preparation and sup- porting software, and Section 6 briefly outlines some other events not included in the list above. For further information on the Australian training programme, including written con- tests, programming contests and joint events with other delegations, the reader is referred to (Burton, 2008). The author is grateful to Bernard Blackham for his valuable comments on this paper. 2. Codebreakers Much of the regular teaching programme in Australia focuses on correct solutions to problems. This is largely through necessity; the training schools are short and there is much material to cover. However, it is critical that students be able to identify incorrect algorithms – in a contest, one cannot afford to spend hours coding up a solution only to discover during testing that it gives the wrong answers. This is particularly important for students with weaker mathematical backgrounds, who find it difficult to prove algorithms correct and instead rely heavily on intuition, examples and testing. In order to identify incorrect algorithms, students must be willing to spend time searching for counterexamples, i.e., specific test cases for which an algorithm gives an incorrect answer. This often requires patience and creativity, since examples that are sim- ple or natural are often not rich enough to show why an algorithm is wrong. For these reasons, the codebreaker was created as a training school event in April 2005, and has been a fixture on the programme ever since. The codebreaker aims to help students develop the following skills: • finding counterexamples to algorithms, which is important during the pen-and- paper stage of designing an algorithm; • generating pathological test cases as real input files, which is important during the final stage of testing a solution. 2.1. Codebreaker Structure The codebreaker runs as a live (and sometimes noisy!) competition, with a running score- board at the front of the room and a contest website that evaluates submissions on the fly (illustrated in Fig. 1). The competition is relatively informal, and is structured as follows: • Students are given three or four problems. These are all problems that the students have seen and thought about before (typically exact