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Australian Journal ISSN 004-8917 AUSTRALIAN JOURNAL Volume 24, Number 2, May 1992 SPECIAL ISSUE ON ALGORITHMS 41 Guest Editorial 42 Algorithmic Issues in the Geometry of Planar Linkage Movement S WHITESIDES 51 Algorithms for Groups J CANNON and GEORGE HA VAS 61 Parallel Search Algorithms for Trees and Graphs PCHAUDHUR1 70 An Overview of Adaptive Sorting A MOFFA T and O PETERSSON SPECIAL FEATURES 50 Book Review Information 78 Book Reviews Published for Australian Computer Society Incorporated Registered by Australia Post, Publication No. NBG 1124 AUSTRALIAN Editor Computer R COOK, The University of Queensland ———■«™——«■—— iiiiiiiiii iiiiiiiiiiii Editorial Co-ordinator LOIS FORDHAM, The University of 77ie Australian Computer Journal is an official publication Queensland of the Australian Computer Society Incorporated. Associate Editors CONTRIBUTIONS to ensure uniformity of style. Referees may MJ LAWRENCE, University of NSW suggest major revisions to be performed by the All material for publication should be sent to: author. C ANDREWS, Mr Rob Cook, Editor Proofs and Reprints: Page proofs of Papers DN WILSON, University of Technology, Australian Computer Journal and Short Communications are sent to the Sydney Centre for Information Technology Research authors for correction prior to publication. Fifty K RAYMOND, The University of Queensland The University of Queensland, Qld 4072 copies of reprints will be supplied to authors J ZELEZNIKOW, Latrobe University without charge. Reprints of individual papers A MOFFAT, Melbourne University Prospective authors may wish to consult may be purchased from Associated Business E SONNENBERG, Melbourne University manuscript preparation guidelines published in Publications, PO Box 440, Broadway, NSW the February 1991 issue. The paragraphs below 2007. Microfilm reprints are available from Editorial Committee briefly summarise the essential details. University Microfilms International, Ann Types of Material: Four regular categories of Arbor/London. J LIONS, JM BENNETT, RA CLARKE, material are published: Papers, Short Commun­ Format: Papers, Short Communications and PC POOLE, AY MONTGOMERY, AJH ications, Letters to the Editor and Book Book Reviews should be typed in double spac­ SALE, CK YUEN Reviews. ing on A4 size paper, with 2.5 cm margins on all Generally speaking, a paper will discuss sig­ four sides. The original, plus two clear bond Office Bearers nificant new results of computing research and paper copies, should be submitted. References GW DOBER, President development, or provide a comprehensive should be cited in standard journal form, and summary of existing computing knowledge generally diagrams should be ink drawn on G TRINDER, Vice-President with the aim of broadening the outlook of Jour­ tracing paper or board with stencil or Letraset RW TISDALL, Vice-President nal readers, or describe important computing lettering. BA UNDERWOOD, Immediate Past President experience or insight. Papers and Short Communications should Short Communications are concise discus­ have a brief Abstract, Key word list and CR RG HEINRICH, National Treasurer sions of computing research or application. categories on the leading page, with authors’ AW GOLDSWORTHY, Executive Director A letter to the Editor will briefly comment affiliation as a footnote. The authors of an and Chief Executive Officer on material previously appearing in the Journal, accepted paper will be asked to supply a brief PO Box 319, Darlinghurst, NSW 2010 or discuss a computing topic of current interest. biographical note for publication with the Telephone (02)211 5855 The term ‘computing’ is interpreted broadly to paper. include descriptions of computer hardware and Copyright © 1992 Australian Computer Publication Board software, and major computer applications Society Inc. such as information systems, expert systems, JM HUGHES, Director computer networks and office automation. Production Management: Associated Business Publications Pty Ltd Refereeing: Papers and Short Communications (ACN002 328 154), Room 104,3 Smail Street, are accepted if recommended by anonymous SUBSCRIPTIONS Ultimo, NSW 2007 (PO Box 440, Broadway, referees. Letters are published at the discretion NSW 2007). Telephone (02) 212 2780, 212 The annual subscription is $40.00. All subscrip­ of the Editor, and Book Reviews are written at 3780. Fax (02)281 4594. tions to the Journal are payable in advance and the Editor’s invitation upon receipt of review should be sent {in Australian currency) to the All advertising enquiries should be referred to the copies of published books. All accepted contri­ Australian Computer Society Inc butions may be subject to minor modifications above address. PO Box 319, Darlinghurst, NSW 2010 A subscription form may be found below. AUSTRALIAN Members: The current issue of the Journal is Subscription/ supplied to personal members and to Corres­ Change of Address Form ponding Institutions. A member joining part­ way through a calendar year is entitled to JOURNAL Photocopies of this form are acceptable. receive one copy of each issue of the Journal ■r published earlier in that calendar year. To NAME ................................................................................................................... ............................. ensure receipt of all issues, members should advise the Branch Honorary Secretary con­ CURRENT ADDRESS........................................................................................................................ cerned, or the National Secretariat, promptly, of any change of address. Membership: Membership of the Society is via a Branch. Branches are autonomous in local matters, and may charge different membership subscriptions. Information may be obtained 0 Please enrol me as a subscriber for 1992.1 enclose a cheque or bank draft for $A40.00. from the following Branch Honorary Secretar­ 0 Please ricord my new address as shown above. I attach the mailing label for the last received ies. ACTGPO Box 446, Canberra 2601. NSW: Suite 602a, 149 Castlereagh Street, Sydney issue*. 2000. Qld: GPO Box 135, Aspley 4034. SA: GPO Box 2423, Adelaide 5001. WA: GPO Box F320, Perth 6001. Vic: Private Bag 4, Rich­ ♦ATTACH LABEL HERE mond 3121. Tas: PO Box 216, Sandy Bay 7005. NT: GPO Box 213, Darwin 0801. Send all correspondence regarding subscriptions to the Printed by MAPS Litho Pty Ltd, Unit 3,8 Aqua­ Circulation Manager, PO Box 319, Darlinghurst, NSW 2010 Australia. tic Drive, Frenchs Forest, NSW 2086. Guest Editorial: Algorithms The theme of this special issue of the journal is algorithms, paper, by Chaudhuri, outlines how some of the fundamen­ the “central concept of computer science”1. Research in tal methods for graph search can be paralellised. Algorithms is having a resurgence in Australia, and the If there are any classic topics in computer science, then four papers in this volume exhibit current popular themes certainly sorting is among them. The search for a good of the research. sorting algorithm began as soon as the first machines were The first paper falls within the subfield of computa­ powered up; today, a section on sorting is an integral part tional geometry, an area which has been very active over of every computer science curriculum. Many proposals for the last few years. Its popularity is driven by the increasing adaptive sorting algorithms have been made in the last 10 number of computational problems with a geometric fla­ years; these are algorithms which sort a list faster if the list vour: in Robotics, Geographical Information Systems, is already “partially sorted”. Moffat and Petersson discuss Graphics and Visualisation. Whitesides presents the com­ the competing formal notions of “partially sorted” and putational geometry approach to motion planning for lin­ give an introduction to algorithms which exploit this kages in a restricted environment. “presortedness”. There has always been a strong collaboration between mathematicians and computer scientists working in algo­ REFERENCE rithms. In the past, the main flow of ideas has been for 1 Les Goldschlager and Andrew Lister, Computer Science, A Modem mathematics to computer science. The second paper, by Introduction, Prentice Hall International Series in Computer Science, 2nd edition 1988, page xi. Cannon and Havas shows, that the flow has become bidi­ rectional. The paper surveys algorithmic methods which have been successful in group theory, one of the fundamen­ Peter Eades Alistair Moffat tal branches of algebra. Professor and Head Senior Lecturer Algorithms research has often focused on improving of Computer Science Department of Computer the resource efficiency of computation. It is clear that for The University of Science many problems, future improvements in time efficiency Newcastle The University of can only be achieved with massive parallelism. The third Melbourne i THE AUSTRALIAN COMPUTER JOURNAL, VOL. 24, No. 2, MAY 1992 41 1 INTRODUCTION Algorithmic Issues This paper considers the planning of planar placements and motions for linkages, especially chains of links. A linkage consists of a collection of rigid rods of given in the Geometry of lengths. Rods may be fastened together at their endpoints, which act like planar universal joints. When the linkage is placed in the plane, its rods are allowed to cross over one Planar Linkage another. As the linkage moves, the rods may rotate freely about their endpoints, subject only to the constraint that Movement distances between certain joints are determined by the existence of rods between them. Sue
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