Additive Pattern Database Heuristics

We explore a method for computing admissible heuristic evaluation functions for search problems. It utilizes pattern databases, which are precomputed tables of the exact cost of solving various subproblems of an existing problem. Unlike standard pattern database heuristics, however, we partition our problems into disjoint subproblems, so that the costs of solving the different subproblems can be added together without overestimating the cost of solving the original problem. Previously, we showed how to statically partition the sliding-tile puzzles into disjoint groups of tiles to compute an admissible heuristic, using the same partition for each state and problem instance. Here we extend the method and show that it applies to other domains as well. We also present another method for additive heuristics which we call dynamically partitioned pattern databases. Here we partition the problem into disjoint subproblems for each state of the search dynamically. We discuss the pros and cons of each of these methods and apply both methods to three different problem domains: the sliding-tile puzzles, the 4-peg Towers of Hanoi problem, and finding an optimal vertex cover of a graph. We find that in some problem domains, static partitioning is most effective, while in others dynamic partitioning is a better choice. In each of these problem domains, either statically partitioned or dynamically partitioned pattern database heuristics are the best known heuristics for the problem

Hackers: a case-study of the social shaping of computing

The study is an examination of hacking, placing the act in the context of theories of technological change. The account of hacking is used to substantiate those theories that emphasise the societal shaping of technology over the notion of technological determinism. The evolution of hacking is traced, showing how it reflects changing trends in the nature of information: the most vivid of these is the conceptualisation of information known as 'cyberspace'. Instead of simply cataloguing the impact of technical changes within computing, and the effects they have had upon information, the study shows how technical change takes place in a process of negotiation and conflict between groups.The two main groups analysed are those of the Computer Underground (CU) and the Computer Security Industry (CSI). The experiences and views of both groups are recounted in what constitute internalist and externalist accounts of hacking and its significance. The internalist account is the evidence provided by hackers themselves. It addresses such issues as what motivates the act of hacking; whether there is an identifiable hacking culture; and why it is almost an exclusively male activity. The externalist account contains the perceptions of hacking held by those outside the activity.The state of computing's security measures and its vulnerability to hacking is described, and evidence is provided of the extent to which hacking gives rise to technical knowledge that could be of potential use in the fixing of security weaknesses. The division within the CSI between those broadly cooperative with hackers and those largely hostile to them is examined, and the reasons why hacking knowledge is not generally utilised are explored. Hackers are prevented from gaining legitimacy within computing in a process referred to as 'closure'. Examples include hackers being stigmatised through the use of analogies that compare their computing activities to conventional crimes such as burglary and tresspass.Stigmatisation is carried out by the CSI who use it in a process of professional boundary formation to distinguish themselves from hackers. It is also used by other authority figures such as Members of Parliament whose involvement in the process of closure takes the form of the anti-hacking legislation they have passed, an analysis of which concludes this study