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Articles 1051 - 1080 of 1102
Full-Text Articles in Computer Sciences
Planar Graphs And Sparse Graphs From Efficient Motion Planning In The Plane, L Paul Chew
Planar Graphs And Sparse Graphs From Efficient Motion Planning In The Plane, L Paul Chew
Computer Science Technical Reports
Given a source, a destination, and a number of obstacles in the plane, the Motion Planning Program is to determine the best path to move an object (a robot) from the source to the destination without colliding with any of the obstacles. For us, motion is restricted to the plane, the robot is represented by a point, and the obstacles are represented by a set of polygons with a total of n vertices among all the polygonal obstacles.
Building Voronoi Diagrams For Convex Polygons In Linear Expected Time, L Paul Chew
Building Voronoi Diagrams For Convex Polygons In Linear Expected Time, L Paul Chew
Computer Science Technical Reports
Let P be a list of points in the plane such that the points of P taken in order form the vertices of a convex polygon. We introduce a simple, linear expected-time algorithm for finding the Voronoi diagram of the points in P. Unlike previous results on expected-time algorithms for Voronoi diagrams, this method does not require any assumptions about the distribution of points. With minor modifications, this method can be used to design fast algorithms for certain problems involving unrestricted sets of points. For example, fast expected-time algorithms can be designed to delete a point from a Voronoi diagram, …
Term Reduction Using Directed Congruence Closure, L Paul Chew
Term Reduction Using Directed Congruence Closure, L Paul Chew
Computer Science Technical Reports
Many problems in computer science can be described in terms of reduction rules that tell how to transform terms. Problems that can be handled in this way include interpreting programs, implementing abstract data types, and proving certain kinds of theorems. A terms is said to have a normal form if it can be transformed, using the reduction rules, into a term to which no further reduction rules apply. In this paper, we extend the Congruence Closure Algorithm, an algorithm for finding the consequences of a finite set of equations, to develop Directed Congruence Closure, a technique for finding the normal …
Applying The Take-Grant Protection Model, Matt Bishop
Applying The Take-Grant Protection Model, Matt Bishop
Computer Science Technical Reports
The Take-Grant Protection Model has in the past been used to model multilevel security hierarchies and simple protection systems. The models are extended to include theft of rights and sharing of information, and additional security policies are examined. The analysis suggests that in some cases the basic rules of the Take-Grant Protection Model should be augmented to represent the policy properly; when appropriate, such modifications are made and their effects with respect to the policy and its Take-Grant representations are discussed.
Administrator's Guide To The Digital Signature Facility "Rover", Matt Bishop
Administrator's Guide To The Digital Signature Facility "Rover", Matt Bishop
Computer Science Technical Reports
This document describes the installation and maintenance of the rover utility, which provides a digital signature capability for internet messages.
Effects Of Replication On Data Availability, Donald B. Johnson, Larry Raab
Effects Of Replication On Data Availability, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
In this paper we examine the effects of replication on the availability of data in a large network. This analysis differs from previous analyses in that it compares the performance of a dynamic consistency control protocol not only to that of other consistency control protocols, but also to the performance of non-replication and to an upper bound on data availability. This analysis also differes in that we gather extensive simulations on large networks subject to partitions at realistically high component reliabilities. We examine the dynamic consistency protocol presented by Jajodia and Mutchler [9, 12] and by Long and Paris[18] along …
Finding Optimal Quorum Assigments For Distributed Databases, Donald B. Johnson, Larry Raab
Finding Optimal Quorum Assigments For Distributed Databases, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
Replication has been studied as a method of increasing the availability of a data item in a distributed database subject to component failures and consequent partitioning. The potential for partitioning requires that a protocol be employed which guarantees that any access to a data item is aware of the most recent update to that data item. By minimizing the number of access requests denied due to this constraint, we maximize availability. In the event that all access requests are reads, placing one copy of the data item at each site clearly leads to maximum availability. The other extreme, all access …
A Proactive Password Checker, Matt Bishop
A Proactive Password Checker, Matt Bishop
Computer Science Technical Reports
Password selection has long been a difficult issue; traditionally, passwords are either assigned by the computer or chosen by the user. When the computer does the assignments, the passwords are often hard to remember; when the User makes the selection, the passwords are often easy to guess. This paper describes a technique, and a mechanism, to allow users to select passwords which to them are easy to remember but to others would be very difficult to guess. The technique is site, user, and group configurable, and allows rapid changing of constraints impossed upon the passwords. Although experience with this technique …
A Comparison Of Consistency Control Protocols, Michael Goldweber, Donald B. Johnson, Larry Raab
A Comparison Of Consistency Control Protocols, Michael Goldweber, Donald B. Johnson, Larry Raab
Computer Science Technical Reports
In this paper we analyze three protocols for maintaining the mutual consistency of replicated objects in a distributed computing environment and compare their performance with that of an oracle protocol whose performance is optimal. We examine these protocols, two dynamic protocols and the majority consensus protocol, via simulations using two measures of availability. The analysis shows that the dynamic protocols, under realistic assumptions, do not perform significantly better than the static voting scheme. Finally we demonstrate that none of these approaches perform as well as our oracle protocol which is shown to be an upper bound on availability.
On The Worst Case Of Three Algorithms For Computing The Jacobi Symbol, Jeffrey Shallit
On The Worst Case Of Three Algorithms For Computing The Jacobi Symbol, Jeffrey Shallit
Computer Science Technical Reports
We study the worst-case behavior of three iterative algorithms- Eisenstein's algorithm, Lebesgue's algorithm, and the "ordinary" Jacobi symbol algorithm - for computing the Jacobi symbol. Each algorithm is similar in format to the Euclidean algorithm for computing gcd (u,v).
Asymptotically Fast Algorithms For Spherical And Related Transforms, James R. Driscoll, Dennis M. Healy
Asymptotically Fast Algorithms For Spherical And Related Transforms, James R. Driscoll, Dennis M. Healy
Computer Science Technical Reports
This paper considers the problem of computing the harmonic expansion of functions defined on the sphere. We begin by proving convolution theorems that relate the convolution of two functions on the sphere to a "multiplication" in the sprectral domain, as well as the multiplication of two functions on the sphere to a "convolution" in the spectral domain. These convolution theorems are then used to develop a sampling theorem on the sphere.
Evaluation Of Concurrent Pools, David Kotz, Carla Ellis
Evaluation Of Concurrent Pools, David Kotz, Carla Ellis
Dartmouth Scholarship
The assignment of resources or tasks to processors in a distributed or parallel system needs to be done in a fashion that helps to balance the load and scales to large configurations. In an architectural model that distinguishes between local and remote data access, it is important to base these allocation functions on a mechanism that preserves locality and avoids high-latency remote references. This paper explores performance considerations affecting the design of such a mechanism, the Concurrent Pools data structure. We evaluate the effectiveness of three different implementations of concurrent pools under a variety of stressful workloads. Our experiments expose …
Prefetching In File Systems For Mimd Multiprocessors, Carla Schlatter Ellis, David Kotz
Prefetching In File Systems For Mimd Multiprocessors, Carla Schlatter Ellis, David Kotz
Dartmouth Scholarship
The problem of providing file I/O to parallel programs has been largely neglected in the development of multiprocessor systems. There are two essential elements of any file system design intended for a highly parallel environment: parallel I/O and effective caching schemes. This paper concentrates on the second aspect of file system design and specifically, on the question of whether prefetching blocks of the file into the block cache can effectively reduce overall execution time of a parallel computation. MIMD multiprocessor architectures have a profound impact on the nature of the workloads they support. In particular, it is the collective behavior …
Prefetching In File Systems For Mimd Multiprocessors, Carla Schlatter Ellis, David Kotz
Prefetching In File Systems For Mimd Multiprocessors, Carla Schlatter Ellis, David Kotz
Dartmouth Scholarship
The problem of providing file I/O to parallel programs has been largely neglected in the development of multiprocessor systems. There are two essential elements of any file system design intended for a highly parallel environment: parallel I/O and effective caching schemes. This paper concentrates on the second aspect of file system design and specifically, on the question of whether prefetching blocks of the file into the block cache can effectively reduce overall execution time of a parallel computation, even under favorable assumptions. \par Experiments have been conducted with an interleaved file system testbed on the Butterfly Plus multiprocessor. Results of …
An Application Of A Fast Data Encryption Standard Implementation, Matt Bishop
An Application Of A Fast Data Encryption Standard Implementation, Matt Bishop
Computer Science Technical Reports
The Data Encryption Standard is used as the basis for the UNIX password encryption scheme. Some of the security of that scheme depends on the speed of the implementation. This paper presents a mathematical formulation of a fast implementation of the DES in software, discusses how the mathematics can be translated into code, and then analyzes the UNIX password scheme to show how these results can be used to implement it. Experimental results are provided for several computers to show that the given method speeds up the computation of a password by roughly 20 times (depending on the specific computer).
Theft Of Information In The Take-Grant Protection Model, Matt Bishop
Theft Of Information In The Take-Grant Protection Model, Matt Bishop
Computer Science Technical Reports
(Revised 5/90). Questions of information flow are in many ways more important than questions of access control, because the goal of many security policies is to thwart the unauthorized release of information, not merely the illicit obtaining of access rights to that information. The Take-Grant Protection Model is an excellent theoretical tool for examining such issues because conditions necessary and sufficienct for information to flow between tow objects, and for rights to object to be obtained or stolen, are known. In this paper we extend these results by examinig the question of information flow from an object the owner of …
The Sharing Of Rights And Information In A Capability-Based Protection System, Matt Bishop
The Sharing Of Rights And Information In A Capability-Based Protection System, Matt Bishop
Computer Science Technical Reports
The paper examines the question of sharing of rights and information in the Take-Grant Protection Model by concentrating on the similarities between the two; in order to do this, we state and prove new theorems for each that specifically show the similarities. The proof for one of the original theorems is also provided. These statements of necessary and sufficient conditions are contrasted to illustrate the proposition that transferring rights and transferring information are fundamentally the same, as one would expect in a capability-based system. We then discuss directions for future research in light of these results.
Evaluation Of Concurrent Pools, David Kotz, Carla Ellis
Evaluation Of Concurrent Pools, David Kotz, Carla Ellis
Dartmouth Scholarship
In a parallel environment, requests for allocation and deallocation of resources or assignment of tasks should be served in a fashion that helps to balance the load and minimize the total parallel runtime. It is important to perform this allocation in a manner that preserves locality and by avoiding remote references (and hence interference with other processes). Concurrent pools, as described by Manber, provide an appropriate data structure for addressing these goals. This paper eveluates the effectiveness of the pool structure under a variety of stressful workloads. It was found that the simpler algorithm than that described by Manber may …
Matching Multiple Patterns From Right To Left, Samuel W. Bent, M A. Sridhar
Matching Multiple Patterns From Right To Left, Samuel W. Bent, M A. Sridhar
Computer Science Technical Reports
We address the problem of matching multiple pattern strings against a text string. Just as the Aho-Corasick algorithm generalizes the Knuth-Morris-Pratt single-pattern algorithm to handle multiple patterns, we exhibit two generalizations of the Boyer-Moore algorithm to handle multiple patterns. In order to obtain worst-case time bounds better than quadratic, our algorithms remember some of the previous history of the matching.
Learning Object-Centered Representations, Peter Anthony Sandon
Learning Object-Centered Representations, Peter Anthony Sandon
Computer Science Technical Reports
When we look at a familiar object from a novel viewpoint, we are usually able to recognize it. In this thesis, we address the problem of learning to recognize objects under transformations associated with viewpoint. Our vision model combines a hierarchical representation of shape features with an explicit representation of the transformation. Shape features are represented in a layered pyramid-shaped subnetwork, while the transformation is explicitly represented in an auxiliary subnetwork. The two connectionist networks are conjunctively combined to allow object- centered shape features to be computed in the upper layers of the network. A simulation of a 2-D translation …
There Is A Planar Graph Almost As Good As The Complete Graph, L Paul Chew
There Is A Planar Graph Almost As Good As The Complete Graph, L Paul Chew
Dartmouth Scholarship
Given a set S of points in the plane, there is a triangulation of S such that a path found within this triangulation has length bounded by a constant times the straight-line distance between the endpoints of the path. Specifically, for any two points a and b of S there is a path along edges of the triangulation with length less that sqrt(10) times [ab], where [ab] is the straight-line Euclidean distance between a and b. The triangulation that has this property is the L1 metric Delauney triangulation for the set S. This result can be applied to motion planning …
Making Mail Friendlier: Adding Macintosh Features And Multimedia Documents In Unix Mail, John R. Meier
Making Mail Friendlier: Adding Macintosh Features And Multimedia Documents In Unix Mail, John R. Meier
Dartmouth College Undergraduate Theses
This paper describes a Macintosh application which acts as a front-end to Unix mail. Features of the Macintosh interface such as icons, menus, and windows replace the command driven interface. Complicated editing commands are replaced with mouse selection and cut, copy, and paste. Message can be composed of text, pictures, and any Macintosh file, because they are encoded into plain text, sent through the mail system,and then unencoded by the receiving end. The designs of the mail server and communications interface are such that mail servers and communications other than Unix mail and a serial line may be easily implemented.
Functions Returning Values Of Dynamic Size, Mark Sherman, Andy Hisgen, Jonathan Rosenberg, David Alex Lamb
Functions Returning Values Of Dynamic Size, Mark Sherman, Andy Hisgen, Jonathan Rosenberg, David Alex Lamb
Computer Science Technical Reports
Modern programming languages, such as Ada (Ichbiah 80), permit the definition of functions that return values whose size can not be determined until the function returns. This paper discusses five implementation techniques that can be used to implement this capability. Comparisons of the techniques are provided and guidelines for selecting a particular technique for a compiler are given.
Computing The Largest Empty Rectangle, B. Chazelle, R. L. Drysdale, D. T. Lee
Computing The Largest Empty Rectangle, B. Chazelle, R. L. Drysdale, D. T. Lee
Dartmouth Scholarship
We consider the following problem: Given a rectangle containing N points, find the largest area subrectangle with sides parallel to those of the original rectangle which contains none of the given points. If the rectangle is a piece of fabric or sheet metal and the points are flaws, this problem is finding the largest-area rectangular piece which can be salvaged. A previously known result [13] takes $O(N^2 )$ worst-case and $O(N\log ^2 N)$ expected time. This paper presents an $O(N\log ^3 N)$ time, $O(N\log N)$ space algorithm to solve this problem. It uses a divide-and-conquer approach similar to the ones …
Maze Game For The Macintosh, Mark Sherman
Maze Game For The Macintosh, Mark Sherman
Computer Science Technical Reports
Maze is a multiperson game using a broadcast protocol on the Macintosh. This disk contains the sources (in TML Pascal) for a Maze game.
Instructions For Using Logic, John W. Scott
Instructions For Using Logic, John W. Scott
Computer Science Technical Reports
This document provides instructions on the use of "Logic", a digital logic simulator for the Macintosh written for Dartmouth students in the Machine Architecture and Digital Electronics courses.
Uses Of Generics In Ada, Mark Sherman
Uses Of Generics In Ada, Mark Sherman
Computer Science Technical Reports
This paper discusses how Ada generic procedures and packages can be used in novel ways to provide general initialization of records, expressions using discriminants, record field hiding and explicit control of parameter binding.
Algorithms For Iterative Array Multiplication, Shinji Nakamura
Algorithms For Iterative Array Multiplication, Shinji Nakamura
Computer Science Technical Reports
Algorithms for the parallel multiplication of two n bit binary numbers by an iterative array of logic cells are discussed. The regular interconnection structures of the multiplier array cell elements, which are ideal for VLSI implementation, are described. The speed and hardware complexity of two new iterative array algorithms, both of which require n cell delays for one n by n bit multiplication, are compared with a straight-forward iterative array algorithm having a 2n cell delay and its higher radix version having an n cell delay.
Qdt (Quickdraw Terminal) Disks, Mark Sherman
Qdt (Quickdraw Terminal) Disks, Mark Sherman
Computer Science Technical Reports
This disk set consists of one disk containing a running version of QDT over a serial line, one disk containing a running version of QDT over Apple Talk (using KSP), and one disk containing the Lisa Pascal sources for creating QDT (either version).
Music Editor, Ed Grosz
Music Editor, Ed Grosz
Computer Science Technical Reports
This disk contains a simple music editor that creates resources that can be used directly by the Macintosh Sound Driver. The MFS formatted disk contains both the runnable program and the Lisa Object Pascal/MacApp Sources.