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• Is using a FSM a good design for general text parsing?

  - by eSKay

  I am reading a file that is filled with hex numbers. I have to identify a particular pattern, say "aaad" (without quotes) from it. Every time I see the pattern, I generate some data to some other file. This would be a very common case in designing programs - parsing and looking for a particular pattern. I have designed it as a Finite State Machine and structured structured it in C using switch-case to change states. This was the first implementation that occured to me. DESIGN: Are there some better designs possible? IMPLEMENTATION: Do you see some problems with using a switch case as I mentioned?

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• The Definition of Regular Languages

  - by AraK

  Good Day, I have tried, and burned my brain to understand the definition of Regular Languages in Discrete Mathematics and its Applications(Rosen) without reaching the goal of understanding why the definition is like that in this book. On page(789), I am rephrasing the definition: Type 3 grammars are defined as: w1 --> w2 Where w1 is a non-terminal, and w2 is of the form: w2 = aB w2 = a Where B is a non-terminal, and a is a terminal. A special case is when w1 is the starting symbol and w2 is lambda(the empty string): w1 = S S --> lambda Two questions I couldn't find an answer for. First, Why can't w2 be of the form Ba. Second, Why lambda is only allowed for the starting symbol only. The book states that, regular languages are equivalent to Finite State Automaton, and we can easily see that a we can build FSA for both cases. I took a look at other resources, and these restrictions don't exist in these resources. Thanks,

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• Dynamically formatting a string

  - by TofuBeer

  Before I wander off and roll my own I was wondering if anyone knows of a way to do the following sort of thing... Currently I am using MessageFormat to create some strings. I now have the requirement that some of those strings will have a variable number of arguments. For example (current code): MessageFormat.format("{0} OR {1}", array[0], array[1]); Now I need something like: // s will have "1 OR 2 OR 3" String s = format(new int[] { 1, 2, 3 }); and: // s will have "1 OR 2 OR 3 OR 4" String s = format(new int[] { 1, 2, 3, 4 }); There are a couple ways I can think of creating the format string, such as having 1 String per number of arguments (there is a finite number of them so this is practical, but seems bad), or build the string dynamically (there are a lot of them so this could be slow). Any other suggestions?

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• fatal error F1002

  - by Ghazooo

  Hi , I have a Fortran 77 program code , I am using fortran power station on XP . I wrote this program because I am studying Master's degree in Mechanics , It is a very long Finite element method code. when I press the compile order it shows me the following message : "fatal error F1002: compiler is out of heap space in pass 2" I googled the problem , and found the following solutions : http://www.microsofttranslator.com/BV.aspx?ref=CSSKB&from=en&to=ar&a=http://support.microsoft.com/kb/112345/en-us?fr=1 But to be honest , I did not understand the solution !!! I think my code is good and clear . Does any one have any suggestions to solve this problem ?? please consider me as a very simple programmer. I will really appreciate that . Thanks in Advance Ghazooo

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• How do I detect a loop in this linked list?

  - by jjujuma

  Say you have a linked list structure in Java. It's made up of Nodes: class Node { Node next; // some user data } and each Node points to the next node, except for the last Node, which has null for next. Say there is a possibility that the list can contain a loop - i.e. the final Node, instead of having a null, has a reference to one of the nodes in the list which came before it. What's the best way of writing boolean hasLoop(Node first) which would return true if the given Node is the first of a list with a loop, and false otherwise? How could you write so that it takes a finite amount of space and a reasonable amount of time?

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• How do I branch an individual file in SVN?

  - by Michael Carman

  The subversion concept of branching appears to be focused on creating an [un]stable fork of the entire repository on which to do development. Is there a mechanism for creating branches of individual files? For a use case, think of a common header (*.h) file that has multiple platform-specific source (*.c) implementations. This type of branch is a permanent one. All of these branches would see ongoing development with occasional cross-branch merging. This is in sharp contrast to unstable development/stable release branches which generally have a finite lifespan. I do not want to branch the entire repository (cheap or not) as it would create an unreasonable amount of maintenance to continuously merge between the trunk and all the branches. At present I'm using ClearCase, which has a different concept of branching that makes this easy. I've been asked to consider transitioning to SVN but this paradigm difference is important. I'm much more concerned about being able to easily create alternate versions for individual files than about things like cutting a stable release branch.

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• XNA Music mixing real-time

  - by Adam L. S.

  I've created a "format" to store segments of music (prelude part, repeated part, ending part) and time information for these segments (offset, scored length) so I can mix it up in real-time as if it were one piece of music, while repeating the repeated part (optionally) indefinitely. This way, the segments can store decay where the next segment is played, while the previous one is finished. (I've created a player for this in Java, and used the Clip class.) I wanted this format, so I can provide a finite length music (for a jukebox feature), while I play infinite length music in-games. However, when I wanted to code a class in XNA that manages this "format" I've noticed, that there is no obvious way to play "Songs" simultaneously/overlapped. How can I do this/what is the best practice, not leaving the XNA framework? (I don't want to create infinite play-lists.)

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• Data Structure for a particular problem??

  - by AGeek

  Hi, Which data structure can perform insertion, deletion and searching operation in O(1) time in the worst case. We may assume the set of elements are integers drawn from a finite set 1,2,...,n, and initialization can take O(n) time. I can only think of implementing a hash table. Implementing it with Trees will not give O(1) time complexity for any of the operation. Or is it possible?? Kindly share your views on this, or any other data structure apart from these.. Thanks..

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• JavaScript data grid for millions of rows

  - by Rudiger

  I need to present millions of rows of data to users in a grid using JavaScript. I don't want the user thinking about viewing finite pieces of data; it should appear that all of the data is available. Rather than downloading all the data at once, small chunks are downloaded as the user comes to them (by scrolling, keying in row numbers, searching). The rows will not be edited through this front end, so read-only answers are acceptable. What JavaScript data grids are best for this kind of seamless paging?

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• Loss of precision - int -> float or double

  - by stan

  I have an exam question i am revising for and the question is for 4 marks "In java we can assign a int to a double or a float". Will this ever loose infromation and why? I have put that because ints are normally of fixed length or size - the precision for sotring data is finite, where storing information in floating point can be infinite, essentially we loose infromation because of this Now i am a little sketchy as to whetehr or not i am hitting the right areas here. I very sure it will loose precision but i cant exactly put my finger on why. Can i getsome help please Thanks

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• Python IPC, popen too slow

  - by UnableToLoad

  i need to run a subprocess (./myProgram) form python script and get output, actually i do this: import subprocess proc = subprocess.Popen('./generate_out', shell=False, stdout=subprocess.PIPE, ) while proc.poll() is None: out = proc.stdout.readline() data = doStuff(out) print(data) but is slow, sometimes pass a lot of time between the output produced by ./generate_out and the print(data), knowing that my doStuff() function is very fast, i think there is some buffer slowing down my pipe... Notes: ./generate_out, generates potentially an unlimited number of lines of finite length each. It seems that when too few chars are put in the pipe between the two processes nothing happens, then when enough is produced i get a huge print (non the expected behaviour!) sometimes i wait many seconds (10-20 and more) between generate_out print and python print) what can i do? maybe communicate() is faster? anithing else? Thank you a lot!

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• array, I/O file and standard deviation (c++)

  - by JohnWong

  double s_deviation(double data[],int cnt, double mean) { int i; double sum= 0; double sdeviation; double x; //x = mean(billy,a_size); for(i=0; i<cnt; i++) { sum += ((data[i]) - (mean)); } sdeviation = sqrt(sum/((double)cnt)); return sdeviation; } When I cout the result from this function, it gave me NaN. I tested the value of (mean) and data[i] using return data[i] and return mean they are valid. when i replaced mean with an actual number, the operation returned a finite number. but with mean as a variable, it produced NaH. I can't see anything wrong with my code at the moment. Again, I am sure mean, data are getting the right number based on those tests. Thank you

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• Question about the mathematical properties of hashes

  - by levand

  Take a commonly used binary hash function - for example, SHA-256. As the name implies, it outputs a 256 bit value. Let A be the set of all possible 256 bit binary values. A is extremely large, but finite. Let B be the set of all possible binary values. B is infinite. Let C be the set of values obtained by running SHA-256 on every member of B. Obviously this can't be done in practice, but I'm guessing we can still do mathematical analysis of it. My Question: By necessity, C ? A. But does C = A?

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• How do I generate a random string of up to a certain length?

  - by slavy13

  I would like to generate a random string (or a series of random strings, repetitions allowed) of length between 1 and n characters from some (finite) alphabet. Each string should be equally likely (in other words, the strings should be uniformly distributed). The uniformity requirement means that an algorithm like this doesn't work: alphabet = "abcdefghijklmnopqrstuvwxyz" len = rand(1, n) s = "" for(i = 0; i < len; ++i) s = s + alphabet[rand(0, 25)] (pseudo code, rand(a, b) returns a integer between a and b, inclusively, each integer equally likely) It doesn't work because shorter lengths are as likely as longer ones, meaning it's more likely to generate a shorter string than a longer one, so the result is not uniform.

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• Cone-Line Segment Intersection 2D

  - by nophnoph

  Hi everyone, I would like to know is there any way to determine if a cone is intersecting with a (finite)line segment. The cone actually a circle located at P(x,y) with theta degree field of view and radius r. The simple visualization can be found here (sorry i can't post the picture here) I'm trying to do it in C# but I don't have any idea how to that, so for now this is what I'm doing : Check if the line segment is intersecting with the circle If the line segment is intersecting with the circle then I check every single point in the line segment using a function I found here. But I don't think this is the best way to do it. Does anyone have an idea? For additional info, I need this function to make some kind of simple vision simulator. Thanks in advance :)

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• How many double numbers are there between 0.0 and 1.0?

  - by polygenelubricants

  This is something that's been on my mind for years, but I never took the time to ask before. Many (pseudo) random number generators generate a random number between 0.0 and 1.0. Mathematically there are infinite numbers in this range, but double is a floating point number, and therefore has a finite precision. So the questions are: Just how many double numbers are there between 0.0 and 1.0? Are there just as many numbers between 1 and 2? Between 100 and 101? Between 10^100 and 10^100+1? Note: if it makes a difference, I'm interested in Java's definition of double in particular.

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• Waiting on threads

  - by Paul Reiners

  I have a method that contains the following (Java) code: doSomeThings(); doSomeOtherThings(); doSomeThings() creates some threads, each of which will run for only a finite amount of time. The problem is that I don't want doSomeOtherThings() to be called until all the threads launched by doSomeThings() are finished. (Also doSomeThings() will call methods that may launch new threads and so on. I don't want to execute doSomeOtherThings() until all these threads have finished.) This is because doSomeThings(), among other things will set myObject to null, while doSomeOtherThings() calls myObject.myMethod() and I do not want myObject to be null at that time. Is there some standard way of doing this kind of thing (in Java)?

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• Getting Started With nServiceBus on VAN Mar 31

  - by van

  Topic: nServiceBus is mature and powerful open source framework that enables to design robust, scalable, message-based, service-oriented architectures. Latest improvements in the configuration API enables developers to quickly get started and build a working simple system that uses messaging infrastructure. The goal of this session is to give a jump start with the framework, introduce basic concepts such as message handlers, Sagas, Pub/Sub, Generic Host and also create a working demo application that uses publish/subscribe messaging. The content of the session is addressed to developers that are interested in learning how to get started using nServiceBus in order to design and build distributed systems. Bio: Bernard Kowalski is currently a Software Developer at Microdesk, one of Autodesk's leading partners in providing variety of Geospatial and Computer-Aided Design solutions. Bernard has experience developing .NET framework-based applications utilizing Windows Forms, Windows Services, ASP.NET MVC, and Web services. In a recent project, Bernard architected and implemented a distributed system based on SOA principles using an open source implementation of an Enterprise Service Bus. Bernard develops software with Agile patterns and practices using Domain Driven Design combined with TDD (Test Driven Development). He is familiar with all of the following APIs: Autodesk Vault/Product Stream API, AutoCAD ActiveX/VBA/.NET API, AutoCAD Mechanical API, Autodesk Inventor API, Autodesk MapGuide Enterprise. Prior to joining Microdesk, Bernard worked as a researcher and teacher at the University of Science and Technology in Krakow, Poland where he was awarded with a PhD in Computer Methods in Materials Science. He also participated in research projects where he developed applications for analysis of hot compression test results using advanced optimization techniques. He also developed Finite Element Method-based programs for thermal and stress analysis using C++ and FORTRAN. Bernard is a member of the Domain Driven Design and ALT.NET user groups in NYC. Virtual ALT.NET (VAN) is the online gathering place of the ALT.NET community. Through conversations, presentations, pair programming and dojos, we strive to improve, explore, and challenge the way we create software. Using net conferencing technology such as Skype and LiveMeeting, we hold regular meetings, open to anyone, usually taking the form of a presentation or an Open Space Technology-style conversation. Please see the Calendar(http://www.virtualaltnet.com/Home/Calendar) to find a VAN group that meets at a time convenient to you, and feel welcome to join a meeting. Past sessions can be found on the Recording page. To stay informed about VAN activities, you can subscribe to the Virtual ALT.NET Google Group and follow the Virtual ALT.NET blog. Times below are Central Standard Time Start Time: Wed, Mar 31, 2010 8:00 PM UTC/GMT -5 hours End Time: Wed, Mar 31, 2010 10:00 PM UTC/GMT -5 hours Attendee URL: http://www.virtualaltnet.com/van Zach Young http://www.virtualaltnet.com

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• Should I be an algorithm developer, or java web frameworks type developer?

  - by Derek

  So - as I see it, there are really two kinds of developers. Those that do frameworks, web services, pretty-making front ends, etc etc. Then there are developers that write the algorithms that solve the problem. That is, unless the problem is "display this raw data in some meaningful way." In that case, the framework/web developer guy might be doing both jobs. So my basic problem is this. I have been an algorithms kind of software developer for a few years now. I double majored in Math and Computer science, and I have a master's in systems engineering. I have never done any web-dev work, with the exception of a couple minor jobs, and some hobby level stuff. I have been job interviewing lately, and this is what happens: Job is listed as "programmer- 5 years of experience with the following: C/C++, Java,Perl, Ruby, ant, blah blah blah" Recruiter calls me, says they want me to come in for interview In the interview, find out they have some webservices development, blah blah blah When asked in the interview, talk about my experience doing algorithms, optimization, blah blah..but very willing to learn new languages, frameworks, etc Get a call back saying "we didn't think you were a fit for the job you interviewed wtih, but our algorithm team got wind of you and wants to bring you on" This has happened to me a couple times now - see a vague-ish job description looking for a "programmer" Go in, find out they are doing some sort of web-based tool, maybe with some hardcore algorithms running in the background. interview with people for the web-based tool, but get an offer from the algorithms people. So the question is - which job is the better job? I basically just want to get a wide berth of experience at this level of my career, but are algorithm developers so much in demand? Even more so than all these supposed hot in demand web developer guys? Will I be ok in the long run if I go into the niche of math based algorithm development, and just little to no, or hobby level web-dev experience? I basically just don't want to pigeon hole myself this early. My salary is already starting to get pretty high - and I can see a company later on saying "we really need a web developer, but we'll hire this 50k/year college guy, instead of this 100k/year experience algorithm guy" Cliffs notes: I have been doing algorithm development. I consider myself to be a "good programmer." I would have no problem picking up web technologies and those sorts of frameworks. During job interviews, I keep getting "we think you've got a good skillset - talk to our algorithm team" instead of wanting me to learn new skills on the job to do their web services or whhatever other new technology they are doing. Edit: Whenever I am talking about algorithm development here - I am talking about the code that produces the answer. Typically I think of more math-based algorithms: solving a financial problem, solving a finite element method, image processing, etc

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• Windows Azure Recipe: High Performance Computing

  - by Clint Edmonson

  One of the most attractive ways to use a cloud platform is for parallel processing. Commonly known as high-performance computing (HPC), this approach relies on executing code on many machines at the same time. On Windows Azure, this means running many role instances simultaneously, all working in parallel to solve some problem. Doing this requires some way to schedule applications, which means distributing their work across these instances. To allow this, Windows Azure provides the HPC Scheduler. This service can work with HPC applications built to use the industry-standard Message Passing Interface (MPI). Software that does finite element analysis, such as car crash simulations, is one example of this type of application, and there are many others. The HPC Scheduler can also be used with so-called embarrassingly parallel applications, such as Monte Carlo simulations. Whatever problem is addressed, the value this component provides is the same: It handles the complex problem of scheduling parallel computing work across many Windows Azure worker role instances. Drivers Elastic compute and storage resources Cost avoidance Solution Here’s a sketch of a solution using our Windows Azure HPC SDK: Ingredients Web Role – this hosts a HPC scheduler web portal to allow web based job submission and management. It also exposes an HTTP web service API to allow other tools (including Visual Studio) to post jobs as well. Worker Role – typically multiple worker roles are enlisted, including at least one head node that schedules jobs to be run among the remaining compute nodes. Database – stores state information about the job queue and resource configuration for the solution. Blobs, Tables, Queues, Caching (optional) – many parallel algorithms persist intermediate and/or permanent data as a result of their processing. These fast, highly reliable, parallelizable storage options are all available to all the jobs being processed. Training Here is a link to online Windows Azure training labs where you can learn more about the individual ingredients described above. (Note: The entire Windows Azure Training Kit can also be downloaded for offline use.) Windows Azure HPC Scheduler (3 labs)  The Windows Azure HPC Scheduler includes modules and features that enable you to launch and manage high-performance computing (HPC) applications and other parallel workloads within a Windows Azure service. The scheduler supports parallel computational tasks such as parametric sweeps, Message Passing Interface (MPI) processes, and service-oriented architecture (SOA) requests across your computing resources in Windows Azure. With the Windows Azure HPC Scheduler SDK, developers can create Windows Azure deployments that support scalable, compute-intensive, parallel applications. See my Windows Azure Resource Guide for more guidance on how to get started, including links web portals, training kits, samples, and blogs related to Windows Azure.

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• How to build a 4x game?

  - by Marco

  I'm trying to study how succefully implement a 4x game. Area of interest: 1) map data: how to store stellars systems (graphs?), how to generate them and so on.. 2) multiplayer: how to organize code in a non graphical server and a client to display it 3) command system: what are patters to catch user and ai decisions and handle them, adding at first "explore" and "colonize" then "combat", "research", "spy" and so on (commands can affect ships, planets, research, etc..) 4) ai system: ai can use commands to expand, upgrade planets and ship I know is a big questions, so help is appreciated :D 1) Map data Best choice is have a graph to model a galaxy. A node is a stellar system and every system have a list of planets. Ship cannot travel outside of predefined paths, like in Ascendancy: http://www.abandonia.com/files/games/221/Ascendancy_2.png Every connection between two stellar systems have a cost, in turns. Generate a galaxy is only a matter of: - dimension: number of stellar systems, - variety: randomize number of planets and types (desertic, earth, etc..), - positions of each stellar system on game space - connections: assure that exist a path between every node, so graph is "connected" (not sure if this a matematically correct term) 2) Multiplayer Game is organized in turns: player 1, player 2, ai1, ai2. Server take care of all data and clients just diplay it and collect data change. Because is a turn game, latency is not a problem :D 3) Command system I would like to design a hierarchy of commands to take care of this aspect: abstract Genericcommand (target) ExploreCommand (Ship) extends genericcommand colonizeCommand (Ship) buildcommand(planet, object) and so on. In my head all this commands are stored in a queue for every planets, ships or reasearch center or spy, and each turn a command is sent to a server to apply command and change data state 4) ai system I don't have any idea about this. Is a big topic and what I want is a simple ai. Something like "expand and fight against everyone". I think about a behaviour tree to control ai moves, so I can develop an ai that try to build ships to expand and then colonize planets, upgrade them throught science and combat enemies. Could be done with a finite state machine too ? any ideas, resources, article are welcome!

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• Clever memory usage through the years

  - by Ben Emmett

  A friend and I were recently talking about the really clever tricks people have used to get the most out of memory. I thought I’d share my favorites, and would love to hear yours too! Interleaving on drum memory Back in the ye olde days before I’d been born (we’re talking the 50s / 60s here), working memory commonly took the form of rotating magnetic drums. These would spin at a constant speed, and a fixed head would read from memory when the correct part of the drum passed it by, a bit like a primitive platter disk. Because each revolution took a few milliseconds, programmers took to manually arranging information non-sequentially on the drum, timing when an instruction or memory address would need to be accessed, then spacing information accordingly around the edge of the drum, thus reducing the access delay. Similar techniques were still used on hard disks and floppy disks into the 90s, but have become irrelevant with modern disk technologies. The Hashlife algorithm Conway’s Game of Life has attracted numerous implementations over the years, but Bill Gosper’s Hashlife algorithm is particularly impressive. Taking advantage of the repetitive nature of many cellular automata, it uses a quadtree structure to store the hashes of pieces of the overall grid. Over time there are fewer and fewer new structures which need to be evaluated, so it starts to run faster with larger grids, drastically outperforming other algorithms both in terms of speed and the size of grid which can be simulated. The actual amount of memory used is huge, but it’s used in a clever way, so makes the list . Elite’s procedural generation Ok, so this isn’t exactly a memory optimization – more a storage optimization – but it gets an honorable mention anyway. When writing Elite, David Braben and Ian Bell wanted to build a rich world which gamers could explore, but their 22K memory was something of a limitation (for comparison that’s about the size of my avatar picture at the top of this page). They procedurally generated all the characteristics of the 2048 planets in their virtual universe, including the names, which were stitched together using a lookup table of parts of names. In fact the original plans were for 2^52 planets, but it was decided that that was probably too many. Oh, and they did that all in assembly language. Other games of the time used similar techniques too – The Sentinel’s landscape generation algorithm being another example. Modern Garbage Collectors Garbage collection in managed languages like Java and .NET ensures that most of the time, developers stop needing to care about how they use and clean up memory as the garbage collector handles it automatically. Achieving this without killing performance is a near-miraculous feet of software engineering. Much like when learning chemistry, you find that every time you think you understand how the garbage collector works, it turns out to be a mere simplification; that there are yet more complexities and heuristics to help it run efficiently. Of course introducing memory problems is still possible (and there are tools like our memory profiler to help if that happens to you) but they’re much, much rarer. A cautionary note In the examples above, there were good and well understood reasons for the optimizations, but cunningly optimized code has usually had to trade away readability and maintainability to achieve its gains. Trying to optimize memory usage without being pretty confident that there’s actually a problem is doing it wrong. So what have I missed? Tell me about the ingenious (or stupid) tricks you’ve seen people use. Ben

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• Suggestions for Summer Intern Application Assignments

  - by orangepips

  As part of our application process we want prospective college interns to complete an assignment on their own - either programming or analytical - to give us something tangible to evaluate such as code or a flowchart. I have two ideas for these assignments, one programming and one analytical, I am interested in gathering feedback about these. Programming Assignment Generate an a month's calendar for a given date. The first row should indicate the days of the week (e.g. Sunday - Saturday). Each subsequent row should contain a week's days. The date supplied should be highlighted (e.g. bolded). I am thinking we'll probably proscribe the output format even more strictly - probably down to what the HTML source should look like including CSS classes. Thinking is this forces answerers to actually do some work if they merely copy a solution from the internet. Analytical Assignment Diagram or describe in prose a system for managing a set of traffic lights for traffic at a four way intersection. Each direction (i.e. North, South, East and West) has two lanes (i.e. right and left). The left lane is turn only and has green arrow light to indicate right of way. The system is able to detect if lanes have cars in them and change the lights accordingly. I would expect a flow chart or some prose describing a finite state machine that deals with each contingency. This would hopefully provide some indication of the applicant's ability to reason through a logic problem of sorts and articulate an approach for solving. Areas Seeking Feedback Is it unreasonable to ask this of applicants? If not, is it better to request before or after a phone screen? Are these questions too hard or easy for a collegiate audience? Any suggestions for alternate questions? Do these seem like good tools for analyzing people who would part of a software development life cycle? Programming language suggestions - I'm thinking Java, Python and/or C# (we're actually a ColdFusion shop).

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• What does path finding in internet routing do and how is it different from A*?

  - by alan2here

  Note: If you don't understand this question then feel free to ask clarification in the comments instead of voting down, it might be that this question needs some more work at the moment. I've been directed here from the Stack Excange chat room Root Access because my question didn't fit on Super User. In many aspects path finding algorithms like A star are very similar to internet routing. For example: A node in an A* path finding system can search for a path though edges between other nodes. A router that's part of the internet can search for a route though cables between other routers. In the case of A*, open and closed lists are kept by the system as a whole, sepratly from any individual node as well as each node being able to temporarily store a state involving several numbers. Routers on the internet seem to have remarkable properties, as I understand it: They are very performant. New nodes can be added at any time that use a free address from a finite (not tree like) address space. It's real routing, like A*, there's never any doubling back for example. Similar IP addresses don't have to be geographically nearby. The network reacts quickly to changes to the networks shape, for example if a line is down. Routers share information and it takes time for new IP's to be registered everywhere, but presumably every router doesn't have to store a list of all the addresses each of it's directions leads most directly to. I'm looking for a basic, general, high level description of the algorithms workings from the point of view of an individual router. Does anyone have one? I presume public internet routers don't use A* as the overheads would be to large, and scale to poorly. I also presume there is a single method worldwide because it seems as if must involve a lot of transferring data to update and communicate a reasonable amount of state between neighboring routers. For example, perhaps the amount of data that needs to be stored in each router scales logarithmically with the number of routers that exist worldwide, the detail and reliability of the routing is reduced over increasing distances, there is increasing backtracking involved in parts of the network that are less geographically uniform or maybe each router really does perform an A* style search, temporarily maintaining open and closed lists when a packet arrives.

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• while(true) and loop-breaking - anti-pattern?

  - by KeithS

  Consider the following code: public void doSomething(int input) { while(true) { TransformInSomeWay(input); if(ProcessingComplete(input)) break; DoSomethingElseTo(input); } } Assume that this process involves a finite but input-dependent number of steps; the loop is designed to terminate on its own as a result of the algorithm, and is not designed to run indefinitely (until cancelled by an outside event). Because the test to see if the loop should end is in the middle of a logical set of steps, the while loop itself currently doesn't check anything meaningful; the check is instead performed at the "proper" place within the conceptual algorithm. I was told that this is bad code, because it is more bug-prone due to the ending condition not being checked by the loop structure. It's more difficult to figure out how you'd exit the loop, and could invite bugs as the breaking condition might be bypassed or omitted accidentally given future changes. Now, the code could be structured as follows: public void doSomething(int input) { TransformInSomeWay(input); while(!ProcessingComplete(input)) { DoSomethingElseTo(input); TransformInSomeWay(input); } } However, this duplicates a call to a method in code, violating DRY; if TransformInSomeWay were later replaced with some other method, both calls would have to be found and changed (and the fact that there are two may be less obvious in a more complex piece of code). You could also write it like: public void doSomething(int input) { var complete = false; while(!complete) { TransformInSomeWay(input); complete = ProcessingComplete(input); if(!complete) { DoSomethingElseTo(input); } } } ... but you now have a variable whose only purpose is to shift the condition-checking to the loop structure, and also has to be checked multiple times to provide the same behavior as the original logic. For my part, I say that given the algorithm this code implements in the real world, the original code is the most readable. If you were going through it yourself, this is the way you'd think about it, and so it would be intuitive to people familiar with the algorithm. So, which is "better"? is it better to give the responsibility of condition checking to the while loop by structuring the logic around the loop? Or is it better to structure the logic in a "natural" way as indicated by requirements or a conceptual description of the algorithm, even though that may mean bypassing the loop's built-in capabilities?

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