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• Recognizing terminals in a CFG production previously not defined as tokens.

  - by kmels

  I'm making a generator of LL(1) parsers, my input is a CoCo/R language specification. I've already got a Scanner generator for that input. Suppose I've got the following specification: COMPILER 1. CHARACTERS digit="0123456789". TOKENS number = digit{digit}. decnumber = digit{digit}"."digit{digit}. PRODUCTIONS Expression = Term{"+"Term|"-"Term}. Term = Factor{"*"Factor|"/"Factor}. Factor = ["-"](Number|"("Expression")"). Number = (number|decnumber). END 1. So, if the parser generated by this grammar receives a word "1+1", it'd be accepted i.e. a parse tree would be found. My question is, the character "+" was never defined in a token, but it appears in the non-terminal "Expression". How should my generated Scanner recognize it? It would not recognize it as a token. Is this a valid input then? Should I add this terminal in TOKENS and then consider an error routine for a Scanner for it to skip it? How does usual language specifications handle this?

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• Use DLL and have it be as trusted as my own application is

  - by Binary255

  Hi, I am using a port of GNU GetOpts, to be specific I am using the one at: http://getopt.codeplex.com I have added the DLL as a reference. But when I run my application I receive an exception: System.IO.FileLoadException was unhandled Message="Could not load file or assembly 'Gnu.Getopt, Version=0.9.1.24287, Culture=neutral, PublicKeyToken=d014b4ccdc53511a' or one of its dependencies. Failed to grant permission to execute. (Exception from HRESULT: 0x80131418)" If it is possible I would like my application to say, "trust this DLL as much as you trust me". Is there a way to do that so I won't have to fiddle with security settings? And if there is not. What is the cleanest way to get the DLL working?

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• recursion tree and binary tree cost calculation

  - by Tony

  Hi all, I've got the following recursion: T(n) = T(n/3) + T(2n/3) + O(n) The height of the tree would be log3/2 of 2. Now the recursion tree for this recurrence is not a complete binary tree. It has missing nodes lower down. This makes sense to me, however I don't understand how the following small omega notation relates to the cost of all leaves in the tree. "... the total cost of all leaves would then be Theta (n^log3/2 of 2) which, since log3/2 of 2 is a constant strictly greater then 1, is small omega(n lg n)." Can someone please help me understand how the Theta(n^log3/2 of 2) becomes small omega(n lg n)?

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• Why Shouldn't I Programmatically Submit Username/Password to Facebook/Twitter/Amazon/etc?

  - by viatropos

  I wish there was a central, fully customizable, open source, universal login system that allowed you to login and manage all of your online accounts (maybe there is?)... I just found RPXNow today after starting to build a Sinatra app to login to Google, Facebook, Twitter, Amazon, OpenID, and EventBrite, and it looks like it might save some time. But I keep wondering, not being an authentication guru, why couldn't I just have a sleek login page saying "Enter username and password, and check your login service", and then in the background either scrape the login page from say EventBrite and programmatically submit the form with Mechanize, or use an API if there was one? It would be so much cleaner and such a better user experience if they didn't have to go through popups and redirects and they could use any previously existing accounts. My question is: What are the reasons why I shouldn't do something like that? I don't know much about the serious details of cookies/sessions/security, so if you could be descriptive or point me to some helpful links that would be awesome. Thanks!

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• How can I monitor if a cookie is being sent to a domain other than the one it originated from?

  - by Brendan Salt

  I am trying to write a program that will verify that all cookies sent out from the machine are in fact going to the domain they came from. This is part of a larger security project to detect cookie based malicious attacks (such as XSS). The main snag for this project is actually detecting the out-going cookies. Can someone point me in the right direction for monitoring out-going HTTP traffic for cookie information? Other information about the project: This is a windows application written in C and numerous scripting languages. Thanks so much for the help.

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• When does an ARM7 processor increase its PC register?

  - by Summer_More_More_Tea

  Hi everyone: I'm thinking about this question for a time: when does an ARM7(with 3 pipelines) processor increase its PC register. I originally thought that after an instruction has been executed, the processor first check is there any exception in the last execution, then increase PC by 2 or 4 depending on current state. If an exception occur, ARM7 will change its running mode, store PC in the LR of current mode and begin to process current exception without modifying the PC register. But it make no sense when analyzing returning instructions. I can not work out why PC will be assigned LR when returning from an undefined-instruction-exception while LR-4 from prefetch-abort-exception, don't both of these exceptions happened at the decoding state? What's more, according to my textbook, PC will always be assigned LR-4 when returning from prefetch-abort-exception no matter what state the processor is(ARM or Thumb) before exception occurs. However, I think PC should be assigned LR-2 if the original state is Thumb, since a Thumb-instruction is 2 bytes long instead of 4 bytes which an ARM-instruction holds, and we just wanna roll-back an instruction in current state. Is there any flaws in my reasoning or something wrong with the textbook. Seems a long question. I really hope anyone can help me get the right answer. Thanks in advance.

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• Unix tree convert to recursive php array

  - by Fordnox

  I have a response from remote server like this: /home/computer/Downloads |-- /home/computer/Downloads/Apple | `-- /home/computer/Downloads/Apple/Pad |-- /home/computer/Downloads/Empty_Folder `-- /home/computer/Downloads/Subfolder |-- /home/computer/Downloads/Subfolder/Empty `-- /home/computer/Downloads/Subfolder/SubSubFolder `-- /home/computer/Downloads/Subfolder/SubSubFolder/Test this is the output for command computer@athome:$ tree -df --noreport -L 5 /home/computer/Downloads/ I would like to parse this string to recursive php array or object, something like this. I would show only part of result to get the idea. array( 'title' => '/home/computer/Downloads', 'children' => array( 0 => array( 'title' => '/home/computer/Downloads/Apple', 'children' => array( ... ) ) ); Response from server can change according to scanned directory. Can someone help me write this function. Please note that this is response from remote server and php functions can not scan any remote dir.

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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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• Is there any benefit to encrypting twice using pgp?

  - by ojblass

  I am asking from a "more secure" perspective. I can imagine a scenario with two required private keys needed for decryption scenarios that may make this an attractive model. This is to settle an argument. My vote is that it is not adding any additional security other than having to compromise two different private keys. I think that if it was any more secure than encrypting it one million times would be the best way to secure informaiton and I don't buy it. So I guess my question becomes is a two locking mechanism equivalent to another one locking mechanism with a single key? Update: Forgive me if the answer is obvious but my bread goes dead as I read books on the topic.

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• IIS reveals internal IP address in content-location field - fix

  - by saille

  Referring: http://support.microsoft.com/kb/q218180/, there is a known issue in IIS4/5/6 whereby it will reveal the internal IP of a web server in the content-location field of the HTTP header. We have IIS 6. I have tried the fix suggested, but it has not worked. The website is configured to send all requests to ASP.NET, and I am wondering if this is why the fix, which addresses IIS configuration, has not worked for us. If this is the case, how would we fix this in ASP.NET? We need to fix this issue in order to pass a security audit.

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• Detecting if a browser is using Private Browsing mode

  - by Steve

  I'm building an extranet for a company paranoid about security. They want to make sure that (among other things) their users are browsing the site with the Private Browsing mode switched on in their web browser so that no cookies or history is kept. I found only this http://jeremiahgrossman.blogspot.com/2009/03/detecting-private-browsing-mode.html and http://serverfault.com/questions/18966/force-safari-to-operate-in-private-mode-and-detect-that-state-from-a-webserver The ideal solution would use no or minimal javascript. Would attempting to set a unique cookie work for all browsers and platforms? Anyone done this before? thanks!

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• Prevent Method call without Exception using @PreAuthorize Annotation

  - by Chepech

  Hi all. We are using Spring Security 3. We have a custom implementation of PermissionEvaluator that has this complex algorithm to grant or deny access at method level on the application. To do that we add a @PreAuthorize annotation to the method we want to protect (obviously). Everything is fine on that. However the behavior that we are looking for is that if a hasPermission call is denied, the protected method call only needs to be skipped, instead we are getting a 403 error each time that happens. Any ideas how to prevent that? You can find a different explanation of the problem here; AccessDeniedException handling during methodSecurityInterception

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• What are the lesser known but cool data structures ?

  - by f3lix

  There a some data structures around that are really cool but are unknown to most programmers. Which are they? Everybody knows linked lists, binary trees, and hashes, but what about Skip lists, Bloom filters for example. I would like to know more data structures that are not so common, but are worth knowing because they rely on great ideas and enrich a programmer's tool box. PS: I am also interested on techniques like Dancing links which make interesting use of the properties of a common data structure. EDIT: Please try to include links to pages describing the data structures in more detail. Also, try to add a couple of words on why a data structures is cool (as Jonas Kölker already pointed out). Also, try to provide one data-structure per answer. This will allow the better data structures to float to the top based on their votes alone.

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• How is a new programming language actually formed/created ?

  - by hory.incpp

  Fortran-Algol-Cpl-Bcpl-C-C++-Java ..... Seems like every language is built upon an ancestor language. My question : New languages extend parent ones or there is some kind of a trick? e.g. System.out.print() in Java ; is it actually printf() in C, and so on (printf is actually .. in Cpl)? If so, doesn't this make every further language be slower and need more memory? What separates a new language from a framework?

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• Do invariant assertions fit into C# programming?

  - by P.Brian.Mackey

  In the book coders at work, the author asks "How do you use invariants in your code". Please explain what this question means. I saw class invariants on wiki, but the example is in Java and I am not skilled enough in Java to relate this example to C#. .NET 4.0 introduces invariance, covariance, and contravariance and is well explained here. Invariance is so broad. The authors usage of the word seems unit test related. For those that read the book, what does the author mean? Are we talking about making an assumption and simply testing the validity after the unit test?

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• Divide and conquer method to compute roots [SOLVED]

  - by hellsoul153

  Hello, Knowing that we can use Divide-and-Conquer algorithm to compute large exponents, for exemple 2 exp 100 = 2 exp(50) * 2 exp(50), which is quite more efficient, is this method efficient using roots ? For exemple 2 exp (1/100) = (2 exp(1/50)) exp(1/50) ? In other words, I'm wondering if (n exp(1/x)) is more efficient to (n exp(1/y)) for x < y and where x and y are integers.

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• 32 vs 64 bit...or, what exactly does 'processing information' mean

  - by jasonbogd

  I am having trouble understanding the difference between a 32 bit processor and 64 bit processor. I know that a 32 bit processor can access 32 bits at a time while a 64 bit processor can access 64 bits at a time. But what exactly does it mean to access a certain number of bits at one time?

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• Are there cross-platform tools to write XSS attacks directly to the database?

  - by Joachim Sauer

  I've recently found this blog entry on a tool that writes XSS attacks directly to the database. It looks like a terribly good way to scan an application for weaknesses in my applications. I've tried to run it on Mono, since my development platform is Linux. Unfortunately it crashes with a System.ArgumentNullException deep inside Microsoft.Practices.EnterpriseLibrary and I seem to be unable to find sufficient information about the software (it seems to be a single-shot project, with no homepage and no further development). Is anyone aware of a similar tool? Preferably it should be: cross-platform (Java, Python, .NET/Mono, even cross-platform C is ok) open source (I really like being able to audit my security tools) able to talk to a wide range of DB products (the big ones are most important: MySQL, Oracle, SQL Server, ...)

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• Is it possible to have regexp that matches all valid regular expressions?

  - by Juha Syrjälä

  Is it possible to detect if a given string is valid regular expression, using just regular expressions? Say I have some strings, that may or may not be a valid regular expressions. I'd like to have a regular expression matches those string that correspond to valid regular expression. Is that possible? Or do I have use some higher level grammar (i.e. context free language) to detect this? Does it affect if I am using some extended version of regexps like Perl regexps? If that is possible, what the regexp matching regexp is?

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• Use Tomcat with Java SecurityManager?

  - by pauline

  I'm writing a web application that is supposed to run on Tomcat on Ubuntu. On Ubuntu, Tomcat is per default configured to run with the Java SecurityManager. Besides my own web application, there will only be some well known third party web applications related to my own, like the BIRT report engine. If one of the web applications fails or gets compromised, it may take down all the others without harm, because they all belong together. What I don't wont to happen is that a compromised web app compromises the system itself, like calling rm -r / Do I need to use the java security manager to achieve this? Or is it only necessary to protect one web app from the other? I'd really like to prevent the effort to create .policy files for all the 3rd party web applications I intend to use.

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• How do I get rid of this drive mount confirmation question when booting the computer?

  - by Dave M G

  With help from this site, I was able to set up an SSHFS connection between two computers on my LAN so that one auto mounts on the other at boot time. Everything works, but there is this annoying confirmation that comes up whenever I boot: An error occurred while mounting /home/dave/Mythbuntu. Press S to skip mounting or M or Manual recovery If I press S, then booting continues, and my drive is mounted as hoped, so it seems like even though I "skipped" it, maybe it tried again and succeeded later in the boot process. I followed the instructions here to set up "if up / if down" scripts, and here is my current /etc/fstab: sshfs#[email protected]:/home/mythbuntu /home/dave/Mythbuntu fuse auto,users,exec,uid=1000,gid=1000,allow_other,reconnect,transform_symlinks,BatchMode=yes 0 0 Although the mounting is working, this step of having to press S every time I boot is obviously kind of a hassle. How do I configure my computer so I don't have to do that, and so that my other computer will still automount?

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• minimum enclosing rectangle of fixed aspect ratio

  - by Ramya Narasimha

  I have an Image with many rectangles at different positions in the image and of different sizes (both overlapping and non-overlapping). I also have a non-negative scores associated with each of these rectangles. My problem now is to find one larger rectangle *of a fixed (given) aspect ratio* that encloses as many of these rectangles as possible. I am looking for an algorithm to do this, if anyone has a solution, even a partial one it would be helpful. Please note that the positions of the rectangles in the image is fixed and cannot be moved around and there is no orientation issue as all of them are upright.

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• Calculating rotation and translation matrices between two odometry positions for monocular linear triangulation

  - by user1298891

  Recently I've been trying to implement a system to identify and triangulate the 3D position of an object in a robotic system. The general outline of the process goes as follows: Identify the object using SURF matching, from a set of "training" images to the actual live feed from the camera Move/rotate the robot a certain amount Identify the object using SURF again in this new view Now I have: a set of corresponding 2D points (same object from the two different views), two odometry locations (position + orientation), and camera intrinsics (focal length, principal point, etc.) since it's been calibrated beforehand, so I should be able to create the 2 projection matrices and triangulate using a basic linear triangulation method as in Hartley & Zissermann's book Multiple View Geometry, pg. 312. Solve the AX = 0 equation for each of the corresponding 2D points, then take the average In practice, the triangulation only works when there's almost no change in rotation; if the robot even rotates a slight bit while moving (due to e.g. wheel slippage) then the estimate is way off. This also applies for simulation. Since I can only post two hyperlinks, here's a link to a page with images from the simulation (on the map, the red square is simulated robot position and orientation, and the yellow square is estimated position of the object using linear triangulation.) So you can see that the estimate is thrown way off even by a little rotation, as in Position 2 on that page (that was 15 degrees; if I rotate it any more then the estimate is completely off the map), even in a simulated environment where a perfect calibration matrix is known. In a real environment when I actually move around with the robot, it's worse. There aren't any problems with obtaining point correspondences, nor with actually solving the AX = 0 equation once I compute the A matrix, so I figure it probably has to do with how I'm setting up the two camera projection matrices, specifically how I'm calculating the translation and rotation matrices from the position/orientation info I have relative to the world frame. How I'm doing that right now is: Rotation matrix is composed by creating a 1x3 matrix [0, (change in orientation angle), 0] and then converting that to a 3x3 one using OpenCV's Rodrigues function Translation matrix is composed by rotating the two points (start angle) degrees and then subtracting the final position from the initial position, in order to get the robot's straight and lateral movement relative to its starting orientation Which results in the first projection matrix being K [I | 0] and the second being K [R | T], with R and T calculated as described above. Is there anything I'm doing really wrong here? Or could it possibly be some other problem? Any help would be greatly appreciated.

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• What is the most easy way to get in advanced Type Theory.

  - by Bubba88

  Of course, by 'advanced' I mean here just something beyond what every programmer does know. I'm currently more-or-less comfortable with the basics and want to understand the most important, most elegant and most practically applicable achievements of modern type theory. I just do not have much time, desire and mental powers to study all the formalistics more thoroughly and that may change in the future. But there is something really attractive for me in that branch, that just forces to ask silly questions like this :) Thank you very much!

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• Algorithm for optimally choosing actions to perform a task

  - by Jules

  There are two data types: tasks and actions. An action costs a certain time to complete, and a set of tasks this actions consists of. A task has a set of actions, and our job is to choose one of them. So: class Task { Set<Action> choices; } class Action { float time; Set<Task> dependencies; } For example the primary task could be "Get a house". The possible actions for this task: "Buy a house" or "Build a house". The action "Build a house" costs 10 hours and has the dependencies "Get bricks" and "Get cement", etcetera. The total time is the sum of all the times of the actions required to perform. We want to choose actions such that the total time is minimal. Note that the dependencies can be diamond shaped. For example "Get bricks" could require "Get a car" (to transport the bricks) and "Get cement" would also require a car. Even if you do "Get bricks" and "Get cement" you only have to count the time it takes to get a car once. Note also that the dependencies can be circular. For example "Money" - "Job" - "Car" - "Money". This is no problem for us, we simply select all of "Money", "Job" and "Car". The total time is simply the sum of the time of these 3 things. Mathematical description: Let actions be the chosen actions. valid(task) = ?action ? task.choices. (action ? actions ? ?tasks ? action.dependencies. valid(task)) time = sum {action.time | action ? actions} minimize time subject to valid(primaryTask)

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