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• Collision detection by sliding against a plane in XNA

  - by Bevin

  Hello, I am attempting to engineer a collision detection algorithm for a custom Minecraft client I'm making. Basically, the entire world is made up of cubes, and the player (or camera) needs to be able to stand on and move against these cubes. The result I want is illustrated in this image: The green line is the player's movement vector. When the player is brushing up against a plane of one of the cubes, I want the vector to change to one that is perpendicular with the plane. The vector should, however, keep all of it's velocity in the plane's direction, yet lose all velocity towards the plane. I hope I've made my question clear. What is the best and most efficient way to implement a collision detection system like this? Also, will a system like this allow for a simple gravity component?

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• Standardizing camera input in OpenCV? (Contrast/Saturation/Brightness etc..)

  - by karpathy

  I am building an application using OpenCV that uses the webcam and runs some vision algorithms. I would like to make this application available on the internet after I am done, but I am concerned about the vast differences in camera settings on every computer, and I am worried that the algorithm may break if the settings are too different from mine. Is there any way, after capturing the frame, to post process it and make sure that the contrast is X, brightness is Y, and saturation is Z? I think the Camera settings themselves can not be changed from code directly using the current OpenCV Python bindings. Would anyone be able to tell me about how I could calculate some of these parameters from the image and adjust them appropriately using OpenCV?

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• Catching KeyboardInterrupt when working with PyGame

  - by Sebastian P.

  I have written a small Python application where I use PyGame for displaying some simple graphics. I have a somewhat simple PyGame loop going in the base of my application, like so: stopEvent = Event() # Just imagine that this eventually sets the stopEvent # as soon as the program is finished with its task. disp = SortDisplay(algorithm, stopEvent) def update(): """ Update loop; updates the screen every few seconds. """ while True: stopEvent.wait(options.delay) disp.update() if stopEvent.isSet(): break disp.step() t = Thread(target=update) t.start() while not stopEvent.isSet(): for event in pygame.event.get(): if event.type == pygame.QUIT: stopEvent.set() It works all fine and dandy for the normal program termination; if the PyGame window gets closed, the application closes; if the application finishes its task, the application closes. The trouble I'm having is, if I Ctrl-C in the Python console, the application throws a KeyboardInterrupt, but keeps on running. The question would therefore be: What have I done wrong in my update loop, and how do I rectify it so a KeyboardInterrupt causes the application to terminate?

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• PHP: Safe way to store decryptable passwords

  - by Jammer

  I'm making an application in PHP and there is a requirement that it must be possible to decrypt the passwords in order to avoid problems in the future with switching user database to different system. What encryption/decryption algorithm would you suggest? Is it good idea to just store the encrypted value and then compare the future authentication attempts to that value? Are the passwords still as safe as MD5/SHA1 when the private key is not available to the attacker (Hidden in USB drive for example)? I should still use salting, right? What encryption libraries should I use for PHP?

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• Whats the deal with python?

  - by gmatt

  My interests in programming lie mainly in algorithms, and lately I have seen many reputable researchers write a lot of their code in python. How easy and convenient is python for scientific computing? Does it have a library of algorithms that compares to matlab's? Is Python a scripting language or does it compile? Is it a great language for prototyping an algorithm? How long would it take me to learn enough of it to be productive provided I know C well and OO programming somewhat? Is it OO based? Sorry for the condensed format of questions, but I'm very curious and was hoping a more experienced programmer could help me out.

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• Need to get pixel averages of a vector sitting on a bitmap...

  - by user346511

  I'm currently involved in a hardware project where I am mapping triangular shaped LED to traditional bitmap images. I'd like to overlay a triangle vector onto an image and get the average pixel data within the bounds of that vector. However, I'm unfamiliar with the math needed to calculate this. Does anyone have an algorithm or a link that could send me in the right direction? I'm not even clear what this type of math is called. I've created a basic image of what I'm trying to capture here: http://imgur.com/Isjip.gif

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• When to trash hashmap contents to avoid performance degradation?

  - by Jack

  Hello, I'm woking on Java with a large (millions) hashmap that is actually built with a capacity of 10.000.000 and a load factor of .75 and it's used to cache some values since cached values become useless with time (not accessed anymore) but I can't remove useless ones while on the way I would like to entirely empty the cache when its performance starts to degrade. How can I decide when it's good to do it? For example, with 10 millions capacity and .75 should I empty it when it reaches 7.5 millions of elements? Because I tried various threshold values but I would like to have an analytic one. I've already tested the fact that emping it when it's quite full is a boost for perfomance (first 2-3 algorithm iterations after the wipe just fill it back, then it starts running faster than before the wipe) Thanks

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• UML and Documenting Simple Diagrams

  - by Jason

  As part of a rewrite of an old Java application into C#, I'm writing an actual Software Design Specification. A problem I run into is when a method is too simple to bother with a Sequence Diagram (it doesn't interact with other objects). As an example, I have a simple POJO called Item, containing the following method: public String getCategoryKey() { StringBuffer value = new StringBuffer("s-"); value.append(this.getModelID()); value.append("-c"); return value; } The purpose and the algorithm for the method needs to be documented. However, a sequence diagram is overkill. How would others document it? (I take no credit/blame for the given method, it's very old code and the author "forgot" to put their name in the Javadoc).

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• k-means clustering in R on very large, sparse matrix?

  - by movingabout

  Hello, I am trying to do some k-means clustering on a very large matrix. The matrix is approximately 500000 rows x 4000 cols yet very sparse (only a couple of "1" values per row). The whole thing does not fit into memory, so I converted it into a sparse ARFF file. But R obviously can't read the sparse ARFF file format. I also have the data as a plain CSV file. Is there any package available in R for loading such sparse matrices efficiently? I'd then use the regular k-means algorithm from the cluster package to proceed. Many thanks

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• How do I restrict the WCF service called by an ASP.NET AJAX page to only allow calls for that page?

  - by NovaJoe

  I have an AjaxControlToolkit DynamicPopulate control that is updated by calls to a WCF service. I know I can check the HttpContext in the service request to see if a user of the page (and thus, the control) is authenticated. However, I don't want anyone clever to be able to call the service directly, even if they're logged in. I want access to the service to be allowed ONLY to requests that are made from the page. Mainly, I don't want anyone to be able to programatically make a large number of calls and then reverse-engineer the algorithm that sits behind the service. Any clever ideas on how this can be done? Maybe I'm over-thinking this? Thanks in advance.

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• Architecture of a secure application that encrypts data in the database.

  - by Przemyslaw Rózycki

  I need to design an application that protects some data in a database against root attack. It means, that even if the aggressor takes control over the machine where data is stored or machine with the application server, he can't read some business critical data from the database. This is a customer's requirement. I'm going to encrypt data with some assymetric algorithm and I need some good ideas, where to store private keys, so that data is secure as well as the application usability was quite comfortable? We can assume, for simplicity, that only one key pair is used.

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• Extracting data points from a matrix and saving them in different matrixes in MATLAB

  - by Hossein

  Hi, I have a 2D Matrix consisting of some coordinates as below(example): Data(X,Y): 45.987543423,5.35000964 52.987544223,5,98765234 Also I have an array consisting of some integers =0 , for example: Cluster(M) 2,0,3,1 each of these numbers in this array corresponds with a row of my 2D Matrix above.For example, it says that row one(coordinate) in the Data Matirx belongs to the cluster 2,second row belongs to cluster 0 and so on. Now I want to have each of the datapoint of each cluster in a separate matrix, for example I want to save datapoints belonging to cluster 1 in a separate matrix, cluster 2 in a separate matrix and so on,.... I can do them manually, but the problem is this has to be an automatic extraction. which means that the number of clusters(range of the numbers in the cluster array varies in each run) so I have to have a general algorithm that does this extraction for me. Can someone help me please? thanks

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• Rearrange a python list into n lists, by column

  - by Ben R

  Trying to solve this at this hour has gotten my mind into a tail-spin: I want to rearrange a list l into a list of n lists, where n is the number of columns. e.g., l = [1,2,3,4,5,6,7,8] n = 5 ==> [[1,6][2,7][3,8][4][5]] another example: l = [1,2,3,4,5,6,7,8,9,10] n = 4 ==> [[1,5,9],[2,6,10],[3,7][4,8] Can someone please help me out with an algorithm? Feel free to use any python awesomeness that's available; I'm sure theres some cool mechanism that's a good fit for this, i just can't think of it.

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• Selecting item from set given distribution

  - by JH

  I have a set of X items such as {blower, mower, stove} and each item has a certain percentage of times it should be selected from the overall set {blower=25%,mower=25%,stove=75%} along with a certain distribution that these items should follow (blower should be selected more at the beginning of selection and stove more at the end). We are given a number of objects to be overall selected (ie 100) and a overall time to do this in (say 100 seconds). I was thinking of using a roulette wheel algorithm where the weights on the wheel are affected by the current distribution as a function of the elapsed time (and the allowed duration) so that simple functions could be used to determine the weight. Are there any common approaches to problems like this that anyone is aware of? Currently i have programmed something similar to this in java using functions such as x^2 (with correct normalization for the weights) to ensure that a good distribution occurs. Other suggestions or common practices would be welcome :-)

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• The easiest way to draw an image?

  - by Benno

  Assume you want to read an image file in a common file format from the hard drive, change the color of one pixel, and display the resulting image to the screen, in C++. Which (open-source) libraries would you recommend to accomplish the above with the least amount of code? Alternatively, which libraries would do the above in the most elegant way possible? A bit of background: I have been reading a lot of computer graphics literature recently, and there are lots of relatively easy, pixel-based algorithms which I'd like to implement. However, while the algorithm itself would usually be straightforward to implement, the necessary amount of frame-work to manipulate an image on a per-pixel basis and display the result stopped me from doing it.

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• iteration on numbers with no 2 same digits

  - by rahmivolkan

  I dont know if it is asked (I couldn't find any). I want to iterate on this kind of numbers implemented on array; int a[10]; int i = 0; for( ; i < 10; i++ ) a[i] = i+1; now the array has "1 2 3 4 5 6 7 8 9 10" and I want to get "1 2 3 4 5 6 7 8 10 9" and then "1 2 3 4 5 6 7 9 8 10" "1 2 3 4 5 6 7 9 10 8" . . . . I tried to get an algorithm but I couldn't figure it out. Is there an easy way to implement "next" iterator for this kind of problems? Thanks in advance

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• Search pattern in string using regex in obj-c

  - by manileo86

  I'm working on a string pattern match algorithm. I use NSRegularExpression for finding the matches. For ex: I've to find all words starting with '#' in a string.. Currently I use the following regex function: static NSRegularExpression *_searchTagRegularExpression; static inline NSRegularExpression * SearchTagRegularExpression() { if (!_searchTagRegularExpression) { _searchTagRegularExpression = [[NSRegularExpression alloc] initWithPattern:@"(?<!\\w)#([\\w\\._-]+)? options:NSRegularExpressionCaseInsensitive error:nil]; } return _searchTagRegularExpression; } and I use it as below: NSRegularExpression *regexp = SearchTagRegularExpression(); [regexp enumerateMatchesInString:searchString options:0 range:stringRange usingBlock:^(NSTextCheckingResult *result, NSMatchingFlags flags, BOOL *stop) { // comes here for every match with range }]; This works properly. But i just want to know if this is the best way. suggest if there's any better alternative...

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• Enumerating all hamiltonian paths from start to end vertex in grid graph

  - by Eric

  Hello, I'm trying to count the number of Hamiltonian paths from a specified start vertex that end at another specified vertex in a grid graph. Right now I have a solution that uses backtracking recursion but is incredibly slow in practice (e.g. O(n!) / 3 hours for 7x7). I've tried a couple of speedup techniques such as maintaining a list of reachable nodes, making sure the end node is still reachable, and checking for isolated nodes, but all of these slowed my solution down. I know that the problem is NP-complete, but it seems like some reasonable speedups should be achievable in the grid structure. Since I'm trying to count all the paths, I'm sure that the search must be exhaustive, but I'm having trouble figuring out how to prune out paths that aren't promising. Does anyone have some suggestions for speeding the search up? Or an alternate search algorithm?

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• R: how to make a unique set of names from a vector of strings?

  - by Mike Dewar

  Hi, I have a vector of strings. Check out my vector, it's awesome: > awesome [1] "a" "b" "c" "d" "d" "e" "f" "f" I'd like to make a new vector that is the same length as awesome but where, if necessary, the strings have been uniqueified. For example, a valid output of my desired function would be > awesome.uniqueified [1] "a" "b" "c" "d.1" "d.2" "e" "f.1" "f.2" Is there an easy, R-thonic and beautiful way to do this? I should say my list in real life (it's not called awesome) contains 25000ish mircoarray probeset identifiers. I'm always nervous when I embark on writing little generic functions (which I'm sure I could do) as I'm sure some R guru has come across this problem in the past, nailed it with some incredible algorithm that doesn't even have to store more than half an element in the vector. I'm just not sure what they might have called it. Probably not uniqueify.

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• PHP 2-way encryption: I need to store passwords that can be retrieved

  - by gAMBOOKa

  I am creating an application that will store passwords, which the user can retrieve and see. The passwords are for a hardware device, so checking against hashes are out of the question. What I need to know is: How do I encrypt and decrypt a password in PHP? What is the safest algorithm to encrypt the passwords with? Where do I store the private key? Instead of storing the private key, is it a good idea to require users to enter the private key any time they need a password decrypted? (Users of this application can be trusted) In what ways can the password be stolen and decrypted? What do I need to be aware of?

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• Serial numbers generation without user data

  - by Sphynx

  This is a followup to this question. The accepted answer is generally sufficient, but requires user to supply personal information (e.g. name) for generating the key. I'm wondering if it's possible to generate different keys based on a common seed, in a way that program would be able to validate if those keys belong to particular product, but without making this process obvious to the end user. I mean it could be a hash of product ID plus some random sequence of characters, but that would allow user to guess potential new keys. There should be some sort of algorithm difficult to guess.

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• Alternative to latex / a way to typeset good looking documents from Java to PDF

  - by drasto

  I'm working on application in Java that will maintain database of song lyrics in plain text and print out some songbooks/chordbooks(that is create PDF file from selected songs). I was planing that the Java application will generate source code for pdflatex and after compiling this source user will get PDF file. Lately I've run into a lot of problems because of latex limitation: fixed memory size (some pictures will also be drawn to PDF) - error when exceeded, no way to query end of line or and of page dynamically, it's very hard to override latex placement algorithm in a complex way,... see also some my other questions regarding latex. I come to conclusion that latex is not good option for automated PDF generation. So I need replacement. I need to be able to typeset: Chords over lyrics when the lyrics are in variable char width so I need to be able to measure text width Chord diagrams that means I'll have to draw quite complex pictures Each song on separate double page Different fonts etc. Thanks for all answers

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• Run java with highest security setting

  - by Ankiov Spetsnaz

  I'm currently writing an in house coding challenge web application and I am wondering if there is any other security precaution I would need to have other than below java option at runtime. java -Djava.security.manager=default Basically, challenges would be more of single threaded math and algorithm focused. So I would need to enable basic data structure objects and disable any file, sockets, threading or any thing that might be not so important. Based on my quick search turning on security manager as above seems to be a solution but since this is a security related I would like to be sure before it goes alive. Is there anything else I could do more?

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• how to diff / align Python lists using arbitrary matching function?

  - by James Tauber

  I'd like to align two lists in a similar way to what difflib.Differ would do except I want to be able to define a match function for comparing items, not just use string equality, and preferably a match function that can return a number between 0.0 and 1.0, not just a boolean. So, for example, say I had the two lists: L1 = [('A', 1), ('B', 3), ('C', 7)] L2 = ['A', 'b', 'C'] and I want to be able to write a match function like this: def match(item1, item2): if item1[0] == item2: return 1.0 elif item1[0].lower() == item2.lower(): return 0.5 else: return 0 and then do: d = Differ(match_func=match) d.compare(L1, L2) and have it diff using the match function. Like difflib, I'd rather the algorithm gave more intuitive Ratcliff-Obershelp type results rather than a purely minimal Levenshtein distance.

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• How to test Text Search accuracy and efficiency?

  - by DEN

  I have created a web application. One of the feature is text search which perform the boolean operator ( NOT, AND, OR) as well. However, I have no idea on calculating the search's accuracy and efficiency. For example: 1 . Probe identification system for a measurement instrument 2 . Pulse-based impedance measurement instrument 3 . Millimeter with filtered measurement mode when the user key in will return the result as below input :measurement instrument Result: 1,2 input : measurement OR instrument NOT milimeter Result: 1,2,3 so, i have no idea on what issue and what algorithm to calculate on the accuracy and efficiency of the text search.. anyone have any idea on that?

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