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  • Shortest-path algorithms which use a space-time tradeoff?

    - by Chris Mounce
    I need to find shortest paths in an unweighted, undirected graph. There are algorithms which can find a shortest path between two nodes, but this can take time. There are also algorithms for computing shortest paths for all pairs of nodes in the graph, but storing such a lookup table would take lots of disk space. What I'm wondering: Is there an algorithm which offers a space-time tradeoff that's somewhere between these two extremes? In other words, is there a way to speed up a shortest-path search, while using less disk space than would be occupied by an all-pairs shortest-path table? I know there are ways to efficiently store lookup tables for this problem, and I already have a couple of ideas for speeding up shortest-path searches using precomputed data. But I don't want to reinvent the wheel if there's already some established algorithm that solves this problem.

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  • Java: confirm method Binary division and find remainder is correct?

    - by cadwag
    I am parsing binary files and have to implement a CRC algorithm to ensure the file is not corrupted. Problem is that I can't seem to get the binary math working when using larger numbers. The example I'm trying to get working: BigInteger G = new BigInteger("11001", 2); BigInteger M = new BigInteger("1110010000", 2); BigInteger R = M.remainder(G); I am expecting: R = "0101" But I am getting: R = "1100" I am assuming the remainder of 0101 is correct since it is given to me in this book I am using as a reference for the CRC algorithm (it's not based in Java), but I can't seem to get it working. I can get small binary calculations to work that I have solved by hand, but not the larger ones. I'll admit that I haven't worked the larger ones by hand yet, that is my next step, but I wanted to see if someone could point out a glaring flaw I have in my code. Can anyone confirm or deny that my methodology is correct? Thanks

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  • Dispelling the UIImage imageNamed: FUD

    - by Roger Nolan
    I see a lot of people saying imageNamed is bad but equal numbers of people saying the performance is good - especially when rendering UITableViews. See this SO question for example or this article on iPhoneDeveloperTips.com UIImage's imageNamed method used to leak so it was best avoided but has been fixed in recent releases. I'd like to understand the caching algorithm better in order to make a reasoned decision about where I can trust the system to cache my images and where I need to go the extra mile and do it myself. My current basic understanding is that it's a simple NSMutableDictionary of UIImages referenced by filename. It gets bigger and when memory runs out it gets a lot smaller. For example, does anyone know for sure that the image cache behind imageNamed does not respond to didReceiveMemoryWarning? It seems unlikely that Apple would not do this. If you have any insight into the caching algorithm, please post it here.

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  • How can it be impossible to "decrypt" an MD5 hash?

    - by Rob
    I was reading a question about MD5, and it made me remember something that boggles me. Very simple question, and I'm sorry if it's not a good one. I just can't understand how you convert something to one thing using some algorithm, and there being no way to convert it back using the algorithm in reverse. So how is this possible? Also, since multiple strings can create the same MD5 hash, due to it being less data than the input string, how would any other hashing system be any better?

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  • Bejeweled Blitz - How does it assert there is always a move?

    - by EvilTeach
    I have been playing Bejeweled Blitz for a while now. Yes, it is an addiction. In thinking about the game, I have observed that on some boards, the bottom runs dry (no moves) leaving only the top part of the board playable. Frequently that part of the board drys up, and one is left with moves in area cleared by the last move. The board never runs completely dry, so clearly the program is doing some sorts of calculation that allows it to choose what to drop to prevent it from running dry. I have noticed in this 'mode' that it is very common for the algorithm to drop jewels which causes more non-dry area to appear in the horizontal area. Perhaps less frequent is a drop which seems designed to open up the bottom part of the board again. So my question is "How would one go about designing an algorithm guarantee that there is always a move available.?"

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  • Estimate gaussian (mixture) density from a set of weighted samples

    - by Christian
    Assume I have a set of weighted samples, where each samples has a corresponding weight between 0 and 1. I'd like to estimate the parameters of a gaussian mixture distribution that is biased towards the samples with higher weight. In the usual non-weighted case gaussian mixture estimation is done via the EM algorithm. Does anyone know an implementation (any language is ok) that permits passing weights? If not, does anyone know how to modify the algorithm to account for the weights? If not, can some one give me a hint on how to incorporate the weights in the initial formula of the maximum-log-likelihood formulation of the problem? Thanks!

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  • SHA1 form .exe file in C#

    - by alibipl
    Hi! I hope that someone could help me with reading exe files in C# and create a SHA1 hash from it. I have tried to read from executable file using StreamReader and BinaryReader. Then using built-in SHA1 algorithm I tried to create a hash but without success. The algorithm results for StreamReader was "AEUj+Ppo5QdHoeboidah3P65N3s=" and for BinaryReader was "rWXzn/CoLLPBWqMCE4qcE3XmUKw=". Can anyone help me to acheive SHA1 hash from exe file? Thx. BTW Sorry for my English ;)

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  • analysis Big Oh notation psuedocode

    - by tesshu
    I'm having trouble getting my head around algorithm analysis. I seem to be okay identifying linear or squared algorithms but am totally lost with nlogn or logn algorithms, these seem to stem mainly from while loops? Heres an example I was looking at: Algorithm Calculate(A,n) Input: Array A of size n t?0 for i?0 to n-1 do if A[i] is an odd number then Q.enqueue(A[i]) else while Q is not empty do t?t+Q.dequeue() while Q is not empty do t?t+Q.dequeue() return t My best guess is the for loop is executed n times, its nested while loop q times making NQ and the final while loop also Q times resulting in O(NQ +Q) which is linear? I am probably totally wrong. Any help would be much appreciated. thanks

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  • Find the closest vector

    - by Alexey Lebedev
    Hello! Recently I wrote the algorithm to quantize an RGB image. Every pixel is represented by an (R,G,B) vector, and quantization codebook is a couple of 3-dimensional vectors. Every pixel of the image needs to be mapped to (say, "replaced by") the codebook pixel closest in terms of euclidean distance (more exactly, squared euclidean). I did it as follows: class EuclideanMetric(DistanceMetric): def __call__(self, x, y): d = x - y return sqrt(sum(d * d, -1)) class Quantizer(object): def __init__(self, codebook, distanceMetric = EuclideanMetric()): self._codebook = codebook self._distMetric = distanceMetric def quantize(self, imageArray): quantizedRaster = zeros(imageArray.shape) X = quantizedRaster.shape[0] Y = quantizedRaster.shape[1] for i in xrange(0, X): print i for j in xrange(0, Y): dist = self._distMetric(imageArray[i,j], self._codebook) code = argmin(dist) quantizedRaster[i,j] = self._codebook[code] return quantizedRaster ...and it works awfully, almost 800 seconds on my Pentium Core Duo 2.2 GHz, 4 Gigs of memory and an image of 2600*2700 pixels:( Is there a way to somewhat optimize this? Maybe the other algorithm or some Python-specific optimizations.

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  • How to prevent Artificial Intelligence from escaping into the internet?

    - by Jason
    I have an interest in artificial intelligence from a evolutionary standpoint. I want to experiment with it somewhat with a high level language like C#, however, I'm stuck on one of the most elementary problems with artificial life -- how to contain it? The best thing I can think of is a virtual machine. Where would I start to create a VM for my budding digital organisms in C#? How could I be sure they couldn't escape into the 'wild'? (I understand this unlikely, by the way.)

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  • Calculating depth and descendants of tree

    - by yuudachi
    Can you guys help me with the algorithm to do these things? I have preorder, inorder, and postorder implemented, and I am given the hint to traverse the tree with one of these orders. I am using dotty to label (or "visit") the nodes. Depth is the number of edges from the root to the bottom leaf, so everytime I move, I add +1 to the depth? Something like that? No idea about the algorithm for descendants. They are asking about the number of nodes a specific node has under itself. These are normal trees btw.

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  • How to generate and encode (for use in GA), random, strict, binary rooted trees with N leaves?

    - by Peter Simon
    First, I am an engineer, not a computer scientist, so I apologize in advance for any misuse of nomenclature and general ignorance of CS background. Here is the motivational background for my question: I am contemplating writing a genetic algorithm optimizer to aid in designing a power divider network (also called a beam forming network, or BFN for short). The BFN is intended to distribute power to each of N radiating elements in an array of antennas. The fraction of the total input power to be delivered to each radiating element has been specified. Topologically speaking, a BFN is a strictly binary, rooted tree. Each of the (N-1) interior nodes of the tree represents the input port of an unequal, binary power splitter. The N leaves of the tree are the power divider outputs. Given a particular power divider topology, one is still free to map the power divider outputs to the array inputs in an arbitrary order. There are N! such permutations of the outputs. There are several considerations in choosing the desired ordering: 1) The power ratio for each binary coupler should be within a specified range of values. 2) The ordering should be chosen to simplify the mechanical routing of the transmission lines connecting the power divider. The number of ouputs N of the BFN may range from, say, 6 to 22. I have already written a genetic algorithm optimizer that, given a particular BFN topology and desired array input power distribution, will search through the N! permutations of the BFN outputs to generate a design with compliant power ratios and good mechanical routing. I would now like to generalize my program to automatically generate and search through the space of possible BFN topologies. As I understand it, for N outputs (leaves of the binary tree), there are $C_{N-1}$ different topologies that can be constructed, where $C_N$ is the Catalan number. I would like to know how to encode an arbitrary tree having N leaves in a way that is consistent with a chromosomal description for use in a genetic algorithm. Also associated with this is the need to generate random instances for filling the initial population, and to implement crossover and mutations operators for this type of chromosome. Any suggestions will be welcome. Please minimize the amount of CS lingo in your reply, since I am not likely to be acquainted with it. Thanks in advance, Peter

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  • Can I encrypt value in C# and use that with SQL Server 2005 symmetric encryption?

    - by Robert Byrne
    To be more specific, if I create a symmetric key with a specific KEY_SOURCE and ALGORITHM (as described here), is there any way that I can set up the same key and algorithm in C# so that I can encrypt data in code, but have that data decrypted by the symmetric key in Sql Server? From the research I've done so far, it seems that the IDENTITY_VALUE for the key is also baked into the cypher text, making things even more complex. I'm thinking about just trying all the various ways I can think of, ie hashing the KEY_SOURCE using different hash algorithms for a key and trying different ways of encrypting the plain text until I get something that works. Or is that just futile? Has anyone else done this, any pointers? UPDATE Just to clarify, I want to use NHibernate on the client side, but theres a bunch of stored procedures on the database side that still perform decryption.

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  • Beagleboard: How do I send/receive data to/from the DSP?

    - by snakile
    I have a beagleboard with TMS320C64x+ DSP. I'm working on an image processing beagleboard application. Here's how it's going to work: The ARM reads an image from a file and put the image in a 2D array. The arm sends the matrix to the DSP. The DSP receives the matrix. The DSP performs the image processing algorithm on the received matrix (the algorithm code uses about 5MB of dynamically allocated memory). The DSP sends the processed image (matrix) to the ARM. The arm received the matrix. The arm saved the processed image to a file. I'v already written the code for steps 1,3,5. What is the easiest way to do steps 3+4 (sending the data)? Code examples are welcome.

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  • Mapping words to numbers with respect to definition

    - by thornate
    As part of a larger project, I need to read in text and represent each word as a number. For example, if the program reads in "Every good boy deserves fruit", then I would get a table that converts 'every' to '1742', 'good' to '977513', etc. Now, obviously I can just use a hashing algorithm to get these numbers. However, it would be more useful if words with similar meanings had numerical values close to each other, so that 'good' becomes '6827' and 'great' becomes '6835', etc. As another option, instead of a simple integer representing each number, it would be even better to have a vector made up of multiple numbers, eg (lexical_category, tense, classification, specific_word) where lexical_category is noun/verb/adjective/etc, tense is future/past/present, classification defines a wide set of general topics and specific_word is much the same as described in the previous paragraph. Does any such an algorithm exist? If not, can you give me any tips on how to get started on developing one myself? I code in C++.

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  • Add C pointer to NSMutableArray

    - by Georges Oates Larsen
    I am writing an Objective-C program that deals with low level image memory. I am using ANSI-C structs for my data storage -- Full blown objects seem overkill seeing as the data I am storing is 100% data, with no methods to operate on that data. Specifically, I am writing a customizable posterization algorithm which relies on an array of colors -- This is where things get tricky. I am storing my colors as structs of three floats, and an integer flag (related to the posterization algorithm specifically). Everyhting is going well, except for one thing... [actual question] I can't figure out how to add pointers to an NSMutableArray! I know how to add an object, but adding a pointer to a struct seems to be more difficult -- I do not want NSMutableArray dereferencing my pointer and treating the struct as some sort of strange object. I want NSMutableArray to add the pointer its self to its collection. How do I go about doing this? Thanks in advance, G

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  • How has test first development changed the way you write software?

    - by Toran Billups
    I've started to find that I can't write software without writing a test first. I ask this subjective question because I want to hear what others in the community think about the reasons I can't go back to writing production code without a test first. If you can't write a test for something you don't understand it Without a regression test you can't clean the code You are going to test it anyway, spend the time to do it right Evolutionary design is possible without fear You actually write less code yourself Fast feedback cycles save time and money Job security (less bugs makes your boss happy) It actually makes my work more enjoyable

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  • Getting pixel averages of a vector sitting atop 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 tagged this as Python, which is preferred, but I'd be happy with the general algorithm!) I've created a basic image of what I'm trying to capture here: http://imgur.com/Isjip.gif

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  • Google App Engine Database Index

    - by fjsj
    I need to store a undirected graph in a Google App Engine database. For optimization purposes, I am thinking to use database indexes. Using Google App Engine, is there any way to define the columns of a database table to create its index? I will need some optimization, since my app uses this stored undirected graph on a content-based filtering for item recommendation. Also, the recommender algorithm updates the weights of some graph's edges. If it is not possible to use database indexes, please suggest another method to reduce query time for the graph table. I believe my algorithm does more data retrieval operations from graph table than write operations. PS: I am using Python.

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  • Computer Vision Website Image Slideshow help

    - by msandbot
    Hi, I'm making an online display of the output of a computer vision algorithm. After running the algorithm I am left with a folder of about 1000 16 bit .tiff files. I need to put those on the website in a list for so that the researchers can click through and find the list. Also there needs to be an image frame with an "animated gif" feel that can be started stopped and played in reverse. Any ideas on the best way to do this? What language to use? I made a simple website in Ruby on Rails but I don't know if it has the capabilities to do what I require.

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