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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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• make a lazy var in scala

  - by ayvango

  Scala does not permit to create laze vars, only lazy vals. It make sense. But I've bumped on use case, where I'd like to have similar capability. I need a lazy variable holder. It may be assigned a value that should be calculated by time-consuming algorithm. But it may be later reassigned to another value and I'd like not to call first value calculation at all. Example assuming there is some magic var definition lazy var value : Int = _ val calc1 : () => Int = ... // some calculation val calc2 : () => Int = ... // other calculation value = calc1 value = calc2 val result : Int = value + 1 This piece of code should only call calc2(), not calc1 I have an idea how I can write this container with implicit conversions and and special container class. I'm curios if is there any embedded scala feature that doesn't require me write unnecessary code

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• Measure CPU performance via JS

  - by Nicholas Kyriakides

  A webapp has as a central component a relatively heavy algorithm that handles geometric operations. There are 2 solutions to make the whole thing accessible from both high-end machines and relatively slower mobile devices. I will use RPC's if i detect that the user machine is ''slow'' or else if i detect that the user machine can handle it OK, then i provide to the webapp the script to handle it client side. Now what would be a reliable way to detect the speed of the user machine? I was thinking of providing a sample script as a test when the page loads and detect the time it took to execute that. Any ideas?

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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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• Visual C++ 9 Linker file size limitation.

  - by Raindog

  It appears that the visual C++ 9 linker has a file allocation algorithm that doubles the size of the file every allocation, so you get 512mb, 1024mb, 2048mb, 4096mb. The problem is that it is using a library that cannot handle files larger 2048MB, and as such crashes with an error such as "cannot read file at is the disk full or write protected". Is there a way to bypass this limitation or otherwise replace the linker with something else that works? A bit of background, I have a code generator that generates a large number of files, ~15k cpp files, I've managed to reduce the number of files to something about 6k to get something that at least completes the linking process, I would like to be able to include all 15k without having to create multiple libs.

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• What FPGAs (Field-Programmable Gate Arrays) can one buy to experiment with at home?

  - by Joe Blow

  What the heck is an FPGA -- where can I buy one? What sort of system do you need to experiment with them? How to program them? Can you "load" if that's the right terms an FPGA using an ordinary mac or perhaps other *nix or windoze computer? Where can I buy some FPGAs today to experiment with ??! Are they expensive this only available to industry or can I buy one today? Does anyone know about this? Thanks! I have become interested in FPGAs after reading this question... Holistic Word Recognition algorithm in detail

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• Rounding a positive number to a power of another number

  - by Sagekilla

  I'm trying to round a number to the next smallest power of another number. The number I'm trying to round is always positive. I'm not particular on which direction it rounds, but I prefer downwards if possible. I would like to be able to round towards arbitrary bases, but the ones I'm most concerned with at the moment is base 2 and fractional powers of 2 like 2^(1/2), 2^(1/4), and so forth. Here's my current algorithm for base 2. The log2 I multiply by is actually the inverse of log2: double roundBaseTwo(double x) { return 1.0 / (1 << (int)((log(x) * log2)) } Any help would be appreciated!

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• Realtime processing and callbacks with Python and C++

  - by Doughy

  I need to write code to do some realtime processing that is fairly computationally complex. I would like to create some Python classes to manage all my scripting, and leave the intensive parts of the algorithm coded in C++ so that they can run as fast as possible. I would like to instantiate the objects in Python, and have the C++ algorithms chime back into the script with callbacks in python. Something like: myObject = MyObject() myObject.setCallback(myCallback) myObject.run() def myCallback(val): """Do something with the value passed back to the python script.""" pass Will this be possible? How can I run a callback in python from a loop that is running in a C++ module? Anyone have a link or a tutorial to help me do this correctly?

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• Scraping html WITHOUT uniquie identifiers using python

  - by Nicholas Law

  I would like to design an algorithm using python that scrapes thousands of pages like this one and this one, gathers all the data and inserts it into a MySQL database. The script will be run on a weekly or bi-weekly basis to update the database of any new information added to each individual page. Ideally I would like a scraper that is easy to work with for table structured data but also data that does not have unique identifiers (ie. id and classes attributes). Which scraper add-on should I use? BeautifulSoup, Scrapy or Mechanize? Are there any particular tutorials/books I should be looking at for this desired result? In the long-run I will be implementing a mobile app that works with all this data through querying the database.

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• Bitmap.Save problems

  - by user284026

  Hello, can anyone tell me if you know to be a problem with Bitmap and steganography for WM 6? I am working on a project and i have to hide a digital signature in a bitmap. The algorithm works perfect, as in untill i have the image on the memory the bitmap contains the modified bytes. But after i save the image (Bitmap.Save()) and I reopen the image, than those bytes are lost. When i say lost i mean they are the orriginal bytes from when the picture was taken. Thank you.

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• troubles with integration on matlab

  - by user648666

  I'd like some help please I really need to solve this problem. Well before anything thank you for your time... My problem: I have a matrix (826x826 double) and I want to integrate this matrix with respect to a vector of (826x1 double) I don't have the functions of any of this. Is there a command or an algorithm to take the integral of a matrix with respect to a vector? Please I really need help, I'm such a newbie at matlab. Sincerely. George

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• finding the total number of distinct shortest paths between 2 nodes in undirected weighted graph in linear time?

  - by logan

  I was wondering, that if there is a weighted graph G(V,E), and I need to find a single shortest path between any two vertices S and T in it then I could have used the Dijkstras algorithm. but I am not sure how this can be done when we need to find all the distinct shortest paths from S to T. Is it solvable on O(n) time? I had one more question like if we assume that the weights of the edges in the graph can assume values only in certain range lets say 1 <=w(e)<=2 will this effect the time complexity?

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• What's the bug in the following code ?

  - by Johannes

  #include <iostream> #include <algorithm> #include <vector> #include <boost/array.hpp> #include <boost/bind.hpp> int main() { boost::array<int, 4> a = {45, 11, 67, 23}; std::vector<int> v(a.begin(), a.end()); std::vector<int> v2; std::transform(v.begin(), v.end(), v2.begin(), boost::bind(std::multiplies<int>(), _1, 2)); std::copy(v2.begin(), v2.end(), std::ostream_iterator<int>(std::cout, " ")); } When run, this gives a creepy segmentation fault. Please tell me where I'm going wrong.

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• what mysql table structure is better

  - by Sergey

  I have very complicated search algorithm on my site, so i decided to make a table with cache or maybe all possible results. I wanna ask what structure would be better, or maybe not the one of them? (mySQL) 1) word VARCHAR, results TEXT or BLOB where i'll store ids of found objects (for example 6 chars for each id) 2) word VARCHAR, result INT, but words are not unique now i think i'll have about 200 000 rows in 1) with 1000-10000 ids each row or 200 000 000+ rows in 2) First way takes more storage memory but i think it would be much faster to find 1 unique row among 200 000, than 1000 rows among 200 mln non unique rows i think about index on word column and no sphinx. So that do YOU think? p.s. as always, sorry for my english if it's not very good.

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• function to efficiently check a change of value in a nested hashmap

  - by zcaudate

  the motivation is for checking what has changed in a deeply nest map, kind of like a reverse of update-in. This is a simple example: (def p1 {:a {:a1 :1 :a2 :2} :b {:b1 :1 :b2 :2}}) (def p2 (update-in p1 [:a :a1] (constantly :updated)) ;; => {:a {:a1 :updated :a2 :2} ;; :b {:b1 :1 :b2 :2}} (what-changed? p1 p2) ;; => {:keys [:a :a1] :value :updated) (what-changed? p2 p1) ;; => {:keys [:a :a1] :value :2) I'm hoping that because clojure maps are persistent data-structures, there may be a smart algorithm to figure this out by looking at the underlying structure as opposed to walking through the nested maps and comparing the difference.

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• Automatic people counting + twittering.

  - by c2h2

  Want to develop a system accurately counting people that go through a normal 1-2m wide door. and twitter whenever people goes in or out and tells how many people remain inside. Now, Twitter part is easy, but people counting is difficult. There is some semi existing counting solution, but they do not quite fit my needs. My idea/algorithm: Should I get some infra-red camera mounting on top of my door and constantly monitoring, and divide the camera image into several grid and calculating they entering and gone? can you give me some suggestion and starting point?

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• Parsing String to TreeNode

  - by Krusu70

  Anyone have a good algorithm how to parse a String to TreeNode in Java? Let's say we have a string s which says how to build a TreeNode. A(B,C) means that A is the name (String) of TreeNode, B is child of A (Treenode), C is sibling of A (TreeNode). So if I call function with string A(B(D,E(F,G)),C) (just a example), then I get a TreeNode equals to: level A (String: name), B - Child (TreeNode), C - Sibling (TreeNode) level B (String: name), D - Child of B (TreeNode), E - Sibling of B (TreeNode) level E (String: name), F - Child of E (TreeNode), G - Sibling of E (TreeNode) The name may not be 1 letter, it could be like real name (many letters).

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• Any chances to imitate times() Ruby method in C#?

  - by Alexander Prokofyev

  Every time I need to do something N times inside an algorithm using C# I write this code for (int i = 0; i < N; i++) { ... } Studying Ruby I have learned about method times() which can be used with the same semantics like this N.times do ... end Code fragment in C# looks more complex and we should declare useless variable i. I tried to write extension method which returns IEnumerable, but I am not satisfied with the result because again I have to declare a cycle variable i. public static class IntExtender { public static IEnumerable Times(this int times) { for (int i = 0; i < times; i++) yield return true; } } ... foreach (var i in 5.Times()) { ... } Is it possible using some new C# 3.0 language features to make N times cycle more elegant?

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• Any tips of how to handle hierarchial trees in relational model?

  - by George

  Hello all. I have a tree structure that can be n-levels deep, without restriction. That means that each node can have another n nodes. What is the best way to retrieve a tree like that without issuing thousands of queries to the database? I looked at a few other models, like flat table model, Preorder Tree Traversal Algorithm, and so. Do you guys have any tips or suggestions of how to implement a efficient tree model? My objective in the real end is to have one or two queries that would spit the whole tree for me. With enough processing i can display the tree in dot net, but that would be in client machine, so, not much of a big deal. Thanks for the attention

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• kmeans based on mapreduce by python

  - by user3616059

  I am going to write a mapper and reducer for the kmeans algorithm, I think the best course of action to do is putting the distance calculator in mapper and sending to reducer with the cluster id as key and coordinates of row as value. In reducer, updating the centroids would be performed. I am writing this by python. As you know, I have to use Hadoop streaming to transfer data between STDIN and STOUT. according to my knowledge, when we print (key + "\t"+value), it will be sent to reducer. Reducer will receive data and it calculates the new centroids but when we print new centroids, I think it does not send them to mapper to calculate new clusters and it just send it to STDOUT and as you know, kmeans is a iterative program. So, my questions is whether Hadoop streaming suffers of doing iterative programs and we should employ MRJOB for iterative programs?

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• How are managed the sequences by JPA and Hibernate?

  - by romaintaz

  Hi all, I am using Hibernate in my project, and many of my entities use a sequence for their technical keys. For example: @Entity @Table(name = "T_MYENTITY") @SequenceGenerator(name = "S_MYENTITY", sequenceName = "S_MYENTITY") public class MyEntity { @Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "S_MYENTITY") @Column(name = "MY_ENTITY_ID") private Long entityId; ... } I have two questions about the ID generated by Hibernate when a new object of this class is persisted: Why SequenceGenerator (from javax.persistence) has a default value of allocationSize set to 50 instead of 1? What are the interests of that? What is the default algorithm used by Hibernate to calculate the generated ID? It seems that Hibernate uses the value returned by the sequence hosted by my Oracle database, but then modify it before assigning it to my entity...

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• Random numbers from binomial distribution

  - by Sarah

  I need to generate quickly lots of random numbers from binomial distributions for dramatically different trial sizes (most, however, will be small). I was hoping not to have to code an algorithm by hand (see, e.g., this related discussion from November), because I'm a novice programmer and don't like reinventing wheels. It appears Boost does not supply a generator for binomially distributed variates, but TR1 and GSL do. Is there a good reason to choose one over the other, or is it better that I write something customized to my situation? I don't know if this makes sense, but I'll alternate between generating numbers from uniform distributions and binomial distributions throughout the program, and I'd like for them to share the same seed and to minimize overhead. I'd love some advice or examples for what I should be considering.

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• Sorting 1000-2000 elements with many cache misses

  - by Soylent Graham

  I have an array of 1000-2000 elements which are pointers to objects. I want to keep my array sorted and obviously I want to do this as quick as possible. They are sorted by a member and not allocated contiguously so assume a cache miss whenever I access the sort-by member. Currently I'm sorting on-demand rather than on-add, but because of the cache misses and [presumably] non-inlining of the member access the inner loop of my quick sort is slow. I'm doing tests and trying things now, (and see what the actual bottleneck is) but can anyone recommend a good alternative to speeding this up? Should I do an insert-sort instead of quicksorting on-demand, or should I try and change my model to make the elements contigious and reduce cache misses? OR, is there a sort algorithm I've not come accross which is good for data that is going to cache miss?

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• Use C function in C++ program; "multiply-defined" error

  - by eom

  I am trying to use this code for the Porter stemming algorithm in a C++ program I've already written. I followed the instructions near the end of the file for using the code as a separate module. I created a file, stem.c, that ends after the definition and has extern int stem(char * p, int i, int j) ... It worked fine in Xcode but it does not work for me on Unix with gcc 4.1.1--strange because usually I have no problem moving between the two. I get the error ld: fatal: symbol `stem(char*, int, int)' is multiply-defined: (file /var/tmp//ccrWWlnb.o type=FUNC; file /var/tmp//cc6rUXka.o type=FUNC); ld: fatal: File processing errors. No output written to cluster I've looked online and it seems like there are many things I could have wrong, but I'm not sure what combination of a header file, extern "C", etc. would work.

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• Beginner C: error: control reaches end of non-void function?

  - by Ting

  I'm trying to make a greedy algorithm and I get this error: greedy2.c:27:1: error: control reaches end of non-void function [-Werror,-Wreturn-type] } ^ 1 error generated. with this code: int man(int argc, char* argv[]) { float amount; do { printf("Input dollar amount owed:\n"); amount = GetFloat(); } while (amount <= 0); int coins = 0; while (amount >= 0.25); { amount = amount - 0.25; coins++; } printf("Number of coins to use: %d\n", coins); } What is wrong with my curly braces, and how do I fix it?

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