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• Is there a name for the technique of using base-2 numbers to encode a list of unique options?

  - by Lunatik

  Apologies for the rather vague nature of this question, I've never been taught programming and Google is rather useless to a self-help guy like me in this case as the key words are pretty ambiguous. I am writing a couple of functions that encode and decode a list of options into a Long so they can easily be passed around the application, you know this kind of thing: 1 - Apple 2 - Orange 4 - Banana 8 - Plum etc. In this case the number 11 would represent Apple, Orange & Plum. I've got it working but I see this used all the time so assume there is a common name for the technique, and no doubt all sorts of best practice and clever algorithms that are at the moment just out of my reach.

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• Algorithm for non-contiguous netmask match

  - by Gianluca

  Hi, I have to write a really really fast algorithm to match an IP address to a list of groups, where each group is defined using a notation like 192.168.0.0/252.255.0.255. As you can see, the bitmask can contain zeros even in the middle, so the traditional "longest prefix match" algorithms won't work. If an IP matches two groups, it will be assigned to the group containing most 1's in the netmask. I'm not working with many entries (let's say < 1000) and I don't want to use a data structure requiring a large memory footprint (let's say 1-2 MB), but it really has to be fast (of course I can't afford a linear search). Do you have any suggestion? Thanks guys. UPDATE: I found something quite interesting at http://www.cse.usf.edu/~ligatti/papers/grouper-conf.pdf, but it's still too memory-hungry for my utopic use case

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• Fastest way of converting a quad to a triangle strip?

  - by Tina Brooks

  What is the fastest way of converting a quadrilateral (made up of foyr x,y points) to a triangle strip? I'm well aware of the general triangulation algorithms that exist, but I need a short, well optimized algorithm that deals with quadrilaterals only. My current algorithm does this, which works for most quads but still gets the points mixed up for some: #define fp(f) bounds.p##f /* Sort four points in ascending order by their Y values */ point_sort4_y(&fp(1), &fp(2), &fp(3), &fp(4)); /* Bottom two */ if (fminf(-fp(1).x, -fp(2).x) == -fp(2).x) { out_quad.p1 = fp(2); out_quad.p2 = fp(1); } else { out_quad.p1 = fp(1); out_quad.p2 = fp(2); } /* Top two */ if (fminf(-fp(3).x, -fp(4).x) == -fp(3).x) { out_quad.p3 = fp(3); out_quad.p4 = fp(4); } else { out_quad.p3 = fp(4); out_quad.p4 = fp(3); }

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• best method of turning millions of x,y,z positions of particles into visualisation

  - by Griff

  I'm interested in different algorithms people use to visualise millions of particles in a box. I know you can use Cloud-In-Cell, adaptive mesh, Kernel smoothing, nearest grid point methods etc to reduce the load in memory but there is very little documentation on how to do these things online. i.e. I have array with: x,y,z 1,2,3 4,5,6 6,7,8 xi,yi,zi for i = 100 million for example. I don't want a package like Mayavi/Paraview to do it, I want to code this myself then load the decomposed matrix into Mayavi (rather than on-the-fly rendering) My poor 8Gb Macbook explodes if I try and use the particle positions. Any tutorials would be appreciated.

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• What is it about Fibonacci numbers?

  - by Ian Bishop

  Fibonacci numbers have become a popular introduction to recursion for Computer Science students and there's a strong argument that they persist within nature. For these reasons, many of us are familiar with them. They also exist within Computer Science elsewhere too; in surprisingly efficient data structures and algorithms based upon the sequence. There are two main examples that come to mind: Fibonacci heaps which have better amortized running time than binomial heaps. Fibonacci search which shares O(log N) running time with binary search on an ordered array. Is there some special property of these numbers that gives them an advantage over other numerical sequences? Is it a density quality? What other possible applications could they have? It seems strange to me as there are many natural number sequences that occur in other recursive problems, but I've never seen a Catalan heap.

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• What's the best way to match a query to a set of keywords?

  - by Ryan Detzel

  Pretty much what you would assume Google does. Advertisers come in and big on keywords, lets say "ipod", "ipod nano", "ipod 60GB", "used ipod", etc. Then we have a query, "I want to buy an ipod nano" or "best place to buy used ipods" what kind of algorithms and systems are used to match those queries to the keyword set. I would imagine that some of those keyword sets are huge, 100k keywords made up of one or more actual words. on top of that queries can be 1-n words as well. Any thoughts, links to wikipedia I can start reading? From what I know already I would use some stemmed hash in disk(CDB?) and a bloom filter to check to see if I should even go to disk.

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• Remove from a std::set<shared_ptr<T>> by T*

  - by Autopulated

  I have a set of shared pointers: std::set<boost::shared_ptr<T>> set; And a pointer: T* p; I would like to efficiently remove the element of set equal to p, but I can't do this with any of the members of set, or any of the standard algorithms, since T* is a completely different type to boost::shared_ptr<T>. A few approaches I can think of are: somehow constructing a new shared_ptr from the pointer that won't take ownership of the pointed to memory (ideal solution, but I can't see how to do this) wrapping / re-implementing shared_ptr so that I can do the above just doing my own binary search over the set Help!

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• Google Suggest - What determines the sort order of suggested queries on google?

  - by John Himmelman

  How is this sort order determined? Is it ranked by popularity, number of results, or a mysterious google algorithm? Does there algorithm take into account the search popularity of a query (using google-trends data or something)? Edit: I found a news article dating back to when google suggest was made public in 2004. Here is an excerpt... How does it work? "Our algorithms use a wide range of information to predict the queries users are most likely to want to see. For example, Google Suggest uses data about the overall popularity of various searches to help rank the refinements it offers." Source: http://www.free-seo-news.com/newsletter138.htm

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• Direct video card access

  - by icemanind

  Guys, I am trying to write a class in C# that can be used as a direct replacement for the C# Bitmap class. What I want to do instead though is perform all graphic functions done on the bitmap using the power of the video card. From what I understand, functions such as DrawLine or DrawArc or DrawText are primitive functions that use simple cpu math algorithms to perform the job. I, instead, want to use the graphics card cpu and memory to do these and other advanced functions, such as skinning a bitmap (applying a texture) and true transparancy. My problem is, in C#, how do I access direct video functions? Is there a library or something I need?

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• Merging photo textures - (from calibrated cameras) - projected onto geometry

  - by freakTheMighty

  I am looking for papers/algorithms for merging projected textures onto geometry. To be more specific, given a set of fully calibrated cameras/photographs and geometry, how can we define a metric for choosing which photograph should be used to texture a given patch of the geometry. I can think of a few attributes one may seek minimize including the angle between the surface normal and the camera, the distance of the camera from the surface, as well as minimizing some parameterization of sharpness. The question is how do these things get combined and are there well established existing solutions?

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• Using a locale-dependent sorting function in Ruby/Rails

  - by knuton

  What is a good approach to sorting an array of strings in accordance with the current locale? For example the standard Array#sort puts "Ä" after "Z", which is not correct in German. I would have expected the gem I18n to offer a hook for defining my own sorting algorithms or providing collation strings or objects. In my imagination, passing this proc or string to the sort function, would make it behave as necessary. I know that this is possible in Python, for example. Google has not helped me this time. Can you? Any advice appreciated!

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• Level of Detail for 3D terrains/models in Mobile Devices (Android / XNA )

  - by afriza

  I am planning to develop for WP7 and Android. What is the better way to display (and traverse) 3D scene/models in term of LoD? The data is planned to be island-wide (Singapore). 1) Real-Time Dynamic Level of Detail Terrain Rendering 2) Discrete LoD 3) Others? And please advice some considerations/algorithms/resources/source codes. something like LoD book also Okay. Side note: I am a beginner in this area but pretty well-versed in C/C++. And I haven't read the LoD book. Related posts: - Distant 3D object rendering [games]

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• How to create digital signature that can not be used to reproduce the message twice

  - by freediver

  I am creating a client-server application and I'd like to send data from server to client securely. Using public/private key algorithms makes sense and in PHP we can use openssl_sign and openssl_verify functions to check that the data came by someone who has the private key. Now imagine that one of the actions sent by server to client is destructive in nature. If somebody uses an HTTP sniffer to catch this command (which will be signed properly) how can I further protect the communication to ensure that only commands coming from our server get processed by the client? I was thinking about using current UTC time as part of the encrypted data but client time might be off. Is there a simple solution to the problem?

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• What should I do to practice?

  - by simion

  I start a year long industrial placement in September where i will be coding in Java predominantly. I am going to use the summer to brush up on my Java as in year one of the degree Java was the main language taught for OOP modules. However this year i have had no Java exposure except for an algorithms module, which was one of eight, so as you can see i am probably getting really rusty!. What i wanted to know is, how does the "real world" java programming differ from university coding and what do you suggest i brush up on that would be different to my normal workings. As a start I definitely need to get familiar with a professional IDE like NetBeans, opposed to having used BlueJ throughout but more specifically what coding practices should I get more familiar with. I appreciate they wont expect me to be a qualified full developer and will give me time, but I would like to hit the ground running as it were, with me having full hopes to secure a permanent position after I finish my degree.

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• O(log N) == O(1) - Why not?

  - by phoku

  Whenever I consider algorithms/data structures I tend to replace the log(N) parts by constants. Oh, I know log(N) diverges - but does it matter in real world applications? log(infinity) < 100 for all practical purposes. I am really curious for real world examples where this doesn't hold. To clarify: I understand O(f(N)) I am curious about real world examples where the asymptotic behaviour matters more than the constants of the actual performance. If log(N) can be replaced by a constant it still can be replaced by a constant in O( N log N). This question is for the sake of (a) entertainment and (b) to gather arguments to use if I run (again) into a controversy about the performance of a design.

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• Lua: Random: Percentage

  - by jargl

  I'm creating a game and currently have to deal with some math.randomness. As I'm not that strong in Lua, how do you think Can you make an algorithm that uses math.random with a given percentage? I mean a function like this: function randomChance( chance ) -- Magic happens here -- Return either 0 or 1 based on the results of math.random end randomChance( 50 ) -- Like a 50-50 chance of "winning", should result in something like math.random( 1, 2 ) == 1 (?) randomChance(20) -- 20% chance to result in a 1 randomChance(0) -- Result always is 0 However I have no clue how to go on, and I completely suck at algorithms I hope you understood my bad explanation of what I'm trying to accomplish

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• What should i do to practise?

  - by simion

  Hi guys I start a year long industrial placement in september where i will be coding in java predominantly. I am going to use the summer to brush up on my java as in year one of the degree java was the main language taught for OOP modules. However this year i have had no java exposure except for an algorithms module, which was one of eight, so as you can see i am probably getting really rusty!. What i wanted to know is, how does the "real world" java programming differ from university coding and what do you suggest i brush up on that would be different to my normal workings. As a start i definatley need to get familiar with a professional IDE like netbeans, opoosed to havign used BlueJ throughout but more specifically what coding practises should i get more familiar with I appreciate they wont expect me to be a qualified full developer and will give me time, but i would like to hit the ground running as it were, with me having full hopes to secure a perminant position after i finish my degree. Thanks for reading

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• Implementation of a distance matrix of a binary tree that is given in the adjacency-list representation

  - by Denise Giubilei

  Given this problem I need an O(n²) implementation of this algorithm: "Let v be an arbitrary leaf in a binary tree T, and w be the only vertex connected to v. If we remove v, we are left with a smaller tree T'. The distance between v and any vertex u in T' is 1 plus the distance between w and u." This is a problem and solution of a Manber's exercise (Exercise 5.12 from U. Manber, Algorithms: A Creative Approach, Addison-Wesley (1989).). The thing is that I can't deal properly with the adjacency-list representation so that the implementation of this algorithm results in a real O(n²) complexity. Any ideas of how the implementation should be? Thanks.

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• java and mysql geting shortest path from to two points

  - by shaharnakash

  hi i have a mysql database that hold id ,name , oneid , twoid , size 1 1 1 2 4 2 2 1 3 1 3 3 2 1 74 4 4 2 4 2 5 5 2 5 12 6 6 4 2 12 7 7 4 6 74 8 8 4 7 12 9 9 3 5 32 10 10 3 8 22 11 11 5 3 66 12 12 5 6 76 13 13 5 9 33 14 14 6 10 11 15 15 6 7 21 16 16 8 3 12 17 17 8 9 10 18 18 9 8 2 19 19 9 10 72 20 20 10 6 31 21 21 10 7 7 22 22 10 9 18 23 23 7 6 8 i want to do Dijkstra algorithm but i cant get the details right if i got the contents to class Conn id ,name , oneid , twoid , size how do i find the path from oneid 1 to twoid 7 and believe me i triad many Dijkstra algorithms so please dont give me only reference

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• implementing type inference

  - by deepblue

  well I see some interesting discussions here about static vs. dynamic typing I generally prefer static typing, due to compile type checking, better documented code,etc. However I do agree that they do clutter up the code if done the way Java does it, for example. so Im about to start building a language of my own and type inference is one of the things that I want to implement, in a functional style language... I do understand that it is a big subject, and Im not trying to create something that has not been done before, just basic inferencing... any pointers on what to read up that will help me with this? preferably something more pragmatic/practical as oppose to more theoretical category theory/type theory texts. If there's a implementation discussion text out here, with data structures/algorithms, that would just be lovely much appreciated

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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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• How to double the size of 8x8 Grid whilst keeping the relative position of certain tiles intact?

  - by ke3pup

  Hi guys I have grid size of size 8x8 , total of 64 Tiles. i'm using this Grid to implement java search algorithms such as BFS and DFS. The Grid has given forbidden Tiles (meaning they can't be traversed or be neighbour of any other tile) and Goal and Start tile. for example Tile 19,20,21,22 and 35, 39 are forbidden and 14 an 43 are the Goal and start node when the program runs. My question is , How can i double the size of the grid, to 16x16 whilst keeping the Relative position of forbidden tiles as well as the Relative position of start and goal Tiles intact? On paper i know i can do this by adding 4 rows and columns to all size but in coding terms i don't know how to make it work? Can someone please give any sort of hints?

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• "Did you mean" feature on a dictionary database

  - by Hazar

  I have a ~300.000 row table; which includes technical terms; queried using PHP and MySQL + FULLTEXT indexes. But when I searching a wrong typed term; for example "hyperpext"; naturally giving no results. I need to "compansate" little writing errors and getting nearest record from database. How I can accomplish such feaure? I know (actually, learned today) about Levenshtein distance, Soundex and Metaphone algorithms but currently not having a solid idea to implement this to querying against database. Best regards. (Sorry about my poor English, I'm trying to do my best)

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• Iterator category

  - by Knowing me knowing you

  In code: //I know that to get this effect (being able to use it with std algorithms) I can inherit like I did in line below: class Iterator //: public std::iterator<std::bidirectional_iterator_tag,T> { private: T** itData_; public: //BUT I WOULD LIKE TO BE ABLE TO DO IT BY HAND AS WELL typedef std::bidirectional_iterator_tag iterator_category; typedef T* value_type;//SHOULD IT BE T AS value_type or T*? typedef std::ptrdiff_t difference_type; typedef T** pointer;//SHOULD IT BE T* AS pointer or T**? typedef T*& reference;//SHOULD IT BE T& AS reference or T*&? }; Basically what I'm asking is if I have my variable of type T** in iterator class is it right assumption that value type for this iterator will be T* and so on as I described in comments in code, right next to relevant lines. Thank you.

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• Do similar passwords have similar hashes?

  - by SLC

  Our computer system at work requires users to change their password every few weeks, and you cannot have the same password as you had previously. It remembers something like 20 of your last passwords. I discovered most people simply increment a digit at the end of their password, so "thisismypassword1" becomes "thisismypassword2" then 3, 4, 5 etc. Since all of these passwords are stored somewhere, I wondered if there was any weakness in the hashes themselves, for standard hashing algorithms used to store passwords like MD5. Could a hacker increase their chances of brute-forcing the password if they have a list of hashes of similar passwords?

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