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• Python regular expression implementation details

  - by Tom

  A question that I answered got me wondering: How are regular expressions implemented in Python? What sort of efficiency guarantees are there? Is the implementation "standard", or is it subject to change? I thought that regular expressions would be implemented as DFAs, and therefore were very efficient (requiring at most one scan of the input string). Laurence Gonsalves raised an interesting point that not all Python regular expressions are regular. (His example is r"(a+)b\1", which matches some number of a's, a b, and then the same number of a's as before). This clearly cannot be implemented with a DFA. So, to reiterate: what are the implementation details and guarantees of Python regular expressions? It would also be nice if someone could give some sort of explanation (in light of the implementation) as to why the regular expressions "cat|catdog" and "catdog|cat" lead to different search results in the string "catdog", as mentioned in the question that I referenced before.

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• Architecture of an image hosting site

  - by kamziro

  I'm sure many here are aware of image hosting sites, like imgur, min.us, photobucket etc. Not that I want to develop one, but besides just uploading the file, organising it in some directory somewhere, what architectural considerations are involved in these sites? Especially when there's millions of page views a day (like imgur, I'd imagine) I'm curious about this because it seems that a lot of sites (say, dating websites etc) would be pretty image intensive. Even if it's not for millions of page views, what are some basic architectural requirements of efficient image deliveries online?

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• MERGE -v- UPSERT

  - by Kevin Ross

  Hi, I have an application I’m writing in access with a SQL server backend. One of the most heavily used parts is where the users selects an answer to a question, a stored procedure is then fired which sees if an answer has already been given, if it has an UPDATE is executed, if not an INSERT is executed. This works just fine but now we have upgraded to SQL server 2008 express I was wondering if it would be better/quicker/more efficient to rewrite this SP to use the new MERGE command. Does anyone have any idea if this is faster than doing a SELECT followed by either an INSERT or UPDATE?

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• hosting environment for delivering FLVs [closed]

  - by Gotys

  What would be the ideal hardware setup for pushing lots of bandwith on a tube site? We have ever-expanding cloud storage where users upload the movies, then we have these web-delivery machines which cache the FLV files on its local harddrives and deliver them to users. Each cache machine can deliver 1200 mbits/s , if it has SAS 8 harddrives. Such a cache machine costs us $550/month for 8x160gb -- so each machine can cache only 160GB at any given time. If we want to cache more then 160gb , we need to add another machine..another $550/month..etc. This is very un-economical so I am wondering if we have any experts here who can figure out a better setup. I've been looking into "gluster FS", but I am not sure if this thing can push a lot of bandwith. Any ideas highly appreciated. Thank you!

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• Removing duplicates without overriding hash method

  - by Javi

  Hello, I have a List which contains a list of objects and I want to remove from this list all the elements which have the same values in two of their attributes. I had though about doing something like this: List<Class1> myList; .... Set<Class1> mySet = new HashSet<Class1>(); mySet.addAll(myList); and overriding hash method in Class1 so it returns a number which depends only in the attributes I want to consider. The problem is that I need to do a different filtering in another part of the application so I can't override hash method in this way (I would need two different hash methods). What's the most efficient way of doing this filtering without overriding hash method? Thanks

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• Performance of DrawingVisual vs Canvas.OnRender for lots of constantly changing shapes

  - by romkyns

  I'm working on a game-like app which has up to a thousand shapes (ellipses and lines) that constantly change at 60fps. Having read an excellent article on rendering many moving shapes, I implemented this using a custom Canvas descendant that overrides OnRender to do the drawing via a DrawingContext. The performance is quite reasonable, although the CPU usage stays high. However, the article suggests that the most efficient approach for constantly moving shapes is to use lots of DrawingVisual instances instead of OnRender. Unfortunately though it doesn't explain why that should be faster for this scenario. Changing the implementation in this way is not a small effort, so I'd like to understand the reasons and whether they are applicable to me before deciding to make the switch. Why could the DrawingVisual approach result in lower CPU usage than the OnRender approach in this scenario?

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• Should I Make These Vectors Classes or Structs in C#

  - by dewald

  I am creating a geometry library in C# and I will need the following immutable types: Vector2f (2 floats - 8 bytes) Vector2d (2 doubles - 16 bytes) Vector3f (3 floats - 12 bytes) Vector3d (3 doubles - 24 bytes) Vector4f (4 floats - 16 bytes) Vector4d (4 doubles - 32 bytes) I am trying to determine whether to make them structs or classes. MSDN suggests only using a struct if the size if going to be no greater than 16 bytes. That reference seems to be from 2005. Is 16 bytes still the max suggested size? I am sure that using structs for the float vectors would be more efficient than using a class, but what should I do about the double vectors? Should I make them structs also to be consistent, or should I make them classes?

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• How to return variable datatypes from a method

  - by Praesagus

  I have a method whose callers require different datatypes. I could repeat the requisite conversions a dozen times by putting them in each of the callers, but it seems much more efficient to do it once in the called method. public String myMethod(String myArg) { return DoSomething(myArg); } public Int32 myMethod(String myArg) { return Convert.ToInt32(DoSomething(myArg)); } private String DoSomething(key) { return SomeList[key]; } If I have multiple methods that pull data from SomeList and have to utilize different data types then in each I have to do a type conversion. Examples might be session variables or query or form requests or any other number of things. In VB I could say Function myMethod(myArg as String) as Variant myMethod=DoSomething(myArg) End Function Sorry if the original post was not very clear. I hope this makes more sense. Thanks

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• php selecting hash using wildcards

  - by tipu

  Say I have a hashmap, $hash = array('fox' => 'some value', 'fort' => 'some value 2', 'fork' => 'some value again); I am trying to accomplish an autocomplete feature. When the user types 'fo', I would like to retrieve, via ajax, the 3 keys from $hash. When the user types 'for', I would like to only retrieve the keys fort and fork. Is this possible? What I was thinking was using binary search to isolate the keys with 'f', instead of brute-force searching. Then continue eliminating the indexes as the user types out their query. Is there a more efficient solution to this?

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• Enum.values() vs EnumSet.allOf( ). Which one is more preferable?

  - by Alexander Pogrebnyak

  I looked under the hood for EnumSet.allOf and it looks very efficient, especially for enums with less than 64 values. Basically all sets share the single array of all possible enum values and the only other piece of information is a bitmask which in case of allOf is set in one swoop. On the other hand Enum.values() seems to be a bit of black magic. Moreover it returns an array, not a collection, so in many cases it must be decorated with Arrays.asList( ) to be usable in any place that expects collection. So, should EnumSet.allOf be more preferable to Enum.values? More specifically, which form of for iterator should be used: for ( final MyEnum val: MyEnum.values( ) ); or for ( final MyEnum val: EnumSet.allOf( MyEnum.values ) );

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• Using malloc/free in Objective-C object

  - by Itamar Katz

  I have a class AudioManager with a member of type AudioBufferList *. (This is a struct declared in the CoreAudio framework). Since AudioBufferList is not a NSObject, I cannot retain it, so I have to alloc/free it (correct me if I'm wrong). My question is, where is the 'right' place to free it? Currently I am doing it in the dealloc method of AudioManager. If I understand correctly, this method is invoked automatically once the release message is sent to the instance of AudioManager --- is that true? Is there any other recommended practice regarding using alloc/free on non-objects members of Objective-C objects? Edit: From Apples documentation: Subclasses must implement their own versions of dealloc to allow the release of any additional memory consumed by the object—such as dynamically allocated storage for data or object instance variables owned by the deallocated object. After performing the class-specific deallocation, the subclass method should incorporate superclass versions of dealloc through a message to super: Which makes things a little bit clearer - but more insights are appreciated.

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• Open source alternative to WebEx WebOffice?

  - by Dieseltime

  I have a client who has been using WebOffice (from WebEx) for a variety of tasks within their small organization. The problem is that they only really need a small subset of the features WebOffice provides (Contact list, Database, and Document Storage). They've asked me to develop a website focused on these three features with the rationalization that this should be more cost-effective, since they currently aren't using many of the features of WebOffice they pay for. What are some open-source alternatives that I could implement for them? Sharepoint sounds like it would be too bloated and Google Apps may not be as collaborative as they would like.

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• Stochastic calculus library in python

  - by LeMiz

  Hello, I am looking for a python library that would allow me to compute stochastic calculus stuff, like the (conditional) expectation of a random process I would define the diffusion. I had a look a at simpy (simpy.sourceforge.net), but it does not seem to cover my needs. This is for quick prototyping and experimentation. In java, I used with some success the (now inactive) http://martingale.berlios.de/Martingale.html library. The problem is not difficult in itself, but there is a lot non trivial, boilerplate things to do (efficient memory use, variable reduction techniques, and so on). Ideally, I would be able to write something like this (just illustrative): def my_diffusion(t, dt, past_values, world, **kwargs): W1, W2 = world.correlated_brownians_pair(correlation=kwargs['rho']) X = past_values[-1] sigma_1 = kwargs['sigma1'] sigma_2 = kwargs['sigma2'] dX = kwargs['mu'] * X * dt + sigma_1 * W1 * X * math.sqrt(dt) + sigma_2 * W2 * X * X * math.sqrt(dt) return X + dX X = RandomProcess(diffusion=my_diffusion, x0 = 1.0) print X.expectancy(T=252, dt = 1./252., N_simul= 50000, world=World(random_generator='sobol'), sigma1 = 0.3, sigma2 = 0.01, rho=-0.1) Does someone knows of something else than reimplementing it in numpy for example ?

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• Core Data multi-threading

  - by JK

  My app starts by presenting a tableview whose datasource is a Core Data SQLite store. When the app starts, a secondary thread with its own store controller and context is created to obtain updates from the web for data in the store. However, any resulting changes to the store are not notified to the fetchedresults controller (I presume because it has its own coordinator) and consequently the table is not updated with store changes. What would be the most efficient way to refresh the context on the main thread? I am considering tracking the objectIDs of any objects changed on the secondary thread, sending those to the main thread when the secondary thread completes and invoking "[context refreshObject:....] Any help would be greatly appreciated.

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• LINQ to SQL vs Entity Framework for an app with a future SQL Azure version

  - by Craig L

  I've got a vertical market Dot Net Framework 1.1 C#/WinForms/SQL Server 2000 application. Currently it uses ADO.Net and Microsoft's SQLHelper for CRUD operations. I've successfully converted it to Dot Net Framework 4 C#/WinForms/ SQL Server 2008. What I'd like to do is also offer my customers the ability to use SQL Azure as a backend storage for their data instead of local/LAN SQL Server. If I know SQL Azure is in my application's future, should I: A. Switch to LINQ to SQL B. Swith to Entity Framework C. Stick with ADO.Net and SQLHelper Thanks !

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• Optimization of running total calculation in SQL for multiple values per join condition

  - by Kiril

  I have the following table (test_table): date value --------------- d1 10.0 d1 20.0 d2 60.0 d2 10.0 d2 -20.0 d3 40.0 I calculate the running total as follows. I use the same query twice, because first I need to calculate the values for a specifi date, and afterwards I can calculate the running total. Otherwise, joining the two tables where date is not unique, I would get too many results from the join: SELECT t1.date, SUM(t2.value) AS total FROM (SELECT date, SUM(value) AS value FROM test_table GROUP BY date) AS t1 JOIN (SELECT date, SUM(value) AS value FROM test_table GROUP BY date) AS t2 ON t1.date >= t2.date GROUP BY t1.date ORDER BY t1.date This gives me (which is fine): date total ------------- d1 30.0 d2 80.0 d3 120.0 BUT, this query isn't very efficient, because I need to change conditions in two places, if necessary. In production, the test_table is a lot bigger ( 4 Mio. rows), and the query takes too much time to complete. Question: How can I avoid using the same query twice?

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• Difference b/w putting condition in JOIN clause versus WHERE clause

  - by user244953

  Suppose I have 3 tables. Sales Rep Rep Code First Name Last Name Phone Email Sales Team Orders Order Number Rep Code Customer Number Order Date Order Status Customer Customer Number Name Address Phone Number I want to get a detailed report of Sales for 2010. I would be doing a join. I am interested in knowing which of the following is more efficient and why ? SELECT O.OrderNum, R.Name, C.Name FROM Order O INNER JOIN Rep R ON O.RepCode = R.RepCode INNER JOIN Customer C ON O.CustomerNumber = C.CustomerNumber WHERE O.OrderDate >= '01/01/2010' OR SELECT O.OrderNum, R.Name, C.Name FROM Order O INNER JOIN Rep R ON (O.RepCode = R.RepCode AND O.OrderDate >= '01/01/2010') INNER JOIN Customer C ON O.CustomerNumber = C.CustomerNumber

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• Scipy sparse... arrays?

  - by spitzanator

  Hey, folks. So, I'm doing some Kmeans classification using numpy arrays that are quite sparse-- lots and lots of zeroes. I figured that I'd use scipy's 'sparse' package to reduce the storage overhead, but I'm a little confused about how to create arrays, not matrices. I've gone through this tutorial on how to create sparse matrices: http://www.scipy.org/SciPy_Tutorial#head-c60163f2fd2bab79edd94be43682414f18b90df7 To mimic an array, I just create a 1xN matrix, but as you may guess, Asp.dot(Bsp) doesn't quite work because you can't multiply two 1xN matrices. I'd have to transpose each array to Nx1, and that's pretty lame, since I'd be doing it for every dot-product calculation. Next up, I tried to create an NxN matrix where column 1 == row 1 (such that you can multiply two matrices and just take the top-left corner as the dot product), but that turned out to be really inefficient. I'd love to use scipy's sparse package as a magic replacement for numpy's array(), but as yet, I'm not really sure what to do. Any advice? Thank you very much!

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• How do I include 2 tables in LocalStorage?

  - by Noor

  I've got a table that you can edit, and I've got a simple code saving that list when you're done with editing it. (the tables have the contenteditable on) The problem I've stumbled upon is that if I double click on enter, the table gets divided into two separate tables with the same ID. This causes the code I'm using to set the localStorage to only store one of the tables (I assume the first).. I've thought of different solutions and I wonder if someone could point out the pro's and con's (if the solutions even works that is). Make a loop that checks the page after tables and stores them into an array of localStorage-items.. I'd have to dynamically create a localStorage item for each table. Take the whole div that the tables are in, and store that in the localStorage, when a user revisits the page, the page checks after the items in storage and displays the whole divs. Any suggestions you have that can beat this :).. (but no cache, it has to be with the localStorage!) Thanks

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• SqlServer2008 - Can I Alter a Scalar Function while it is referenced in many places

  - by Casey C.

  We have a scalar function that returns a DateTime. It performs a couple of quick table selects to get its return value. This function is already in use throughout the database - in default constraints, stored procs, etc. I would like to change the implementation of the function (to remove the table hits and make it more efficient) but apparently I can't do that while it is referenced by other objects in the database. Will I actually need to update every object in the database that references it to remove the reference, update the function and then update all those objects to restore the reference to the function? Thanks for any insight you can give.

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• Arguments On a Console eMbedded Visual C++ Application

  - by Nathan Campos

  I'm trying to develop a simple application that will read some files, targeted for Windows CE. For this I'm using Microsoft eMbedded Visual C++ 3. This program(that is for console) will be called like this: /Storage Card/Test coms file.cmss As you can see, file.cmss is the first argument, but on my main I have a condition to show the help(the normal, how to use the program) if the arguments are smaller than 2: if(argc < 2) { showhelp(); return 0; } But when I execute the program on the command-line of Windows CE(using all the necessary arguments) I got the showHelp() content. Then I've checked all the code, but it's entirelly correct. But I think that eVC++ don't use argc and argv[] for arguments, then I want some help on how to determine the arguments on it.

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• How to rotate an SSE/AVX vector

  - by user1584773

  I need to perform a rotate operation with as little clock cycle as possible. In the first case let's assume __m128i as source and dest type source: || A0 || A1 || A2 || A3 || dest : || A1 || A2 || A3 || A0 || dest = (__m128i)_mm_shuffle_epi32((__m128i)source, _MM_SHUFFLE(0,3,2,1)); Now I want to do the same whit AVX intrinsics So let's assume this time __m256i as source and dest type source: || A0 || A1 || A2 || A3 || A4 || A5 || A6 || A7 || dest : || A1 || A2 || A3 || A4 || A5 || A6 || A7 || A0 || The Avx intrinsics is missing most of the corresponding SSE integer operations. Maybe there is some way go get the desider output working with the floating point version. I've tryed with: dest = (__m256i)_mm256_shuffle_ps((__m256)source, (__m256)source, _MM_SHUFFLE(0,3,2,1)); but what I get is: || A0 || A2 || A3 || A4 || A5 || A6 || A7 || A1 || Any Idea on how to solve this in an efficient way? (without mixing SSE and AVX operation and without "manually" inverting A0 and A1 Thanks in advance!

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• Huge dataset point in polygon in .net (collision detection)

  - by Rickard Liljeberg

  I have a pretty big mesh with polygons, usually triangles but sometimes rectangles. Each point in my mesh has a value (value has nothing to do with coordinates). Now I am creating a second mesh in the same coordinate-space as the old mesh. I now want to interpolate out values for all points (vertices) in the new mesh using the values from the old mesh. Now I could loop each polygon in the new mesh and detect which old vertices are in each polygon by making 2d collision detection (altho even this I don't get to function properly so if anyone has simple and fast code for 2d collision detection (triangle is enough) I would gladly see it). However to my main point again. looping each old vertice for each new polygon seems less than efficient. is there a better way?

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• Client side page permissions - Javascript / jquery / cookies / other?

  - by Ozaki

  TLDR Using plain HTML / Javascript. Want to block access to some pages (doesn't have to be super secure just to stop some peeking eyes). I thought of simply doing this by setting a cookie for each page they are allowed to visit with a value of true but thats a bit messy. Although it would work. Is there a way to set an array of values to a cookie so I can read the cookie and if a name of a page is in there then allow access with an IF statement or so on each of my pages. If they dont have the cookie just to replace my #content (entirepage) to "sorry no" etc. For example: $.cookie("Access","page1, page2, page3",{ expires: 1 }); Am already using JQuery, Jquery cookie. etc. I am up for anyway of doing this cookie idea is just an example So what do you think would be the best / most efficient way of managing this?

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• How do I regenerate statistics in Openx?

  - by Martin Bauer

  ue to faulty hardware, statistics generated over a 2 week period were significantly higher than normal (10000 times higher than normal). After moving the application to a new server, the problem rectified itself. The issue I have is that there are 2 weeks of stats that are clearly wrong. I have checked the raw impressions table for the affected fortnight and it seems to be correct (ie. stats per banner per day match the average for the previous month). Looking at the intermediate & summary impressions tables, the values are inflated. I understand from the openx forum (http://forum.openx.org/index.php?s=7796fd9dae40e020a010773746f3ada9&showtopic=503424297) it's possible to regenerate stats from the raw data but it will only regenerate stats per hour, meaning regenerating stats for 2 weeks would be very time consuming. Is there another, more efficient way to regenerate the stats from the raw data for the affected fortnight?

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