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• Algorithm for redirecting the traffic

  - by TechGeeky

  I was going through the interview questions and found out the below question which I am not able to answer it. Can anyone provide some sort of algorithm for this problem how can I solve it? There are a cluster of stateless servers all serving the same pages. The servers are hosting 5 web pages- p1.html, p2.html, p3.html, p4.html and p5.html p1.html just redirects users to the other 4 pages Requests to p1.html should result in 10% of users being redirected to p2.html, 5% of users redirected to p3.html, 20% of users redirected to p4.html, and 65% of users redirected to p5.html. Users do not need to stick to the page they are first redirected to. They could end up on a different page with every request to p1.html Write a function/pseudocode that would be invoked with every request to p1.html and redirect the correct percentage of users to the correct page. Any suggestions will be of great help.

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• does a switch idiom make sense in this case?

  - by the ungoverned

  I'm writing a parser/handler for a network protocol; the protocol is predefined and I am writing an adapter, in python. In the process of decoding the incoming messages, I've been considering using the idiom I've seen suggested elsewhere for "switch" in python: use a hash table whose keys are the field you want to match on (a string in this case) and whose values are callable expressions: self.switchTab = { 'N': self.handleN, 'M': self.handleM, ... } Where self.handleN, etc., are methods on the current class. The actual switch looks like this: self.switchTab[selector]() According to some profiling I've done with cProfile (and Python 2.5.2) this is actually a little bit faster than a chain of if..elif... statements. My question is, do folks think this is a reasonable choice? I can't imagine that re-framing this in terms of objects and polymorphism would be as fast, and I think the code looks reasonably clear to a reader.

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• Best solution for language documentation.

  - by Simone Margaritelli

  I'm developing a new object oriented scripting language and the project itself is quite ready for audience now, so i'm starting to think about a serious (not as "drafty" as it is right now) way of document its grammar, functions from standard library and standard library classes. I've looked a bit around and almost every language hash its own web application for the documentation, Python uses Sphinx for instance. Which is the best PHP (don't have the time/will to install mod_who_knows_what on my server) application to accomplish this? I've used mediawiki a bit but i found its tag system a little bit hard to use in this context. Thanks for your answers.

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• Why use hashing to create pathnames for large collections of files?

  - by Stephen

  Hi, I noticed a number of cases where an application or database stored collections of files/blobs using a has to determine the path and filename. I believe the intended outcome is a situation where the path never gets too deep, or the folders ever get too full - too many files (or folders) in a folder making for slower access. EDIT: Examples are often Digital libraries or repositories, though the simplest example I can think of (that can be installed in about 30s) is the Zotero document/citation database. Why do this? EDIT: thanks Mat for the answer - does this technique of using a hash to create a file path have a name? Is it a pattern? I'd like to read more, but have failed to find anything in the ACM Digital Library

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• Killing the mysqld process

  - by Josh K

  I have a table with ~800k rows. I ran an update users set hash = SHA1(CONCAT({about eight fields})) where 1; Now I have a hung Sequel Pro process and I'm not sure about the mysqld process. This is two questions: What harm can possibly come from killing these programs? I'm working on a separate database, so no damage should come to other databases on the system, right? Assume you had to update a table like this. What would be a quicker / more reliable method of updating without writing a separate script. I just checked with phpMyAdmin and it appears as though the query is complete. I still have Sequel Pro using 100% of both my cores though...

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• How to tell process id within Python

  - by R S

  Hey, I am working with a cluster system over linux (www.mosix.org) that allows me to run jobs and have the system run them on different computers. Jobs are run like so: mosrun ls & This will naturally create the process and run it on the background, returning the process id, like so: [1] 29199 Later it will return. I am writing a Python infrastructure that would run jobs and control them. For that I want to run jobs using the mosrun program as above, and save the process ID of the spawned process (29199 in this case). This naturally cannot be done using os.system or commands.getoutput, as the printed ID is not what the process prints to output... Any clues? Edit: Since the python script is only meant to initially run the script, the scripts need to run longer than the python shell. I guess it means the mosrun process cannot be the script's "son process". Any suggestions? Thanks

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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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• How does this Perl grep work to determine the union of several hashes?

  - by titaniumdecoy

  I don't understand the last line of this function from Programming Perl 3e. Here's how you might write a function that does a kind of set intersection by returning a list of keys occurring in all the hashes passed to it: @common = inter( \%foo, \%bar, \%joe ); sub inter { my %seen; for my $href (@_) { while (my $k = each %$href) { $seen{$k}++; } } return grep { $seen{$_} == @_ } keys %seen; } I understand that %seen is a hash which maps each key to the number of times it was encountered in any of the hashes provided to the function.

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• Whats the best data-structure for storing 2-tuple (a, b) which support adding, deleting tuples and c

  - by bhups

  Hi So here is my problem. I want to store 2-tuple (key, val) and want to perform following operations: - keys are strings and values are Integers - multiple keys can have same value - adding new tuples - updating any key with new value (any new value or updated value is greater than the previous one, like timestamps) - fetching all the keys with values less than or greater than given value - deleting tuples. Hash seems to be the obvious choice for updating the key's value but then lookups via values will be going to take longer (O(n)). The other option is balanced binary search tree with key and value switched. So now lookups via values will be fast (O(lg(n))) but updating a key will take (O(n)). So is there any data-structure which can be used to address these issues? Thanks.

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• tomcat session replication without multicast

  - by Andreas Petersson

  i am planning to use 2 dedicated root servers rented at a hosting provider. those machines will run tomcat 6 in a cluster. if i will add additional machines later on - it is unlikely that they will be accessible with multicast, because they will be located in different subnets. is it possible to run tomcat without multicast? all tutorials for tomcat 6 clustering include multicast heartbeat. are there any alternatives to SimpleTcpCluster? or are other alternatives more appropriate in this situation?

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• What is the best way to implement this composite GetHashCode()

  - by Frank Krueger

  I have a simple class: public class TileName { int Zoom, X, Y; public override bool Equals (object obj) { var o = obj as TileName; return (o != null) && (o.Zoom == Zoom) && (o.X == X) && (o.Y == Y); } public override int GetHashCode () { return (Zoom + X + Y).GetHashCode(); } } I was curious if I would get a better distribution of hash codes if I instead did something like: public override int GetHashCode () { return Zoom.GetHashCode() + X.GetHashCode() + Y.GetHashCode(); } This class is going to be used as a Dictionary key, so I do want to make sure there is a decent distribution.

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• What's the best way of using a pair (triple, etc) of values as one value in C#?

  - by Yacoder

  That is, I'd like to have a tuple of values. The use case on my mind: Dictionary<Pair<string, int>, object> or Dictionary<Triple<string, int, int>, object> Are there built-in types like Pair or Triple? Or what's the best way of implementing it? Update There are some general-purpose tuples implementations described in the answers, but for tuples used as keys in dictionaries you should additionaly verify correct calculation of the hash code. Some more info on that in another question. Update 2 I guess it is also worth reminding, that when you use some value as a key in dictionary, it should be immutable.

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• Dynamically render partial templates using mustache

  - by btakita

  Is there a way to dynamically inject partial templates (and have it work the same way in both Ruby & Javascript)? Basically, I'm trying to render different types of objects in a list. The best I can come up with is this: <div class="items"> {{#items}} <div class="item"> {{#is_message}} {{< message}} {{/is_message}} {{^is_message}} {{#is_picture}} {{< picture}} {{/is_picture}} {{^is_picture}} {{/is_picture}} {{/is_message}} </div> {{/items}} </div> For obvious reasons, I'm not super-psyched about this approach. Is there a better way? Also note that the different types of models for the views can have non-similar fields. I suppose I could always go to the lowest common denominator and have the data hash contain the html, however I would rather use the mustache templates.

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• how to store passwords in database?

  - by rgksugan

  I use jsp and servlets in my web application. i need to store passwords in the database. I found that hashing will be the best way to do that. I used this code to do it. java.security.MessageDigest d = null; d = java.security.MessageDigest.getInstance("SHA-1"); d.reset(); d.update(pass.getBytes("UTF-8")); byte b[] = d.digest(); String tmp = (new BASE64Encoder()).encode(b); When i tried to print the value of tmp, i get some other value.i guess its the hash value of the password. But when i persist this data to the database the original password gets saved there other than the value in tmp.. What is the problem???

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• Separating an Array into a comma seperated string with quotes

  - by user548744

  I'm manually building an SQL query where I'm using an Array in the params hash for an SQL IN statement, like: ("WHERE my_field IN('blue','green','red')"). So I need to take the contents of the array and output them into a string where each element is single quoted and comma seperated (and with no ending comma). So if the array was: my_array = ['blue','green','red'] I'd need a string that looked like: "'blue','green','red'" I'm pretty new to Ruby/Rails but came up with something that worked: if !params[:colors].nil? @categories_array = params[:colors][:categories] @categories_string ="" for x in @categories_array @categories_string += "'" + x + "'," end @categories_string.chop! #remove the last comma end So, I'm good but curious as to what a proper and more consise way of doing this would look like? Thanks

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• Tool for response time analysis on JBoss server?

  - by Ariel Vardi

  I am running a pretty high traffic cluster of JBoss servers serving REST requests and I am interested in tools reading the access logs in Tomcat format (with %D parameter) to provide a detailed analysis of the response time on a per-call basis. Ideally this tool would generate a chart showing the progression of the response time throughout the day, hour per hour, then a weekly view with averages on the day, and monthly with average on the weeks (CACTI style). I've looked for such tools and couldn't find anything. Is any of you guys aware of something close to that before I start writing my own? I haven't looked into CACTI extensions yet, but that be an option?

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• ActiveRecord Create (not !) Throwing Exception on Validation

  - by myferalprofessor

  So I'm using ActiveRecord model validations to validate a form in a RESTful application. I have a create action that does: @association = Association.new and the receiving end of the form creates a data hash of attributes from the form parameters to save to the database using: @association = user.associations.create(data) I want to simply render the create action if validation fails. The problem is that the .create (not !) method is throwing an exception in cases where the model validation fails. Example: validates_format_of :url, :with => /(^$)|(^(http|https):\/\/[a-z0-9]+([\-\.]{1}[a-z0-9]+)*\.[a-z]{2,5}(([0-9]{1,5})?\/.*)?$)/ix, :message => "Your url doesn't seem valid." in the model produces: ActiveRecord::RecordInvalid Exception: Validation failed: Url Your url doesn't seem valid. I thought .create! is supposed throw an exception whereas .create is not. Am I missing something here? Ruby 1.8.7 patchlevel 173 & rails 2.3.3

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• Confusing Perl code

  - by titaniumdecoy

  I don't understand the last line of this function from Programming Perl 3e. Here's how you might write a function that does a kind of set intersection by returning a list of keys occurring in all the hashes passed to it: @common = inter( \%foo, \%bar, \%joe ); sub inter { my %seen; for my $href (@_) { while (my $k = each %$href) { $seen{$k}++; } } return grep { $seen{$_} == @_ } keys %seen; } I understand that %seen is a hash which maps each key to the number of times it was encountered in any of the hashes provided to the function.

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• PHP Captcha without session

  - by Anton N

  Ok, here is an issue: in the project i'm working on, we can't rely on server-side sessions for any functionality. The problem is that common captcha solutions from preventing robotic submits require session to store the string to match captcha against. The question is - is there any way to solve the problem without using sessions? What comes to my mind - is serving hidden form field, containing some hash, along with captcha input field, so that server then can match these two values together. But how can we make this method secure, so that it couldn't be used to break captcha easily.

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• Should I make sure arguments aren't null before using them in a function.

  - by Nathan W

  The title may not really explain what I'm really trying to get at, couldn't really think of a way to describe what I mean. I was wondering if it is good practice to check the arguments that a function accepts for nulls or empty before using them. I have this function which just wraps some hash creation like so. Public Shared Function GenerateHash(ByVal FilePath As IO.FileInfo) As String If (FilePath Is Nothing) Then Throw New ArgumentNullException("FilePath") End If Dim _sha As New Security.Cryptography.MD5CryptoServiceProvider Dim _Hash = Convert.ToBase64String(_sha.ComputeHash(New IO.FileStream(FilePath.FullName, IO.FileMode.Open, IO.FileAccess.Read))) Return _Hash End Function As you can see I just takes a IO.Fileinfo as an argument, at the start of the function I am checking to make sure that it is not nothing. I'm wondering is this good practice or should I just let it get to the actual hasher and then throw the exception because it is null.? Thanks.

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• two HashMap iteration

  - by user431276

  I have two HashMaps and I can iterate both hashmaps with following code Iterator it = mp.entrySet().iterator(); while (it.hasNext()) { Map.Entry pairs = (Map.Entry)it.next(); String firstVal = pairs.getValue(); } Iterator it2 = mp2.entrySet().iterator(); while (it2.hasNext()) { Map.Entry pairs2 = (Map.Entry)it.next(); String SecondVal = pairs2.getValue(); } myFunction(firstVal, SecondVal) Is there anyway to iterate two hashmaps at the same time without using two loops? Currently, I have a method that accepts two parameters and each parameter value is stored in first and second hashmap. I have to iterate first hash then second to get values. I think there must be a good way to do it but I don't know :( P.S: there could be some errors in above code as this is just an example to explain my problem. Each iterator is a method in original program and accept one parameter. I couldn't copy past real time functions as they are HUGE !

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• relating data stored in NoSQL DB to data stored in SQL DB

  - by seanbrant

  Whats the best way to use a SQL DB along side a NoSQL DB? I want to keep my users and other data in postgres but have some data that would be better suited for a NoSQL DB like redis. I see a lot of talk about switching to NoSQL but little talk on integrating it with existing systems. I think it would be foolish to throw the baby out with the bath water and ditch SQL all together, unless it makes things easier to maintain and develop. I'm wondering what the best approach is for relating data stored in SQL to my data in redis. I was thinking of something along the line of this. User object stored in SQL Book object in redis, key sh1 hash of value, value is a JSON string Relations stored in redis, key User.pk:books, value redis set of sha1's Anyone have experience, tips, better ways?

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• Can't find netbooted for Kerrighed pxe boot with Ubuntu Lucid Server

  - by Pengin

  I'm following installtion guides for pxe booting and kerrighed. I can't find the package nfsbooted for Ubuntu 10.04. Where did it go? Context: At work I have access to 8 mini-ITX PCs and am trying to build a cluster. My plans include trying Condor, GridGain, Hadoop, and recently Kerrighed has caught my eye. (I reaslise these are all for different kinds of things, I'm just evaluating). Ideally, I'd like to have all the nodes network boot from a single server, since that seems so much easier to manage, plus I can 'borrow' additional PCs for a while without touching their HD. I've been getting on great with Ubuntu Lucid Server (10.04), trying to follow the only guides I can find to get pxe booting (and ultimately kerrighed) to work. This guide is for Ubuntu 8.04 and this one is for Debian. They both refer to a package I can't seem to find, nfsbooted. Has this package been replaced? Am I doing something daft?

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• Semicolon in object variable name

  - by milkfilk

  There's a common LDAP attribute called userCertificate;binary. It actually has a semi-colon in the attribute name. In ruby, I turn an LDAP entry into a OpenStruct object called 'struct'. struct.class = OpenStruct But of course ruby thinks it's an end-of-line character. ? struct.userCertificate;binary NameError: undefined local variable or method `binary' for main:Object from (irb):52 from :0 IRB knows that the local variable is there, because it gives me struct.userCertificate;binary from the tab auto-completion. I can also see the class variable when calling struct.methods on it. struct.methods = ... "send", "methods", "userCertificate;binary=", "hash", ... It's definitely there, I can see the contents if I print the whole variable to_s(). But how can I access the local variable when it has a semicolon in it? I have workarounds for this but I thought it was an interesting problem to post.

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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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