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• Java Refuses to Start - Could not reserve enough space for object heap

  - by Randyaa

  Background We have a pool of aproximately 20 linux blades. Some are running Suse, some are running Redhat. ALL share NAS space which contains the following 3 folders: /NAS/app/java - a symlink that points to an installation of a Java JDK. Currently version 1.5.0_10 /NAS/app/lib - a symlink that points to a version of our application. /NAS/data - directory where our output is written All our machines have 2 processors (hyperthreaded) with 4gb of physical memory and 4gb of swap space. We limit the number of 'jobs' each machine can process at a given time to 6 (this number likely needs to change, but that does not enter into the current problem so please ignore it for the time being). Some of our jobs set a Max Heap size of 512mb, some others reserve a Max Heap size of 2048mb. Again, we realize we could go over our available memory if 6 jobs started on the same machine with the heap size set to 2048, but to our knowledge this has not yet occurred. The Problem Once and a while a Job will fail immediately with the following message: Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine. We used to chalk this up to too many jobs running at the same time on the same machine. The problem happened infrequently enough (MAYBE once a month) that we'd just restart it and everything would be fine. The problem has recently gotten much worse. All of our jobs which request a max heap size of 2048m fail immediately almost every time and need to get restarted several times before completing. We've gone out to individual machines and tried executing them manually with the same result. Debugging It turns out that the problem only exists for our SuSE boxes. The reason it has been happening more frequently is becuase we've been adding more machines, and the new ones are SuSE. 'cat /proc/version' on the SuSE boxes give us: Linux version 2.6.5-7.244-bigsmp (geeko@buildhost) (gcc version 3.3.3 (SuSE Linux)) #1 SMP Mon Dec 12 18:32:25 UTC 2005 'cat /proc/version' on the RedHat boxes give us: Linux version 2.4.21-32.0.1.ELsmp ([email protected]) (gcc version 3.2.3 20030502 (Red Hat Linux 3.2.3-52)) #1 SMP Tue May 17 17:52:23 EDT 2005 'uname -a' gives us the following on BOTH types of machines: UTC 2005 i686 i686 i386 GNU/Linux No jobs are running on the machine, and no other processes are utilizing much memory. All of the processes currently running might be using 100mb total. 'top' currently shows the following: Mem: 4146528k total, 3536360k used, 610168k free, 132136k buffers Swap: 4194288k total, 0k used, 4194288k free, 3283908k cached 'vmstat' currently shows the following: procs -----------memory---------- ---swap-- -----io---- --system-- ----cpu---- r b swpd free buff cache si so bi bo in cs us sy id wa 0 0 0 610292 132136 3283908 0 0 0 2 26 15 0 0 100 0 If we kick off a job with the following command line (Max Heap of 1850mb) it starts fine: java/bin/java -Xmx1850M -cp helloworld.jar HelloWorld Hello World If we bump up the max heap size to 1875mb it fails: java/bin/java -Xmx1875M -cp helloworld.jar HelloWorld Error occurred during initialization of VM Could not reserve enough space for object heap Could not create the Java virtual machine. It's quite clear that the memory currently being used is for Buffering/Caching and that's why so little is being displayed as 'free'. What isn't clear is why there is a magical 1850mb line where anything higher means Java can't start. Any explanations would be greatly appreciated.

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• Evidence-Based-Scheduling - are estimations only as accurate as the work-plan they're based on?

  - by Assaf Lavie

  I've been using FogBugz's Evidence Based Scheduling (for the uninitiated, Joel explains) for a while now and there's an inherent problem I can't seem to work around. The system is good at telling me the probability that a given project will be delivered at some date, given the detailed list of tasks that comprise the project. However, it does not take into account the fact that during development additional tasks always pop up. Now, there's the garbage-can approach of creating a generic task/scheduled-item for "last minute hacks" or "integration tasks", or what have you, but that clearly goes against the idea of aggregating the estimates of many small cases. It's often the case that during the development stage of a project you realize that there's a whole area your planning didn't cover, because, well, that's the nature of developing stuff that hasn't been developed before. So now your ~3 month project may very well turn into a 6 month project, but not because your estimations were off (you could be the best estimator in the world, for those task the comprised your initial work plan); rather because you ended up adding a whole bunch of new tasks that weren't there to begin with. EBS doesn't help you with that. It could, theoretically (I guess). It could, perhaps, measure the amount of work you add to a project over time and take that into consideration when estimating the time remaining on a given project. Just a thought. In other words, EBS works on a task basis, but not on a project/release basis - but the latter is what's important. It's what your boss typically cares about - delivery date, not the time it takes to finish each task along the way, and not the time it would have taken, if your planning was perfect. So the question is (yes, there's a question here, don't close it): What's your methodology when it comes to using EBS in FogBugz and how do you solve the problem above, which seems to be a main cause of schedule delays and mispredictions? Edit Some more thoughts after reading a few answers: If it comes down to having to choose which delivery date you're comfortable presenting to your higher-ups by squinting at the delivery-probability graph and choosing 80%, or 95%, or 60% (based on what, exactly?) then we've resorted to plain old buffering/factoring of our estimates. In which case, couldn't we have skipped the meticulous case by case hour-sized estimation effort step? By forcing ourselves to break down tasks that take more than a day into smaller chunks of work haven't we just deluded ourselves into thinking our planning is as tight and thorough as it could be? People may be consistently bad estimators that do not even learn from their past mistakes. In that respect, having an EBS system is certainly better than not having one. But what can we do about the fact that we're not that good in planning as well? I'm not sure it's a problem that can be solved by a similar system. Our estimates are wrong because of tendencies to be overly optimistic/pessimistic about certain tasks, and because of neglect to account for systematic delays (e.g. sick days, major bug crisis) - and usually not because we lack knowledge about the work that needs to be done. Our planning, on the other hand, is often incomplete because we simply don't have enough knowledge in this early stage; and I don't see how an EBS-like system could fill that gap. So we're back to methodology. We need to find a way to accommodate bad or incomplete work plans that's better than voodoo-multiplication.

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• Throwing a C++ exception from inside a Linux Signal handler

  - by SoapBox

  As a thought experiment more than anything I am trying to get a C++ exception thrown "from" a linux signal handler for SIGSEGV. (I'm aware this is not a solution to any real world SIGSEGV and should never actually be done, but I thought I would try it out after being asked about it, and now I can't get it out of my head until I figure out how to do it.) Below is the closest I have come, but instead of the signal being caught properly, terminate() is being called as if no try/catch block is available. Anyone know why? Or know a way I can actually get a C++ exception from a signal handler? The code (beware, the self modifying asm limits this to running on x86_64 if you're trying to test it): #include <iostream> #include <stdexcept> #include <signal.h> #include <stdint.h> #include <errno.h> #include <string.h> #include <sys/mman.h> using namespace std; uint64_t oldaddr = 0; void thrower() { cout << "Inside thrower" << endl; throw std::runtime_error("SIGSEGV"); } void segv_handler(int sig, siginfo_t *info, void *pctx) { ucontext_t *context = (ucontext_t *)pctx; cout << "Inside SIGSEGV handler" << endl; oldaddr = context->uc_mcontext.gregs[REG_RIP]; uint32_t pageSize = (uint32_t)sysconf(_SC_PAGESIZE); uint64_t bottomOfOldPage = (oldaddr/pageSize) * pageSize; mprotect((void*)bottomOfOldPage, pageSize*2, PROT_READ|PROT_WRITE|PROT_EXEC); // 48 B8 xx xx xx xx xx xx xx xx = mov rax, xxxx *((uint8_t*)(oldaddr+0)) = 0x48; *((uint8_t*)(oldaddr+1)) = 0xB8; *((int64_t*)(oldaddr+2)) = (int64_t)thrower; // FF E0 = jmp rax *((uint8_t*)(oldaddr+10)) = 0xFF; *((uint8_t*)(oldaddr+11)) = 0xE0; } void func() { try { *(uint32_t*)0x1234 = 123456789; } catch (...) { cout << "caught inside func" << endl; throw; } } int main() { cout << "Top of main" << endl; struct sigaction action, old_action; action.sa_sigaction = segv_handler; sigemptyset(&action.sa_mask); action.sa_flags = SA_SIGINFO | SA_RESTART | SA_NODEFER; if (sigaction(SIGSEGV, &action, &old_action)<0) cerr << "Error setting handler : " << strerror(errno) << endl; try { func(); } catch (std::exception &e) { cout << "Caught : " << e.what() << endl; } cout << "Bottom of main" << endl << endl; } The actual output: Top of main Inside SIGSEGV handler Inside thrower terminate called after throwing an instance of 'std::runtime_error' what(): SIGSEGV Aborted Expected output: Top of main Inside thrower caught inside func Caught : SIGSEGV Bottom of main

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• Django: TypeError: 'str' object is not callable, referer: http://xxx

  - by user705415

  I've been wondering why when I set the settings.py of my django project 'arvindemo' debug = Flase and deploy it on Apache with mod_wsgi, I got the 500 Internal Server Error. Env: Django 1.4.0 Python 2.7.2 mod_wsgi 2.8 OS centOS Here is the recap: Visit the homepage, go to sub page A/B/C/D, and fill some forms, then submit it to the Apache server. Once click 'submit' button, I will get the '500 Internal Server Error', and the error_log listed below(Traceback): [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] Traceback (most recent call last): [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/python2.7/lib/python2.7/site-packages/django/core/handlers/wsgi.py", line 241, in __call__ [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] response = self.get_response(request) [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/python2.7/lib/python2.7/site-packages/django/core/handlers/base.py", line 179, in get_response [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] response = self.handle_uncaught_exception(request, resolver, sys.exc_info()) [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/python2.7/lib/python2.7/site-packages/django/core/handlers/base.py", line 224, in handle_uncaught_exception [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] if resolver.urlconf_module is None: [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/python2.7/lib/python2.7/site-packages/django/core/urlresolvers.py", line 323, in urlconf_module [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] self._urlconf_module = import_module(self.urlconf_name) [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/python2.7/lib/python2.7/site-packages/django/utils/importlib.py", line 35, in import_module [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] __import__(name) [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] File "/opt/web/django/arvindemo/arvindemo/../arvindemo/urls.py", line 23, in <module> [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] url(r'^submitPage$', name=submitPage), [Tue Apr 10 10:07:20 2012] [error] [client 122.198.133.250] TypeError: url() takes at least 2 arguments (2 given) When using django runserver, I set arvindemo.settings debug = True, everything is OK. But things changed once I set debug = Flase. Here is my views.py from django.http import HttpResponseRedirect from django.http import HttpResponse, HttpResponseServerError from django.shortcuts import render_to_response import datetime, string from user_info.models import * from django.template import Context, loader, RequestContext import settings def hello(request): return HttpResponse("hello girl") def helpPage(request): return render_to_response('kktHelp.html') def server_error(request, template_name='500.html'): return render_to_response(template_name, context_instance = RequestContext(request) ) def page404(request): return render_to_response('404.html') def submitPage(request): post = request.POST Mall = 'goodsName' Contest = 'ojs' Presentation = 'addr' WeatherReport = 'city' Habit = 'task' if Mall in post: return submitMall(request) elif Contest in post: return submitContest(request) elif Presentation in post: return submitPresentation(request) elif Habit in post: return submitHabit(request) elif WeatherReport in post: return submitWeather(request) else: return HttpResponse(request.POST) return HttpResponseRedirect('404') def submitXXX(): ..... def xxxx(): .... Here comes the urls.py from django.conf.urls import patterns, include, url from views import * from django.conf import settings handler500 = 'server_error' urlpatterns = patterns('', url(r'^hello/$', hello), # hello world url(r'^$', homePage), url(r'^time/$', getTime), url(r'^time/plus/(\d{1,2})/$', hoursAhead), url(r'^Ttime/$', templateGetTime), url(r'^Mall$', templateMall), url(r'^Contest$', templateContest), url(r'^Presentation$', templatePresentation), url(r'^Habit$', templateHabit), url(r'^Weather$', templateWeather), url(r'^Help$', helpPage), url(r'^404$', page404), url(r'^500$', server_error), url(r'^submitPage$', submitPage), url(r'^submitMall$', submitMall), url(r'^submitContest$', submitContest), url(r'^submitPresentation$', submitPresentation), url(r'^submitHabit$', submitHabit), url(r'^submitWeather$', submitWeather), url(r'^terms$', terms), url(r'^privacy$', privacy), url(r'^thanks$', thanks), url(r'^about$', about), url(r'^static/(?P<path>.*)$','django.views.static.serve',{'document_root':settings.STATICFILES_DIRS}), ) I'm sure there is no syntax error in my django project,cause when I use django runserver, everything is fine. Anyone can help ? Best regards

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• What container type provides better (average) performance than std::map?

  - by Truncheon

  In the following example a std::map structure is filled with 26 values from A - Z (for key) and 0 - 26 for value. The time taken (on my system) to lookup the last entry (10000000 times) is roughly 250 ms for the vector, and 125 ms for the map. (I compiled using release mode, with O3 option turned on for g++ 4.4) But if for some odd reason I wanted better performance than the std::map, what data structures and functions would I need to consider using? I apologize if the answer seems obvious to you, but I haven't had much experience in the performance critical aspects of C++ programming. UPDATE: This example is rather trivial and hides the true complexity of what I'm trying to achieve. My real world project is a simple scripting language that uses a parser, data tree, and interpreter (instead of a VM stack system). I need to use some kind of data structure (perhaps map) to store the variables names created by script programmers. These are likely to be pretty randomly named, so I need a lookup method that can quickly find a particular key within a (probably) fairly large list of names. #include <ctime> #include <map> #include <vector> #include <iostream> struct mystruct { char key; int value; mystruct(char k = 0, int v = 0) : key(k), value(v) { } }; int find(const std::vector<mystruct>& ref, char key) { for (std::vector<mystruct>::const_iterator i = ref.begin(); i != ref.end(); ++i) if (i->key == key) return i->value; return -1; } int main() { std::map<char, int> mymap; std::vector<mystruct> myvec; for (int i = 'a'; i < 'a' + 26; ++i) { mymap[i] = i - 'a'; myvec.push_back(mystruct(i, i - 'a')); } int pre = clock(); for (int i = 0; i < 10000000; ++i) { find(myvec, 'z'); } std::cout << "linear scan: milli " << clock() - pre << "\n"; pre = clock(); for (int i = 0; i < 10000000; ++i) { mymap['z']; } std::cout << "map scan: milli " << clock() - pre << "\n"; return 0; }

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• Where do I start ?

  - by Panthe

  Brief History: Just graduated high school, learned a bit of python and C++, have no friends with any helpful computer knowledge at all. Out of anyone i met in my school years I was probably the biggest nerd, but no one really knew. I consider my self to have a vast amount of knowledge on computers and tech then the average person. built/fixed tons of computers, and ability to troubleshoot pretty much any problem I came across. Now that high school is over, Ive really been thinking about my career. Loving, living computers for the past 15 years of my life I decided to take my ability's and try to learn computer programming, why I didn't start earlier I don't know, seems to be big mistake on my part... Doing some research I concluded that Python was the first programming language I should learn, since it was high level and easier to understand then C++ and Java. I also knew that to become good at what I did I needed to know more then just 2 or 3 languages, which didn't seem like a big problem considering once I learned the way Python worked, mainly syntax changed, and the rest would come naturally. I watched a couple of youtube videos, downloaded some book pdf's and snooped around from some tutorials here and there to get the hang of what to do. A two solid weeks had passed of trying to understand the syntax, create small programs that used the basic functions and understanding how it worked, I think i have got the hang of it. It breaks down into what ive been dealing with all this time (although i kinda knew) is that, input,output, loops, functions and other things derived from 0's and 1's storing data and recalling it, ect. (A VERY BASIC IDEA). Ive been able to create small programs, Hangman, file storing, temperature conversion, Caeser Cipher decode/encoding, Fibonacci Sequence and more, which i can create and understand how each work. Being 2 weeks into this, I have learned alot. Nothing at all compared to what i should be learning in the years to come if i get a grip on what I'm doing. While doing these programs I wont stop untill I've done doing a practice problem on a book, which embarresing enough will take me a couple hour depending on the complexity of it. I absolutly will not put aside the challenge until its complete, WHICH CAN BE EXTREMELY DRAINING, ive tried most problems without cheating and reached success, which makes me feel extremely proud of my self after completing something after much trial and error. After all this I have met the demon, alogrithm's which seem to be key to effiecent code. I cant seem to rap my head around some of the computer codes people put out there using numbers, and sometimes even basic functions, I have been able to understand them after a while but i know there are alot more complex things to come, considering my self smart, functions that require complex codes, actually hurt my brain. NOTHING EVER IN LIFE HURT MY BRAIN....... not even math classes in highschool, trying to understand some of the stuff people put out there makes me feel like i have a mental disadvantage lol... i still walk forward though, crossing my fingers that the understanding will come with time. Sorry if is this is long i just wish someone takes all these things into consideration when answering my question. even through all these downsides im still pushing through and continuing to try and get good at this, i know reading these tutorials wont make me any good unless i can become creative and make my own, understand other peoples programs, so this leads me to the simple question i could have asked in the beginning..... WHERE IN THE WORLD DO I START ? Ive been trying to find out how to understand some of the open source projects, how i can work with experianced coders to learn from them and help them, but i dont think thats even possible by the way how far people's knowledge is compared to me, i have no freinds who i can learn from, can someone help me and guide me into the right direction.. i have a huge motivation to get good at coding, anything information would be extremely helpful

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• Trying get dynamic content hole-punched through Magento's Full Page Cache

  - by rlflow

  I am using Magento Enterprise 1.10.1.1 and need to get some dynamic content on our product pages. I am inserting the current time in a block to quickly see if it is working, but can't seem to get through full page cache. I have tried a variety of implementations found here: http://tweetorials.tumblr.com/post/10160075026/ee-full-page-cache-hole-punching http://oggettoweb.com/blog/customizations-compatible-magento-full-page-cache/ http://magentophp.blogspot.com/2011/02/magento-enterprise-full-page-caching.html (http://www.exploremagento.com/magento/simple-custom-module.php - custom module) Any solutions, thoughts, comments, advice is welcome. here is my code: app/code/local/Fido/Example/etc/config.xml <?xml version="1.0"?> <config> <modules> <Fido_Example> <version>0.1.0</version> </Fido_Example> </modules> <global> <blocks> <fido_example> <class>Fido_Example_Block</class> </fido_example> </blocks> </global> </config> app/code/local/Fido/Example/etc/cache.xml <?xml version="1.0" encoding="UTF-8"?> <config> <placeholders> <fido_example> <block>fido_example/view</block> <name>example</name> <placeholder>CACHE_TEST</placeholder> <container>Fido_Example_Model_Container_Cachetest</container> <cache_lifetime>86400</cache_lifetime> </fido_example> </placeholders> </config> app/code/local/Fido/Example/Block/View.php <?php /** * Example View block * * @codepool Local * @category Fido * @package Fido_Example * @module Example */ class Fido_Example_Block_View extends Mage_Core_Block_Template { private $message; private $att; protected function createMessage($msg) { $this->message = $msg; } public function receiveMessage() { if($this->message != '') { return $this->message; } else { $this->createMessage('Hello World'); return $this->message; } } protected function _toHtml() { $html = parent::_toHtml(); if($this->att = $this->getMyCustom() && $this->getMyCustom() != '') { $html .= '<br />'.$this->att; } else { $now = date('m-d-Y h:i:s A'); $html .= $now; $html .= '<br />' ; } return $html; } } app/code/local/Fido/Example/Model/Container/Cachetest.php <?php class Fido_Example_Model_Container_Cachetest extends Enterprise_PageCache_Model_Container_Abstract { protected function _getCacheId() { return 'HOMEPAGE_PRODUCTS' . md5($this->_placeholder->getAttribute('cache_id') . $this->_getIdentifier()); } protected function _renderBlock() { $blockClass = $this->_placeholder->getAttribute('block'); $template = $this->_placeholder->getAttribute('template'); $block = new $blockClass; $block->setTemplate($template); return $block->toHtml(); } protected function _saveCache($data, $id, $tags = array(), $lifetime = null) { return false; } } app/design/frontend/enterprise/[mytheme]/template/example/view.phtml <?php /** * Fido view template * * @see Fido_Example_Block_View * */ ?> <div> <?php echo $this->receiveMessage(); ?> </span> </div> snippet from app/design/frontend/enterprise/[mytheme]/layout/catalog.xml <reference name="content"> <block type="catalog/product_view" name="product.info" template="catalog/product/view.phtml"> <block type="fido_example/view" name="product.info.example" as="example" template="example/view.phtml" />

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• jQuery: Div elements are not showing up

  - by Legend

  I am adapting the Coverflow technique to work with a div. Following is the html: <html> <head> <meta http-equiv="Content-type" content="text/html; charset=utf-8" /> <style type="text/css" media="screen"> body,html { margin: 0; padding: 0; background: #000; height: 100%; color: #eee; font-family: Arial; font-size: 10px; } div.magnifyme { height: 80px; padding: 80px; position: absolute; top: 0px; left: 0px; width: 8000px; } div.wrapper { margin: 0px; height: 470px; /*border: 2px solid #999;*/ overflow: hidden; padding-left: 40px; right: 1px; width: 824px; position: relative; } div.container {position: relative; width: 854px; height: 480px; background: #000; margin: auto;} div.nav {position: absolute; top: 10px; width: 20%; height: 10%; right: 1px; } div.magnifyme div { position: absolute; width: 300px; height: 280px; float: left; margin: 5px; position: relative; border: 2px solid #999; background: #500; } </style> <script type="text/javascript" src="jquery-1.3.2.js"></script> <script type="text/javascript" src="ui.coverflow.js"></script> <script type="text/javascript"> $(function() { $("div.magnifyme").coverflow(); $("#add").click(function() { location.reload(); $(".magnifyme").append("<div id=\"div5\">hello world</div>"); $("div.magnifyme").coverflow(); }); }); </script> </head> <body> <div class="container"> <div class="wrapper"> <div class="magnifyme"> <div id="div0">This is div 0</div> <div id="div1">This is div 1</div> <div id="div2">This is div 2</div> <div id="div3">This is div 3</div> <div id="div4">This is div 4</div> </div> </div> <div class="nav"> <button type="button" id="add">Add to Deck</button> </div> </div> </body> </html> The coverflow function (included as a js file in the head section) is here. When I click the button, I was expecting it to add a DIV to the already present deck. For some reason, it doesn't show the newly added DIV. I tried calling the coverflow() function after I added the new element but that didn't work either. Is there any way I can make this work?

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• xsl:include fails after upgrading PHP version; is libxml/libxslt version mismatch the issue?

  - by Wiseguy

  I'm running Windows XP with the precompiled PHP binaries available from windows.php.net. I upgraded from PHP 5.2.5 to PHP 5.2.16, and now the xsl:includes in some of my stylesheets stopped working. Testing each version in succession, I discovered that it worked up through 5.2.8 and does not work in 5.2.9+. I now get the following three errors for each xsl:include. Warning: XSLTProcessor::importStylesheet() [xsltprocessor.importstylesheet]: I/O warning : failed to load external entity "file%3A/C%3A/path/to/included/stylesheet.xsl" in ... on line 227 Warning: XSLTProcessor::importStylesheet() [xsltprocessor.importstylesheet]: compilation error: file file%3A//C%3A/path/to/included/stylesheet.xsl line 36 element include in ... on line 227 Warning: XSLTProcessor::importStylesheet() [xsltprocessor.importstylesheet]: xsl:include : unable to load file%3A/C%3A/path/to/included/stylesheet.xsl in ... on line 227 I presume this is because it cannot find the specified file. Many of the includes are in the same directory as the stylesheet being transformed and have no directory in the path, i.e. <xsl:include href="fileInSameDir.xsl">. Interestingly, in the first and third errors, it is displaying the file:// protocol with only one slash instead of the correct two. I'm guessing that's the problem. (When I hard-code a full path using "file:/" it fails, but when I hard-code a full path with "file://" it works.) But what could cause that? A bug in libxslt/libxml? I also found an apparent version mismatch between libxml and the version of libxml that libxslt was compiled against. 5.2.5 libxml Version = 2.6.26 libxslt compiled against libxml Version = 2.6.26 5.2.8 libxml Version = 2.6.32 libxslt compiled against libxml Version = 2.6.32 === it breaks in versions starting at 5.2.9 === 5.2.9 libxml Version = 2.7.3 libxslt compiled against libxml Version = 2.6.32 5.2.16 libxml Version = 2.7.7 libxslt compiled against libxml Version = 2.6.32 Up until PHP 5.2.9, libxslt was compiled against the same version of libxml that was included with PHP. But starting with PHP 5.2.9, libxslt was compiled against an older version of libxml than that which was included with PHP. Is this a problem with the distributed binaries or just a coincidence? To test this, I imagine PHP could be built with different versions of libxml/libxslt to see which combinations work or don't. Unfortunately, I'm out of my element in a Windows world, and building PHP on Windows seems over my head. Regrettably, I've been thus far unable to reproduce this problem with an example outside my app, so I'm struggling to narrow it down and can't submit a specific bug. So, do you think it is caused by a version mismatch problem in the distributed binaries? a bug introduced in PHP 5.2.9? a bug introduced in libxml 2.7? something else? I'm stumped. Any thoughts that could point me in the right direction are greatly appreciated. Thanks.

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• Help with Silverlight Sockets and Message delivery

  - by pixel3cs

  There are 4 months since I stopped developing my Silverlight Multiplayer Chess game. The problem was a bug wich I couldn't reproduce. Sice I got some free time this week I managed to discover the problem and I am now able to reproduce the bug. It seems that if I send 10 messages from client, one after another, with no delay between them, just like in the below example // when I press Enter, the client will 10 messages with no delay between them private void textBox_KeyDown(object sender, KeyEventArgs e) { if (e.Key == Key.Enter && textBox.Text.Length > 0) { for (int i = 0; i < 10; i++) { MessageBuilder mb = new MessageBuilder(); mb.Writer.Write((byte)GameCommands.NewChatMessageInTable); mb.Writer.Write(string.Format("{0}{2}: {1}", ClientVars.PlayerNickname, textBox.Text, i)); SendChatMessageEvent(mb.GetMessage()); //System.Threading.Thread.Sleep(100); } textBox.Text = string.Empty; } } // the method used by client to send a message to server public void SendData(Message message) { if (socket.Connected) { SocketAsyncEventArgs myMsg = new SocketAsyncEventArgs(); myMsg.RemoteEndPoint = socket.RemoteEndPoint; byte[] buffer = message.Buffer; myMsg.SetBuffer(buffer, 0, buffer.Length); socket.SendAsync(myMsg); } else { string err = "Server does not respond. You are disconnected."; socket.Close(); uiContext.Post(this.uiClient.ProcessOnErrorData, err); } } // the method used by server to receive data from client private void OnDataReceived(IAsyncResult async) { ClientSocketPacket client = async.AsyncState as ClientSocketPacket; int count = 0; try { if (client.Socket.Connected) count = client.Socket.EndReceive(async); // THE PROBLEM IS HERE // IF SERVER WAS RECEIVE ALL MESSAGES SEPARATELY, ONE BY ONE, THE COUNT // WAS ALWAYS 15, BUT BECAUSE THE SERVER RECEIVE 3 MESSAGES IN 1, THE COUNT // IS SOMETIME 45 } catch { HandleException(client); } client.MessageStream.Write(client.Buffer, 0, count); Message message; while (client.MessageStream.Read(out message)) { message.Tag = client; ThreadPool.QueueUserWorkItem(new WaitCallback(this.processingThreadEvent.ServerGotData), message); totalReceivedBytes += message.Buffer.Length; } try { if (client.Socket.Connected) client.Socket.BeginReceive(client.Buffer, 0, client.Buffer.Length, 0, new AsyncCallback(OnDataReceived), client); } catch { HandleException(client); } } there are sent only 3 big messages, and every big message contain 3 or 4 small messages. This is not the behavior I want. If I put a 100 milliseconds delay between message delivery, everything is work fine, but in a real world scenario users can send messages to server even at 1 millisecond between them. Are there any settings to be done in order to make the client send only one message at a time, or Even if I receive 3 messages in 1, are they full messages all the time (I dont't want to receive 2.5 messages in one big message) ? because if they are, I can read them and treat this new situation

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• Languages and VMs: Features that are hard to optimize and why

  - by mrjoltcola

  I'm doing a survey of features in preparation for a research project. Name a mainstream language or language feature that is hard to optimize, and why the feature is or isn't worth the price paid, or instead, just debunk my theories below with anecdotal evidence. Before anyone flags this as subjective, I am asking for specific examples of languages or features, and ideas for optimization of these features, or important features that I haven't considered. Also, any references to implementations that prove my theories right or wrong. Top on my list of hard to optimize features and my theories (some of my theories are untested and are based on thought experiments): 1) Runtime method overloading (aka multi-method dispatch or signature based dispatch). Is it hard to optimize when combined with features that allow runtime recompilation or method addition. Or is it just hard, anyway? Call site caching is a common optimization for many runtime systems, but multi-methods add additional complexity as well as making it less practical to inline methods. 2) Type morphing / variants (aka value based typing as opposed to variable based) Traditional optimizations simply cannot be applied when you don't know if the type of someting can change in a basic block. Combined with multi-methods, inlining must be done carefully if at all, and probably only for a given threshold of size of the callee. ie. it is easy to consider inlining simple property fetches (getters / setters) but inlining complex methods may result in code bloat. The other issue is I cannot just assign a variant to a register and JIT it to the native instructions because I have to carry around the type info, or every variable needs 2 registers instead of 1. On IA-32 this is inconvenient, even if improved with x64's extra registers. This is probably my favorite feature of dynamic languages, as it simplifies so many things from the programmer's perspective. 3) First class continuations - There are multiple ways to implement them, and I have done so in both of the most common approaches, one being stack copying and the other as implementing the runtime to use continuation passing style, cactus stacks, copy-on-write stack frames, and garbage collection. First class continuations have resource management issues, ie. we must save everything, in case the continuation is resumed, and I'm not aware if any languages support leaving a continuation with "intent" (ie. "I am not coming back here, so you may discard this copy of the world"). Having programmed in the threading model and the contination model, I know both can accomplish the same thing, but continuations' elegance imposes considerable complexity on the runtime and also may affect cache efficienty (locality of stack changes more with use of continuations and co-routines). The other issue is they just don't map to hardware. Optimizing continuations is optimizing for the less-common case, and as we know, the common case should be fast, and the less-common cases should be correct. 4) Pointer arithmetic and ability to mask pointers (storing in integers, etc.) Had to throw this in, but I could actually live without this quite easily. My feelings are that many of the high-level features, particularly in dynamic languages just don't map to hardware. Microprocessor implementations have billions of dollars of research behind the optimizations on the chip, yet the choice of language feature(s) may marginalize many of these features (features like caching, aliasing top of stack to register, instruction parallelism, return address buffers, loop buffers and branch prediction). Macro-applications of micro-features don't necessarily pan out like some developers like to think, and implementing many languages in a VM ends up mapping native ops into function calls (ie. the more dynamic a language is the more we must lookup/cache at runtime, nothing can be assumed, so our instruction mix is made up of a higher percentage of non-local branching than traditional, statically compiled code) and the only thing we can really JIT well is expression evaluation of non-dynamic types and operations on constant or immediate types. It is my gut feeling that bytecode virtual machines and JIT cores are perhaps not always justified for certain languages because of this. I welcome your answers.

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• Javascript phsyics in a 2d space

  - by eroo

  So, I am working on teaching myself Canvas (HTML5) and have most of a simple game engine coded up. It is a 2d representation of a space scene (planets, stars, celestial bodies, etc). My default "Sprite" class has a frame listener like such: "baseClass" contains a function that allows inheritance and applies "a" to "this.a". So, "var aTest = new Sprite({foo: 'bar'});" would make "aTest.foo = 'bar'". This is how I expose my objects to each other. { Sprite = baseClass.extend({ init: function(a){ baseClass.init(this, a); this.fields = new Array(); // list of fields of gravity one is in. Not sure if this is a good idea. this.addFL(function(tick){ // this will change to be independent of framerate soon. // and this is where I need help // gobjs is an array of all the Sprite objects in the "world". for(i = 0; i < gobjs.length; i++){ // Make sure its got setup correctly, make sure it -wants- gravity, and make sure it's not -this- sprite. if(typeof(gobjs[i].a) != undefined && !gobjs[i].a.ignoreGravity && gobjs[i].id != this.id){ // Check if it's within a certain range (obviously, gravity doesn't work this way... But I plan on having a large "space" area, // And I can't very well have all objects accounted for at all times, can I? if(this.distanceTo(gobjs[i]) < this.s.size*10 && gobjs[i].fields.indexOf(this.id) == -1){ gobjs[i].fields.push(this.id); } } } for(i = 0; i < this.fields.length; i++){ distance = this.distanceTo(gobjs[this.fields[i]]); angletosun = this.angleTo(gobjs[this.fields[i]])*(180/Math.PI); // .angleTo works very well, returning the angle in radians, which I convert to degrees here. // I have no idea what should happen here, although through trial and error (and attempting to read Maths papers on gravity (eeeeek!)), this sort of mimics gravity. // angle is its orientation, currently I assign a constant velocity to one of my objects, and leave the other static (it ignores gravity, but still emits it). this.a.angle = angletosun+(75+(distance*-1)/5); //todo: omg learn math if(this.distanceTo(gobjs[this.fields[i]]) > gobjs[this.fields[i]].a.size*10) this.fields.splice(i); // out of range, stop effecting. } }); } }); } Thanks in advance. The real trick is that one line: { this.a.angle = angletosun+(75+(distance*-1)/5); } This is more a physics question than Javascript, but I've searched and searched and read way to many wiki articles on orbital mathematics. It gets over my head very quickly. Edit: There is a weirdness with the SO formatting; forgives me, I is noobie.

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• Decentralized synchronized secure data storage

  - by Alberich

  Introduction Hi, I am going to ask a question which seems utopic for me, but I need to know if there is a way to achieve what I need. And if not, I need to know why not. The idea Suppose I have a database structure, in MySql. I want to create some solution to allow anyone (no matter who, no matter where) to have a synchronized copy (updated clone) of this database (with its content) Well, and it is not going to be just one synchronized copy, it could (and should) be a multiple replication (supposing the basic, this means, for example, ten copies all over the world) And, the most important thing: It must be secure. By secure I mean only real-accepted transactions will be synchronized with all the others (no matter how many) database copies/clones. Note: Since it would be quite difficult to make the synchronization in real-time, I will design everything to make this feature dispensable. So it is not required. My auto-suggestion This is how I am thinking to manage it: Time identifiers and Updates checking: Every action (insert, update, delete...) will be stored as the action instruction itself, associated to the time identifier. [I think better than a DATETIME field, it'll be an INT one, with the number of miliseconds passed from 1st january 2013 on, for example]. So each copy is going to ask to the "neighbour copy" for new actions done since last update, and execute them after checking they are allowed. Problem 1: the "neighbour copy" could be outdated too. Solution 1: do not ask just one neighbour, create a random list with some of the copies/clones and ask them for news (I could avoid the list and ask ALL the clones for updates, but this will be inefficient if clones number ascends too much). Problem 2: Real-time global synchronization is not active. What if... Someone at CLONE_ENTERPRISING inserts a row into TABLE. ... this row goes to every clone ... Someone at CLONE_FIXEMALL deletes this row. ... and at the same time, somewhere in an outdated clone ... Someone at CLONE_DROPOUT edits this row (now inexistent at the other clones) Solution 2: easy stuff, force a GLOBAL synchronization before doing any new "depending-on-third-data action" (edit, for example). This global synch. will be unnecessary when making an INSERT, for instance. Note: Well, someone could have some fun, and make the same insert in two clones... since they're not getting updated in real-time, this row will exist twice. But, it's the same as when we have one single database, in some needed cases we check if there is an existing same-row before doing the final action. Not a problem. Problem 3: It is possible to edit the code and do not filter actions, so someone could spread instructions to delete everything, or just make some trolling activity. This is not a problem, since good clones will always be somewhere. Those who got bad won't interest anymore. I really appreciate if you read. I know this is not the perfect solution, it has possibly hundred of holes, but it is my basic start. I will now appreciate anything you can teach me now. Thanks a lot. PS.: It could be that all this I am trying already exists and has its own name. Sorry for asking then (I'd anyway thank this name, if it exists)

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• Recommended integration mechanism for bi-directional, authenticated, encrypted connection in C clien

  - by rcampbell

  Let me first give an example. Imagine you have a single server running a JVM application. This server keeps a collection of N equations, once for each client: Client #1: 2x Client #2: 1 + y Client #3: z/4 This server includes an HTTP interface so that random visitors can type https://www.acme.com/client/3 int their browsers and see the latest evaluated result of z/4. The tricky part is that either the client or the server may change the variable value at any time, informing the other party immediately. More specifically, Client #3 - a C app - can initially tell the server that z = 20. An hour later that same client informs the server that z = 23. Likewise the server can later inform the client that z = 28. As caf pointed out in the comments, there can be a race condition when values are changed by the client and server simultaneously. The solution would be for both client and server to send the operation performed in their message, which would need to be executed by the other party. To keep things simple, let's limit the operations to (commutative) addition, allowing us to disregard message ordering. For example, the client seeds the server with z = 20: server:z=20, client:z=20 server sends {+3} message (so z=23 locally) & client sends {-2} message (so z=18 locally) at the exact same time server receives {-2} message at some point, adds to his local copy so z=21 client receives {+3} message at some point, adds to his local copy so z=21 As long as all messages are eventually evaluated by both parties, the correct answer will eventually be given to the users of the client and server since we limited ourselves to commutative operations (addition of 3 and -2). This does mean that both client and server can be returning incorrect answers in the time it takes for messages to be exchanged and processed. While undesirable, I believe this is unavoidable. Some possible implementations of this idea include: Open an encrypted, always on TCP socket connection for communication Pros: no additional infrastructure needed, client and server know immediately if there is a problem (disconnect) with the other party, fairly straightforward (except the the encryption), native support from both JVM and C platforms Cons: pretty low-level so you end up writing a lot yourself (protocol, delivery verification, retry-on-failure logic), probably have a lot of firewall headaches during client app installation Asynchronous messaging (ex: ActiveMQ) Pros: transactional, both C & Java integration, free up the client and server apps from needing retry logic or delivery verification, pretty straightforward encryption, easy extensibility via message filters/routers/etc Cons: need additional infrastructure (message server) which must never fail, Database or file system as asynchronous integration point Same pros/cons as above but messier RESTful Web Service Pros: simple, possible reuse of the server's existing REST API, SSL figures out the encryption problem for you (maybe use RSA key a la GitHub for authentication?) Cons: Client now needs to run a C HTTP REST server w/SSL, client and server need retry logic. Axis2 has both a Java and C version, but you may be limited to SOAP. What other techniques should I be evaluating? What real world experiences have you had with these mechanisms? Which do you recommend for this problem and why?

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• android program crashing (new to platform)

  - by mutio

  So it is my first real Android program (!hello world), but i do have java experience.The program compiles fine, but on running it crashes as soon as it opens (tried debugging, but it crashes before it hits my breakpoint). Was looking for any advice from anyone who is more experienced with android. package org.me.tipcalculator; import android.app.Activity; import android.os.Bundle; import android.widget.TextView; import android.view.View; import android.widget.Button; import android.widget.EditText; import java.text.NumberFormat; import android.util.Log; public class TipCalculator extends Activity { public static final String tag = "TipCalculator"; /** Called when the activity is first created. */ @Override public void onCreate(Bundle icicle) { super.onCreate(icicle); setContentView(R.layout.main); final EditText mealpricefield = (EditText) findViewById(R.id.mealprice); final TextView answerfield = (TextView) findViewById(R.id.answer); final Button button = (Button) findViewById(R.id.calculate); button.setOnClickListener(new Button.OnClickListener() { public void onClick(View v) { try { Log.i(tag, "onClick invoked."); String mealprice = mealpricefield.getText().toString(); Log.i(tag, "mealprice is [" + mealprice + "]"); String answer = ""; if (mealprice.indexOf("$") == -1) { mealprice = "$" + mealprice; } float fmp = 0.0F; NumberFormat nf = java.text.NumberFormat.getCurrencyInstance(); fmp = nf.parse(mealprice).floatValue(); fmp *= 1.2; Log.i(tag, "Total Meal Price (unformatted) is [" + fmp + "]"); answer = "Full Price, including 20% Tip: " + nf.format(fmp); answerfield.setText(answer); Log.i(tag, "onClick Complete"); } catch(java.text.ParseException pe){ Log.i (tag ,"Parse exception caught"); answerfield.setText("Failed to parse amount?"); } catch(Exception e){ Log.e (tag ,"Failed to Calculate Tip:" + e.getMessage()); e.printStackTrace(); answerfield.setText(e.getMessage()); } } } ); } Just in case it helps heres the xml <?xml version="1.0" encoding="UTF-8"?> <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent"> <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:text="Android Tip Calculator"/> <EditText android:id="@+id/mealprice" android:layout_width="fill_parent" android:layout_height="wrap_content" android:autoText="true"/> <Button android:id="@+id/calculate" android:layout_width="wrap_content" android:layout_height="wrap_content" android:text="Calculate Tip"/> <TextView android:id= "@+id/answer" android:layout_width="fill_parent" android:layout_height="wrap_content" android:text=""/> </LinearLayout>

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• Are there supposed to be more restrictions on operator->* overloads?

  - by Potatoswatter

  I was perusing section 13.5 after refuting the notion that built-in operators do not participate in overload resolution, and noticed that there is no section on operator->*. It is just a generic binary operator. Its brethren, operator->, operator*, and operator[], are all required to be non-static member functions. This precludes definition of a free function overload to an operator commonly used to obtain a reference from an object. But the uncommon operator->* is left out. In particular, operator[] has many similarities. It is binary (they missed a golden opportunity to make it n-ary), and it accepts some kind of container on the left and some kind of locator on the right. Its special-rules section, 13.5.5, doesn't seem to have any actual effect except to outlaw free functions. (And that restriction even precludes support for commutativity!) So, for example, this is perfectly legal (in C++0x, remove obvious stuff to translate to C++03): #include <utility> #include <iostream> #include <type_traits> using namespace std; template< class F, class S > typename common_type< F,S >::type operator->*( pair<F,S> const &l, bool r ) { return r? l.second : l.first; } template< class T > T & operator->*( pair<T,T> &l, bool r ) { return r? l.second : l.first; } template< class T > T & operator->*( bool l, pair<T,T> &r ) { return l? r.second : r.first; } int main() { auto x = make_pair( 1, 2.3 ); cerr << x->*false << " " << x->*4 << endl; auto y = make_pair( 5, 6 ); y->*(0) = 7; y->*0->*y = 8; // evaluates to 7->*y = y.second cerr << y.first << " " << y.second << endl; } I can certainly imagine myself giving into temp[la]tation. For example, scaled indexes for vector: v->*matrix_width[5] = x; Did the standards committee forget to prevent this, was it considered too ugly to bother, or are there real-world use cases?

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• Please recommend me intermediate-to-advanced Python books to buy.

  - by anonnoir

  I'm in the final year, final semester of my law degree, and will be graduating very soon. (April, to be specific.) But before I begin practice, I plan to take 2 two months off, purely for serious programming study. So I'm currently looking for some Python-related books, gauged intermediate to advanced, which are interesting (because of the subject matter itself) and possibly useful to my future line of work. I've identified 2 possible purchases at the moment: Natural Language Processing with Python. The law deals mostly with words, and I've quite a number of ideas as to where I might go with NLP. Data extraction, summaries, client management systems linked with document templates, etc. Programming Collective Intelligence. This book fascinates me, because I've always liked the idea of machine learning (and I'm currently studying it by the side too, for fun). I'd like to build/play around with Web 2.0 applications; and who knows if I can apply some of the things I learn to my legal work. (E.g. Playground experiments to determine how and under what circumstances judges might be biased, by forcing algorithms to pore through judgments and calculate similarities, etc.) Please feel free to criticize my current choices, but do at least offer or recommend other books that I should read in their place. My budget can deal with 4 books, max. These books will be used heavily throughout the 2 months; I will be reading them back to back, absorbing the explanations given, and hacking away at their code. Also, the books themselves should satisfy 2 main criteria: Application. The book must teach how to solve problems. I like reading theory, but I want to build things and solve problems first. Even playful applications are fine, because games and experiments always have real-world applications sooner or later. Readability. I like reading technical books, no matter how difficult they are. I enjoy the effort and the feeling that you're learning something. But the book shouldn't contain code or explanations that are too cryptic or erratic. Even if it's difficult, the book's content should be accessible with focused reading. Note: I realize that I am somewhat of a beginner to the whole programming thing, so please don't put me down. But from experience, I think it's better to aim up and leave my comfort zone when learning new things, rather than to just remain stagnant the way I am. (At least the difficulty gives me focus: i.e. if a programmer can be that good, perhaps if I sustain my own efforts I too can be as good as him someday.) If anything, I'm also a very determined person, so two months of day-to-night intensive programming study with nothing else on my mind should, I think, give me a bit of a fighting chance to push my programming skills to a much higher level.

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• Redis Cookbook Chat Recipe

  - by Tommy Kennedy

  I am a new starter to Node.Js and Redis. I got the Redis cookbook and was trying out the Chat client & Server recipe. I was wondering if anybody got the code to work or if there is some bug in the code. I dont see where the sent messages from the client get invoked on the server. Any help would be great. Regards, Tom Client Code: <?php ?> <html> <head> <title></title> <script src="http://192.168.0.118:8000/socket.io/socket.io.js"></script> <script src="https://ajax.googleapis.com/ajax/libs/jquery/1.6.1/jquery.min.js"></script> <script> var socket = io.connect('192.168.0.118',{port:8000}); socket.on('message', function(data){ alert(data); //var li = new Element('li').insert(data); //$('messages').insert({top: li}); }); </script> </head> <body> <ul id="messages"> <!-- chat messages go here --> </ul> <form id="chatform" action=""> <input id="chattext" type="text" value="" /> <input type="submit" value="Send" /> </form> <script> $('#chatform').submit(function() { socket.emit('message', 'test'); //$('chattext').val()); $('chattext').val(""); // cleanup the field return false; }); </script> </body> </html> Server Code: var http = require('http'); io = require('socket.io'); redis = require('redis'); rc = redis.createClient(); //rc1 = redis.createClient(); rc.on("connect",function(){ rc.subscribe("chat"); console.log("In Chat Stream"); }); rc.on("message",function (channel,message){ console.log("Sending hope: " + message); //rc1.publish("chat","hope"); socketio.sockets.emit('message',message); }); server = http.createServer(function(req,res){ res.writeHead(200,{'content-type':'text/html'}); res.end('<h1>hello world</h1>'); }); server.listen(8000); var socketio = io.listen(server);

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• Segfault on copy constructor for string

  - by user2756569

  I'm getting a segfault on a line where I'm creating a c++ string with the copy constructor. I've looked at some of the similar issues, but they're all due to passing in a bad c++ string object. I'm just passing in a raw string, so I'm not sure what my issue is. I'll paste the relevant snippets of code (it's taken from several different files, so it might look a bit jumbled). The segfault occurs in the 4th line of the default constructor for the Species class. Species::Species(string _type) { program_length = 0; cout << _type << " 1\n"; cout << type << " 2\n"; type = string(_type); } Grid::Grid(int _width, int _height) { *wall = Species("wall"); *empty = Species("empty"); turn_number = 0; width = _width; height = _height; for(int a= 0; a < 100; a++) for(int b = 0; b< 100; b++) { Creature empty_creature = Creature(*empty,a,b,NORTH,this); (Grid::map)[a][b] = empty_creature; } } int main() { Grid world = Grid(8,8); } class Grid { protected: Creature map[100][100]; int width,height; int turn_number; Species *empty; Species *wall; public: Grid(); Grid(int _width, int _height); void addCreature(Species &_species, int x, int y, Direction orientation); void addWall(int x, int y); void takeTurn(); void infect(int x, int y, Direction orientation, Species &_species); void hop(int x, int y, Direction orientation); bool ifWall(int x, int y, Direction orientation); bool ifEnemy(int x, int y, Direction orientation, Species &_species); bool ifEmpty(int x, int y, Direction orientation); void print(); }; class Species { protected: int program_length; string program[100]; string type; public: species(string _type); void addInstruction(string instruction); bool isWall(); bool isEmpty(); bool isEnemy(Species _enemy); string instructionAt(int index); string getType(); };

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• How do I learn algorithms?

  - by Panthe

  Brief History: Just graduated high school, learned a bit of python and C++, have no friends with any helpful computer knowledge at all. Out of anyone i met in my school years I was probably the biggest nerd, but no one really knew. I consider my self to have a vast amount of knowledge on computers and tech then the average person. built/fixed tons of computers, and ability to troubleshoot pretty much any problem I came across. Now that high school is over, Ive really been thinking about my career. Loving, living computers for the past 15 years of my life I decided to take my ability's and try to learn computer programming, why I didn't start earlier I don't know, seems to be big mistake on my part... Doing some research I concluded that Python was the first programming language I should learn, since it was high level and easier to understand then C++ and Java. I also knew that to become good at what I did I needed to know more then just 2 or 3 languages, which didn't seem like a big problem considering once I learned the way Python worked, mainly syntax changed, and the rest would come naturally. I watched a couple of youtube videos, downloaded some book pdf's and snooped around from some tutorials here and there to get the hang of what to do. A two solid weeks had passed of trying to understand the syntax, create small programs that used the basic functions and understanding how it worked, I think i have got the hang of it. It breaks down into what ive been dealing with all this time (although i kinda knew) is that, input,output, loops, functions and other things derived from 0's and 1's storing data and recalling it, ect. (A VERY BASIC IDEA). Ive been able to create small programs, Hangman, file storing, temperature conversion, Caeser Cipher decode/encoding, Fibonacci Sequence and more, which i can create and understand how each work. Being 2 weeks into this, I have learned alot. Nothing at all compared to what i should be lear ning in the years to come if i get a grip on what I'm doing. While doing these programs I wont stop untill I've done doing a practice problem on a book, which embarresing enough will take me a couple hour depending on the complexity of it. I absolutly will not put aside the challenge until its complete, WHICH CAN BE EXTREMELY DRAINING, ive tried most problems without cheating and reached success, which makes me feel extremely proud of my self after completing something after much trial and error. After all this I have met the demon, alogrithm's which seem to be key to effiecent code. I cant seem to rap my head around some of the computer codes people put out there using numbers, and sometimes even basic functions, I have been able to understand them after a while but i know there are alot more complex things to come, considering my self smart, functions that require complex codes, actually hurt my brain. NOTHING EVER IN LIFE HURT MY BRAIN....... not even math classes in highschool, trying to understand some of the stuff people put out there makes me feel like i have a mental disadvantage lol... i still walk forward though, crossing my fingers that the understanding will come with time. Sorry if is this is long i just wish someone takes all these things into consideration when answering my question. even through all these downsides im still pushing through and continuing to try and get good at this, i know reading these tutorials wont make me any good unless i can become creative and make my own, understand other peoples programs, so this leads me to the simple question i could have asked in the beginning..... WHERE IN THE WORLD DO I START ? Ive been trying to find out how to understand some of the open source projects, how i can work with experianced coders to learn from them and help them, but i dont think thats even possible by the way how far people's knowledge is compared to me, i have no freinds who i can learn from, can someone help me and guide me into the right direction.. i have a huge motivation to get good at coding, anything information would be extremely helpful

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• My xcode mapview wont work when combined with webview

  - by user1715702

  I have a small problem with my mapview. When combining the mapview code with the code for webview, the app does not zoom in on my position correctly (just gives me a world overview where i´m supposed to be somewhere in California - wish I was). And it doesn´t show the pin that I have placed on a specific location. These things works perfectly fine, as long as the code does not contain anything concerning webview. Below you´ll find the code. If someone can help me to solve this, I would be som thankful! ViewController.h #import <UIKit/UIKit.h> #import <MapKit/MapKit.h> #define METERS_PER_MILE 1609.344 @interface ViewController : UIViewController <MKMapViewDelegate>{ BOOL _doneInitialZoom; IBOutlet UIWebView *webView; } @property (weak, nonatomic) IBOutlet MKMapView *_mapView; @property (nonatomic, retain) UIWebView *webView; @end ViewController.m #import "ViewController.h" #import "NewClass.h" @interface ViewController () @end @implementation ViewController @synthesize _mapView; @synthesize webView; - (void)viewDidLoad { [super viewDidLoad]; [_mapView setMapType:MKMapTypeStandard]; [_mapView setZoomEnabled:YES]; [_mapView setScrollEnabled:YES]; MKCoordinateRegion region = { {0.0,0.0} , {0.0,0.0} }; region.center.latitude = 61.097557; region.center.longitude = 12.126545; region.span.latitudeDelta = 0.01f; region.span.longitudeDelta = 0.01f; [_mapView setRegion:region animated:YES]; newClass *ann = [[newClass alloc] init]; ann.title = @"Hjem"; ann.subtitle = @"Her bor jeg"; ann.coordinate = region.center; [_mapView addAnnotation:ann]; NSString *urlAddress = @"http://google.no"; //Create a URL object. NSURL *url = [NSURL URLWithString:urlAddress]; //URL Requst Object NSURLRequest *requestObj = [NSURLRequest requestWithURL:url]; //Load the request in the UIWebView. [webView loadRequest:requestObj]; } - (void)viewDidUnload { [self setWebView:nil]; [self set_mapView:nil]; [super viewDidUnload]; // Release any retained subviews of the main view. } - (BOOL)shouldAutorotateToInterfaceOrientation:(UIInterfaceOrientation)interfaceOrientation { return (interfaceOrientation != UIInterfaceOrientationPortraitUpsideDown); } - (void)viewWillAppear:(BOOL)animated { // 1 CLLocationCoordinate2D zoomLocation; zoomLocation.latitude = 61.097557; zoomLocation.longitude = 12.126545; // 2 MKCoordinateRegion viewRegion = MKCoordinateRegionMakeWithDistance(zoomLocation, 0.5*METERS_PER_MILE, 0.5*METERS_PER_MILE); // 3 MKCoordinateRegion adjustedRegion = [_mapView regionThatFits:viewRegion]; // 4 [_mapView setRegion:adjustedRegion animated:YES]; } @end NewClass.h #import <UIKit/UIKit.h> #import <MapKit/MKAnnotation.h> @interface newClass : NSObject{ CLLocationCoordinate2D coordinate; NSString *title; NSString *subtitle; } @property (nonatomic, assign) CLLocationCoordinate2D coordinate; @property (nonatomic, copy) NSString *title; @property (nonatomic, copy) NSString *subtitle; @end NewClass.m #import "NewClass.h" @implementation newClass @synthesize coordinate, title, subtitle; @end

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• Termite colony simulator using java

  - by ashii

  hi everyone, i hve to design a simulator that will maintain an environment, which consists of a collection of patches arranged in a rectangular grid of arbitrary size. Each patch contains zero or more wood chips. A patch may be occupied by one or more termites or predators, which are mobile entities that live within the world and behave according to simple rules. A TERMITE can pick up a wood chip from the patch that it is currently on, or drop a wood chip that it is carrying. Termites travel around the grid by moving randomly from their current patch to a neighbouring patch, in one of four possible directions. New termites may hatch from eggs, and this is simulated by the appearance of a new termite at a random patch within the environment. A PREDATOR moves in a similar way to termites, and if a predator moves onto a patch that is occupied by a termite, then the predator eats the termite. At initialization, the termites, predators, and wood chips are distributed randomly in the environment. Simulation then proceeds in a loop, and the new state of the environment is obtained at each iteration. i have designed the arena using jpanel but im not able to randomnly place wood,termite and predator in that arena. can any one help me out?? my code for the arena is as following: 01 import java.awt.*; 02 import javax.swing.*; 03 04 public class Arena extends JPanel 05 { 06 private static final int Rows = 8; 07 private static final int Cols = 8; 08 public void paint(Graphics g) 09 { 10 Dimension d = this.getSize(); 11 // don't draw both sets of squares, when you can draw one 12 // fill in the entire thing with one color 13 g.setColor(Color.WHITE); 14 // make the background 15 g.fillRect(0,0,d.width,d.height); 16 // draw only black 17 g.setColor(Color.BLACK); 18 // pick a square size based on the smallest dimension 19 int sqsize = ((d.width<d.height) ? d.width/Cols : d.height/Rows); 20 // loop for rows 21 for (int row=0; row<Rows; row++) 22 { 23 int y = row*sqsize; // y stays same for entire row, set here 24 int x = (row%2)*sqsize; // x starts at 0 or one square in 25 for (int i=0; i<Cols/2; i++) 26 { 27 // you will only be drawing half the squares per row 28 // draw square 29 g.fillRect(x,y,sqsize,sqsize); 30 // move two square sizes over 31 x += sqsize*2; 32 } 33 } 34 35 } 36 37 38 39 public void update(Graphics g) { paint(g); } 40 41 42 43 public static void main (String[] args) 44 { 45 46 JFrame frame = new JFrame("Arena"); 47 frame.setSize(600,400); 48 frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); 49 frame.setContentPane(new Arena()); 50 frame.setVisible(true); 51 } 52 53 }

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• Mbean registered but not found in mbean Server

  - by Prisco

  I have a problem about the mbeans. I have created a simple mbean and I have registered it on the default mBeanServer that is run (Via eclipse or java -jar mbean.jar) and in the same process if I try to fouund the mbean registered with a simple query: for (ObjectInstance instance : mbs.queryMBeans(ObjectNameMbean, null)) { System.out.println(instance.toString()); } the query retuerns my mbean, but if I start another process and try to search this mbean registered the mbeas is not found! why? The approch is : (Process that is running) public static void main(String[] args) throws Exception { MBeanServer mbeanServer =ManagementFactory.getPlatformMBeanServer(); ObjectName objectName = new ObjectName(ObjectNameMbean); Simple simple = new Simple (1, 0); mbeanServer.registerMBean(simple, objectName); while (true) { wait (Is this necessary?) } } So this is the first process that is running (that has the only pourpose to registry the mbean, because there is another process that want to read these informations. So I start another process to search this mbean but nothing. I 'm not using jboss but the local Java virtual Machine but my scope is to deploy this simple application in one ejb (autostart) and another ejb will read all informations. All suggestions are really apprecciated. This example should be more useful : Object Hello: public class Hello implements HelloMBean { public void sayHello() { System.out.println("hello, world"); } public int add(int x, int y) { return x + y; } public String getName() { return this.name; } public int getCacheSize() { return this.cacheSize; } public synchronized void setCacheSize(int size) { this.cacheSize = size; System.out.println("Cache size now " + this.cacheSize); } private final String name = "Reginald"; private int cacheSize = DEFAULT_CACHE_SIZE; private static final int DEFAULT_CACHE_SIZE = 200; } Interface HelloBean (implemented by Hello) public interface HelloMBean { public void sayHello(); public int add(int x, int y); public String getName(); public int getCacheSize(); public void setCacheSize(int size); } Simple Main import java.lang.management.ManagementFactory; import java.util.logging.Logger; import javax.management.MBeanServer; import javax.management.ObjectName; public class Main { static Logger aLog = Logger.getLogger("MBeanTest"); public static void main(String[] args) { try{ MBeanServer mbs = ManagementFactory.getPlatformMBeanServer(); ObjectName name = new ObjectName("ApplicationDomain:type=Hello"); Hello mbean = new Hello(); mbs.registerMBean(mbean, name); // System.out.println(mbs.getAttribute(name, "Name")); aLog.info("Waiting forever..."); Thread.sleep(Long.MAX_VALUE); } catch(Exception x){ x.printStackTrace(); aLog.info("exception"); } } } So now I have exported this project as jar file and run it as "java -jar helloBean.jar" and by eclipse I have modified the main class to read informations of this read (Example "Name" attribute) by using the same objectname used to registry it . Main to read : public static void main(String[] args) { try{ MBeanServer mbs = ManagementFactory.getPlatformMBeanServer(); ObjectName name = new ObjectName("ApplicationDomain:type=Hello"); System.out.println(mbs.getAttribute(name, "Name")); } catch(Exception x){ x.printStackTrace(); aLog.info("exception"); } } But nothing, the bean is not found.

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• Faulty to use memcache together with a php web-browser-game in this way?

  - by Crowye

  Background We are currently working on a strategy web-browser game based on php, html and javascript. The plan is to have 10,000+ users playing within the same world. Currently we are using memcached to: store json static data, language files store changeable serialized php class objects (such as armies, inventorys, unit-containers, buildings, etc) In the back we have a mysql server running and holding all the game data aswell. When a object is loaded through our ObjectLoader it loads in this order: checks a static hashmap in the script for the object checks memcache if it has already been loaded into it otherwise loads from database, and saves it into memcache and the static temp hashmap We have built the whole game using a class-object-oriented approach where functionality is always made between objects. Beause of this we think we have managed to get a nice structure, and with the help of memcached we have received good request times from client-server when interacting with the game. I'm aware that memcache is not synchronized, and also is not commonly used for holding a full game in memory. In the beginning after a server's startup the load times when loading objects into memcache for the first time will be high, but after the server's been online for a while and most loads are from memcache, the loads will be well reduced. Currently we are saving changed objects into memcache and database at the same time. Earlier we had an idea to save objects into db only after a certain time or at intervals, but due to risk inconsistency if the memcache/server went down, we skipped it for now. Client requests to server often return object's status simple json-format without changing the object, which in turn is represented in the browser visually with images and javascript. But from time to time depending on when an object was last updated, it updates them with new information (e.g. a build-queue holding planned buildings time-progress is increased, and/or planned-queue-items-array has changed). Questions: Do you see how this could work or are we walking in blindness here? Do you expect us to have a lot of inconsistency issues if someone loads and updates the a memcache objects while someone else does the same? Is it even doable to do it in the way he have done it? Seems to be working fine atm, but so far we have only been 4 people online at the same time.. Is some other cache program more fit for this class-object approach than memcached? Is there any other tips you have for this situation? UPDATE Since it is simply a "normal webpage" (no applet, flash, etc), we are implementing the game so that the server is the only one holding a "real game-state".. the state of the different javascript-objects on the client is more like a approximative version of the server's game state. From time to time and before you do certain things important things, the client's visual state is updated to the server's state (e.g. the client things he can afford a barracks, asks the server to build a barracks, server updates current resources according to income-data on server and then tries to build a barracks or casts an error-message, and then sends the current server-state on resources, buildings back to the client).. It is not a fast-paced game lika real strategy game. More like a quite slow 3-4 months playtime game, where buildings can take +1 minute up to several days to complete.

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• How can I determine/use $(this) in js callback script

  - by Rabbott

  I am using Rails and jQuery, making an ajax call initiated by clicking a link. I setup my application.js file to look like the one proposed here and it works great. The problem I'm having is how can I use $(this) in my say.. update.js.erb file to represent the link I clicked? I don't want to have to assign an ID to every one, then recompile that id in the callback script.. EDIT To give a simple example of something similar to what I'm trying to do (and much easier to explain): If a user clicks on a link, that deletes that element from a list, the controller would handle the callback, and the callback (which is in question here) would delete the element I clicked on, so in the callback delete.js.erb would just say $(this).fadeOut(); This is why I want to use $(this) so that I dont have to assign an ID to every element (which would be the end of the world, just more verbose markup) application.js jQuery.ajaxSetup({ 'beforeSend': function(xhr) {xhr.setRequestHeader("Accept", "text/javascript,application/javascript,text/html")} }) function _ajax_request(url, data, callback, type, method) { if (jQuery.isFunction(data)) { callback = data; data = {}; } return jQuery.ajax({ type: method, url: url, data: data, success: callback, dataType: type }); } jQuery.extend({ put: function(url, data, callback, type) { return _ajax_request(url, data, callback, type, 'PUT'); }, delete_: function(url, data, callback, type) { return _ajax_request(url, data, callback, type, 'DELETE'); } }); jQuery.fn.submitWithAjax = function() { this.unbind('submit', false); this.submit(function() { $.post(this.action, $(this).serialize(), null, "script"); return false; }) return this; }; // Send data via get if <acronym title="JavaScript">JS</acronym> enabled jQuery.fn.getWithAjax = function() { this.unbind('click', false); this.click(function() { $.get($(this).attr("href"), $(this).serialize(), null, "script"); return false; }) return this; }; // Send data via Post if <acronym title="JavaScript">JS</acronym> enabled jQuery.fn.postWithAjax = function() { this.unbind('click', false); this.click(function() { $.post($(this).attr("href"), $(this).serialize(), null, "script"); return false; }) return this; }; jQuery.fn.putWithAjax = function() { this.unbind('click', false); this.click(function() { $.put($(this).attr("href"), $(this).serialize(), null, "script"); return false; }) return this; }; jQuery.fn.deleteWithAjax = function() { this.removeAttr('onclick'); this.unbind('click', false); this.click(function() { $.delete_($(this).attr("href"), $(this).serialize(), null, "script"); return false; }) return this; }; // This will "ajaxify" the links function ajaxLinks(){ $('.ajaxForm').submitWithAjax(); $('a.get').getWithAjax(); $('a.post').postWithAjax(); $('a.put').putWithAjax(); $('a.delete').deleteWithAjax(); } show.html.erb <%= link_to 'Link Title', article_path(a, :sentiment => Article::Sentiment['Neutral']), :class => 'put' %> The combination of the two things will call update.js.erb in rails, the code in that file is used as the callback of the ajax ($.put in this case) update.js.erb // user feedback $("#notice").html('<%= flash[:notice] %>'); // update the background color $(this OR e.target).attr("color", "red");

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