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Showing posts from January 24, 2010

anti-bot: CAPTCHA!

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perm url with updates: http://xahlee.org/js/captcha.htmlAnti-bot Test: CAPTCHA!Xah Lee, 2010-01-29You've seen on the web CAPTCHAs Like this: It is a test designed to prevent bot that spam websites. Software can be written to automatically fill web forms. That means, they can leave blog comments or create new web accounts. So, spamers use these software to create hundreds of comments or accounts by the seconds, and leave their advertisement or otherwise walware.To prevent that, one needs something that computers can not do. Some sort of bot test. So, you have the distorted image, which computers cannot recognize well yet.The name CAPTCHA is supposed to be: Completely Automated Public Turing test to tell Computers and Humans Apart.Google's reCAPTCHAThere is a captcha service called reCAPTCHA, now owned by Google and is free, at http://recaptcha.net/. It allows web masters to put captchas on their site.There is one aspect about reCAPTCHA that's interesting. The distorted text …

interesting tidbits about domain names

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perm url with updates: http://xahlee.org/js/domain_names.htmlDomain Names, Cybersquatting, Resell MarketXah Lee, 2010-01-22Some interestig tidbits about domains today.Domain Name Registry vs RigistrarSo what's the diff between Domain name registry and Domain name registrar? Here are quotes:A domain name registry, is a database of all domain names registered in a top-level domain. A registry operator, also called a Network Information Center (NIC), is the part of the Domain Name System (DNS) of the Internet that keeps the database of domain names, and generates the zone files which convert domain names to IP addresses. Each NIC is an organisation that manages the registration of Domain names within the top-level domains for which it is responsible, controls the policies of domain name allocation, and technically operates its top-level domain. It is potentially distinct from a domain name registrar. [1]A domain name registrar is an organization or commercial entity, accredited by th…

Blocking Image Leechers

perm url with updates: http://xahlee.org/js/image_leechers.htmlBlocking Image LeechersXah Lee, 2010-01-22This page gives some tips about preventing websites that use images from your website.Image leeching is a problem. Basically, some other website using inline image with the image at your website. Besides copyright issues, it causes bandwidth problem on your site. There are a lot websites these days that allow its users to insert images from a url. The user may not be aware that it is a problem, since most of them are not technical person, and they simply wanted to show their friends some images they found.Image leeching often takes significant bandwidth from yoursite. If you have a image, let's say a beautiful girl. Many sites that are porn or othewise shady sites, such as 4chan or lots others that are infested by teens and highschool and college students, gamers, they have huge amounts of traffic for rather useless content (mostly teen drivels and bantering). If they insert o…

Random Notes On Nicolas Bourbaki

perm url with updates: http://xahlee.org/math/nicolas_bourbaki.htmlRandom Notes On Nicolas BourbakiXah Lee, 2010-01-28Spent about 3 hours reading about Bourbaki.Nicolas Bourbaki is a influential math group, used to be mysterious. I didn't know much about the group until recent years, from Wikipedia. “MacTutor History of Mathematics Archive” written by J J O'Connor and E F Robertson, University of St Andrews. SourceSourceRead also, Twenty-Five Years with Nicolas Bourbaki 1949–1973 (2008-07), by Armand Borel, from AMS Volume 45, Number 3. http://www.ams.org/notices/199803/borel.pdfSome juicy quotes:... Cartan was frequently bugging Weil with questions on how to present this material, so that at some point, to get it over with once and for all, Weil suggested they write themselves a new Traité d’Analyse. This suggestion was spread around, and soon a group of about ten mathematicians began to meet regularly to plan this treatise. It was soon decided that the work would be collect…

File Aliases Considered A Plague

perm url with updates: http://xahlee.org/UnixResource_dir/writ/hardlink_softlink_alias_junction_plague.htmlFile Aliases Considered A PlagueXah Lee, 2009-09-01Learned today about Windows's Junction. Basically, it's a file aliasing mechanism in NTFS much like unix's hardlink, softlink, and also Mac OS X's HFS+'s alias.NTFS junction pointAlias (Mac OS)Symbolic linkHard linkAll these i have avoided like a plague in software. They create a lot problems.I noticed this Windows juncture because i am using rsync thru cygwin to copy files in my Documents folder to my Mac. Rsync keeps telling me permission denied. Here's the error: xah@xah-PC ~ $ rsync -z -r -v -t --exclude=".DS_Store" --delete --rsh="ssh -l xah" ~/Documents/ xah@169.254.153.147:~/Documents_PC/ Password: building file list ... rsync: opendir "/cygdrive/c/Users/xah/Documents/My Music" failed: Permission denied (13) rsync: opendir "/cygdrive/c/Users/xah/Documents/My Pictur…

NoSQL Movement

perm url with updates: http://xahlee.org/comp/nosql.htmlThe NoSQL MovementXah Lee, 2010-01-26In the past few years, there's new fashionable thinking about anti relational database, now blessed with a rhyming term: NoSQL. Basically, it considers that relational database is outdated, and not “horizontally” scalable. I'm quite dubious of these claims.According to Wikipedia Scalability article, verticle scalability means adding more resource to a single node, such as more cpu, memory. (You can easily do this by running your db server on a more powerful machine.), and “Horizontal scalability” means adding more machines. (and indeed, this is not simple with sql databases, but again, it is the same situation with any software, not just database. To add more machines to run one single software, the software must have some sort of grid computing infrastructure built-in. This is not a problem of the software per se, it is just the way things are. It is not a problem of databases.)I'…

python 3 adoption

perm url with updates: http://xahlee.org/comp/python3.html.Python 3 AdoptionXah Lee, 2010-01-26Some notes of Wikipedia readings related to Python.Unladen Swallow, a new project from Google. It is a new python compiler with the goal of 5 times faster than the de facto standand implementation CPython. Also note Stackless Python, which has already been used in some major commercial projects.Was looking into what's new in Python 3. See: http://docs.python.org/dev/3.0/whatsnew/3.0.html. From a quick reading, i don't really like it. Here's some highlights:Print is now a function. Great, much improvement.Many functions that return lists now returns “Views” or “Iterators” Instead. A fucking fuck all fucked up shit. A extraneous “oop engineering” complication. (See: Lambda in Python 3000) The cmp() function used in sort is basically gone, users are now supposed to use the “key” parameter instead. This is a flying-face-fuck to computer science. This would be the most serious fuckup …

Learning Notes Of Symmetric Space and Differential Geometry Topics

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perm url with updates: http://xahlee.org/math/symmetric_space.htmlLearning Notes Of Symmetric Space and Differential Geometry TopicsXah Lee, 2010-01-27Spent a hour chatting with Richard Palais on voice yesterday. He is teaching me some math about transvections. This page is some learning notes on some differential geometry related topics spurred from the chat.Spent about 6 hours reading Wikipedia and writing this.Here's Wikipedia article Symmetric space, some quotes:Symmetric SpaceIn differential geometry, representation theory and harmonic analysis, a symmetric space is a smooth manifold whose group of symmetries contains an "inversion symmetry" about every point. There are two ways to make this precise, via Riemannian geometry or via Lie theory; the Lie theoretic definition is more general and more algebraic.In Riemannian geometry, the inversions are geodesic symmetries, and these are required to be isometries, leading to the notion of a Riemannian symmetric space...He…

css problems

perm url with updates: http://xahlee.org/js/css_problems.htmlCSS ProblemsXah Lee, 2010-01-28Some random thoughts about CSS.Am reading the Wikipedia article on Cascading Style Sheets again. Here's a interesting quote:While new additions to CSS3 provide a stronger, more robust layout feature-set, CSS is still very much rooted as a styling language, not a layout language. This problem has also meant that creating fluid layouts is still very much done by hand-coding CSS, and make the development of a standards-based WYSIWYG editor more difficult than expected.This is so very much true. For example, if you want text to flow in 2 columns, basically you have to manually move the the text to the appropriate block. (as opposed to, for example, text being auto word wrapped by a specified width when the text is long. See: CSS Text Wrapping)Also, although you can make a page's layout using CSS instead of Tables, but if you want more fine grained layout, such as using nested tables, CSS pr…

unix pipe as functional language

perm url with updates: http://xahlee.org/comp/unix_pipes_and_functional_lang.html.Unix Pipe As Functional LanguageXah Lee, 2010-01-25Found the following juicy interview snippet today:Is there a connection between the idea of composing programs together from the command line throught pipes and the idea of writing little languages, each for a specific domain?Alfred Aho: I think there's a connection. Certainly in the early days of Unix, pipes facilitated function composition on the command line. You could take an input, perform some transformation on it, and then pipe the output into another program. ... When you say “function composition”, that brings to mind the mathematical approach of function composition.Alfred Aho: That's exactly what I mean. Was that mathematical formalism in mind at the invention of the pipe, or was that a metaphor added later when someone realized it worked the same way?Alfred Aho: I think it was right there from the start. Doug McIlroy, at least in my boo…