A.I.

Posted by Michał ‘mina86’ Nazarewicz on 22nd of September 2013

While cleaning Tiny Applications Collection a little I’ve dropped both artificial intelligence scripts. However, not wanting to let them disappear, I’ve decided to post them here for posterity.

The first one is an eight line of code version that might just be what Sid wrote as his first program ever:

#!/usr/bin/perl -wWtT
while (<>) {
	if (/[aeiouyAEIOUY][^a-zA-Z]*$/) {
		print "Yes.\n";
	} elsif (!/^\s*$/) {
		print "No.\n";
	}
}

The second one is an ‘improved’ six-line version akin to Pitr’s code:

#!/usr/bin/perl -wWtTn
if (/[aeiouyAEIOUY][^a-zA-Z]*$/) {
	print "No!\n";
} elsif (!/^\s*$/) {
	print "Yes.\n";
}

Standard-agnostic HTML code

Posted by Michał ‘mina86’ Nazarewicz on 17th of July 2013

HTML has gone quite a long way since its inception. This means a lot of new features but also some small incompatibilities which may pose issues in certain situations. For instance, when posting a code snippet for others to include on their websites, it’s best if it works correctly on as many sites as possible which implies being compatible with as many versions of HTML as possible. But how to create a snippet that works both in HTML and XHTML? Here are a few tips:

CSS sprites as background

Posted by Michał ‘mina86’ Nazarewicz on 6th of May 2013

CSS sprites aren’t anything new. They have been around for years, and are one of the methods to optimise website’s load time. The idea is to incorporate multiple images into one and in this way decrease number of round trips between the server and the browser.

In its traditional use, CSS sprites work as a replacement for images and cannot be used as a background. Alas that is exactly what I wanted to do with a quote and flag icons like the following:

Example block quote with a quote icon and two paragraphs with flags
Update: This website has evolved slightly since 2013. The flags are no longer used (replaced by content negotiation) and quote sprite icon has been replaced by an SVG. While I no longer use this technique, it is of course still valid.

After some playing around I’ve finally figured out how to get this working. Even though there are some caveats, sprites can be used as a top-left no-repeat background image as well.

SSL and dropping ‘www.’ with mod_rewrite

Posted by Michał ‘mina86’ Nazarewicz on 10th of February 2013

Surprisingly I couldn’t find any HTTPS-aware examples how to drop the www. prefix from web hosts in Apache, so I had to come up with one myself. Firstly, the following lines need to find their way to the end of Apache configuration file (/etc/httpd/conf/httpd.conf or something):

RewriteEngine on
RewriteCond %{HTTPS} off
RewriteCond %{HTTP_HOST} ^www\.(.*)$
RewriteRule ^(.*)$ http://%1$1 [L,R=301]

Secondly, analogous lines need to be added inside of the <VirtualHost _default_:443> directive of mod_ssl configuration file (/etc/httpd/conf.d/ssl.conf or similar), like so:

<VirtualHost _default_:443>
	# … various directives …

	# Here’s what needs to be added:
	RewriteEngine on
	RewriteCond %{HTTP_HOST} ^www\.(.*)$
	RewriteRule ^(.*)$ https://%1$1 [L,R=301]
</VirtualHost>

Now, after a restart, Apache will drop the www. prefix for both secure and insecure connections.

CMA po polsku

Posted by Michał ‘mina86’ Nazarewicz on 22nd of January 2013

Będę miał dzisiaj przyjemność prezentować Contiguous Memory Allocator na seminarium dyplomowym (CMA jest bowiem tematem mojej pracy). Dostępny jest również referat opisujący z większą ilością detali zawartość slajdów. Aktualizacja: Z racji, że jestem już po obronie, dodaję również link do pracy inżynierskiej.

Prezentacja nie opisuje sposobu użycia CMA zatem po więcej szczegółów odsyłam do materiałów z LCE 2012. No i oczywiście jest również strona mina86.com/cma/, z której linkuję do przeróżnych materiałów dotyczących CMA.

CMA on LCE/ELCE 2012

Posted by Michał ‘mina86’ Nazarewicz on 7th of November 2012

LinuxCon / Embedded Linux Conference Europe 2012 is nearly over, and I had a pleasure of talking about the Contiguous Memory Allocator. The slides from the talk are embedded below, are available for download from Google Drive, and their source code can be accessed at GitHub.

Unfortunately, in contrast to other LCE/ELCE conferences, talks were not recorded, so the video of the presentation is not available.

For more links regarding CMA, I have set up a resource page at mina86.com/cma/. Beside the link to the final CMA patchset and to the LCE/ELCE presentation, it links to various articles and patches relating to CMA directly or indirectly.

LazyProxy in Python

Posted by Michał ‘mina86’ Nazarewicz on 8th of July 2012

Paths of destiny lead mysterious ways. Not so long ago, I was a hard-core C hacker and now, I spend a lot of the time coding in Python.

In somehow related news, I have discovered that my search-foo is not good enough, when I was unable to find a decent implementations of several design patterns in Python.

What I needed was a generic proxy that would defer initialisation of an object to the moment it is first used. Here is what I came up with:

class LazyProxy(object):
    __slots__ = '__get'

    def __init__(self, cls, *args, **kw):
        object.__setattr__(self, '_LazyProxy__get',
                           lambda: self.__set(cls(*args, **kw)))

    def __set(self, obj):
        object.__setattr__(self, '_LazyProxy__get', lambda: obj)
        return obj

    def __getattr__(self, name):
        return getattr(self.__get(), name)

    def __setattr__(self, name, value):
        return setattr(self.__get(), name, value)

    def __delattr__(self, name):
        return delattr(self.__get(), name)

Deep Dive into Contiguous Memory Allocator

Posted by Michał ‘mina86’ Nazarewicz on 10th of June 2012

This is an extended version of an LWN article on CMA. It contains more detail on how to use CMA and a lot of boring code samples.

Contiguous Memory Allocator (or CMA) has been developed to allow big physically contiguous memory allocations. By initialising early at boot time and with some fairly intrusive changes to Linux memory management, it is able to allocate big memory chunks without a need to grab memory for exclusive use.

Simple in principle, it grew to be a quite complicated system which requires cooperation between boot-time allocator, buddy system, DMA subsystem, and some architecture-specific code. Still, all that complexity is usually hidden away and normal users won’t be exposed to it. Depending on perspective, CMA appears slightly different and there are different things to be done and look for.

Null: The never-ending story

Posted by Michał ‘mina86’ Nazarewicz on 27th of March 2011

I have already mentioned some problems with the null pointer but my recent discovery knocked my socks off.

By now, it should come with no surprise to anyone that 0 in pointer context acts as a null pointer (no matter of its actual representation). Moreover, it takes only a tiny bit of experimenting to figure out that expressions like (int)0 do as well. The latter is in itself a bit of a pita but it is conforming to the C++ standard which says:

Prime numbers less than 100

Posted by Michał ‘mina86’ Nazarewicz on 12th of December 2010

Anyone working in a big corporation must have been hit by some ‘funny’ mail from a coworker that helps everyone gets through the day. No different at my office — at one point all engineers have been challenged to write the shortest code in C that prints all prime numbers (and only prime numbers) less than a hundred each on separate line.

This is an interesting brain-teaser so posting it here so others may choose to think about it while their code’s compiling.

Of course, a ‘C program’ needs not to be taken too seriously — depending on not too far fetched undefined behaviours of given implementation is all right (but please do not use system or exec family of calls; not that I can see how that would help).

By the way, if you’re interested in how this challenge looks solved in Rust, I’ve described that as well.