Adventures in Computer Programming – bakert@gmail.com
yubnub.org is a commandline for the web. Better than that, it’s collaborative. This means you don’t need to create the command to subscribe to a feed at bloglines, it’s already been done (it’s called blub).
It’s like one shell shared by thousands so that every contribution can build on all the others. Like so much on the web it’s messy and somewhat fragile but it’s brilliant nonetheless. Check it out.
You can access the X clipboard in Vim by using the ‘+’ buffer.
So to paste from your system clipboard into Vim use:
"+p
And to copy the current line to system clipboard use:
"+Y
On Ubuntu 5.10 (“Breezy Badger”) this only works (in both vim and gvim) once you have installed gvim:
sudo apt-get install gnome-vim
I got a very intriguing email yesterday. It read:
Mr Baker,
I wonder if you can throw any light on the fact that my virus killer has warned me that there is a new internet connection on my router.
The IP address is 169.254.0.0
As you are a computer programmer I would like you to explain to me how you came to get a web address through my router when it is securely firewalled and why you would wish to do so?.
On selecting the address I was sent to the Bluebones.net website.
Your IP address should not be anywher near my computer system and I wish to know from you why it is before making any further judgements or decisions on what to do about it. I should be glad if you would proffer an explanation as urgent.
For a moment I thought I must have some horrible virus that was using my machine to go out and infect others. Then I thought about it for a minute and I realised what must have happened.
169.254.0.0 is not a valid IP address. It is part of a block reserved by IANA for showing errors in getting DHCP addresses.
So the browser of my “victim” must have tried to visit 169.254.0.0, failed to find a machine on that address and automatically taken him to the top search result for “169.254.0.0” which must have just happened to be my post, ‘Network Client Assigned 169.154 Address‘
Mystery solved. Probably.
You should use
$_SERVER['SCRIPT_NAME']
not
$_SERVER['PATH_INFO']
in PHP. PATH_INFO has been monkeyed with in some versions (and when running as FastCGI?) so sometimes it appears as PATH_INFO and other times as ORIG_PATH_INFO and PATH_INFO does not exist in the $_SERVER array. SCRIPT_NAME seems to have had a far less chequered history. Sometimes I think there could be a new beautiful version of PHP without little gotchas like this, but then that wouldn’t really be PHP would it?
There are a LOT of cinemas in London (over 100) and yet everyone from easycinema to Google thinks that the best way to search for a film to see is by cinema.
London Cinema is a better way.
This is a work in progress so any feedback greatfully received at bakert+web@gmail.com.
“Hello, may I speak with the person there with responsibility for the telephone?”
Hang on, aren’t I registered with the wonderful Telephone Preference Service making any cold callers liable to a £20,000 fine? Yes, I am but this man does not sound English and this is a terribly odd sounding line if it is coming from inside the UK. Oh dear, I think I’ve just come face to face with a new phenomenon.
I guess voice-over-ip (internet telephone calls) means that it is now possible for companies not based in the UK to start cold calling into the UK. So now we need an international agreement on spam telephone calls?
All possible subsets of a set is called a “powerset”. So, for the set { 1, 2, 3 } the powerset is:
{}, { 1 }, { 2 }, { 3 }, { 1, 2 }, { 1, 3 }, { 2, 3 } and { 1, 2, 3 }
One way to generate this set of sets is with the following recursive algorithm:
If S isn't empty:
pick the first element and call it "head"
call the rest of S "tail"
for each element e of the powerset of tail:
one element of the powerset of S is e
another is e plus head
Some code to generate this in PHP (apart from the empty set) is:
function powerset($stack, $set) {
if (! $set) {
return array();
}
$new_stack = $stack;
$tail = $set;
$head = array_shift($tail);
$powerset_of_tail = powerset($new_stack, $tail);
foreach ($powerset_of_tail as $e) {
array_push($new_stack, $e);
array_push($new_stack, array_merge($e, array($head)));
}
array_push($new_stack, array($head));
return $new_stack;
}
We can add in the empty set and wrap the recursive code in a convenience function:
function get_powerset($set) {
$powerset = powerset(array(), $set);
array_push($powerset, array());
return $powerset;
}
$set = array(1, 2, 3);
$powerset = get_powerset($set);
I’ve added hyperlinks to my “streetmap for mobiles” servlet so that you can get to the images of the surrounding area too.
This won’t work in lots of phones so I’ve put this version at:
http://bluebones.net/map/advanced
and left the old one-image-only version available at:
If you do try out either version I’d be interested how it works and what phone/PDA you use. Add your comments below.
Following on from my attempt to make a practical minimax with alpha-beta pruning example available, Joshua S. Gilpatrick has simplified my Tic-Tac-Toe implentation to showcase the alpha-beta pruning more clearly and also added a graphical board. You might want to download his code or visit the permanent Tic-Tac-Toe page where both sets of code are available.
IBM’s Blue Gene/L, is the world’s fastest supercomputer. It is a 32-rack installation of the BlueGene system at the Livermore National Research Laboratory in California.
Blue Gene is a commercial product and buying even one rack automatically puts you in the Top 500 supercomputers list.
Blue Gene differs in approach from a number of previous “fastest ever computers” in that it uses low-power 700MHz processors rather than the latest-and-greatest chips coming off the production lines. This enables them to jam so much power into a small area and, more importantly, to be able to cool it.
Blue Gene/L covers just 2,500 square feet, uses just 1.5MW electricity which costs $100M dollars a year. These are low figures compared to previous fastest computers.
Blue Gene uses custom software based on Linux. The key is reliability. If a normal system has 1 failure per month then the Blue Gene/L would have a failure every 40 seconds. As Manesh Gupta (who worked on the software) says, “we ruled out Microsoft Windows”. This is achieved by using “system-on-a-chip design”, keeping the client access system separate from the main computational machine, using 1 of every 2 processors for network comms and keeping daemons and other “processor noise” producers to an absolute minimum.
To take full advantage of Blue Gene applications have to fully parallelised, use at least 32 processes (up to 64,000 for Blue Gene/L) and use no more than 511MB RAM per process. Programs can be compiled for the system from FORTRAN 95, C, C++ and MPI. The programs must also use static linking. GNU compilers have been ported.
Blue Gene/L has been used to produce a simulation of the behaviour of 2 million atoms. This took 48 hours, produced results that took a week to render in visualisation tools and will take the physicist involved a year to analyse and report on. The simulation was faster than the real world experiment would have been.
George Chiu and Manesh Gupta delivered a talk on Blue Gene at Imperial College today in a fairly hot lecture room where I put these notes together. Thanks guys.