Graham White: My Notes

I've had my Squeezebox Duet at home for quite some time now, since I was experimenting with using a slug as a home media server. I've been waiting for my replacement for the slug for some time now and it's still not arrived so the home media server project has stalled somewhat at the moment. However, I do still have the Squeezebox set up and even though I'm not really using it right now here's what you get in the box when you buy one...

The packaging feels very nice indeed, as it should along with the rest of the stuff in the box given the price if I'm honest. Once you slide the box out of it's card sleeve you're faced with a rather sleek minimal looking flip top black box:


The next thing is to flip that lid up and you see a very neatly laid out set of Squeezebox gear which I've positioned after a little unpacking:


So in the box we have a whole array of different stuff. There is, of course, the squeezebox remote and the receiver boxes, these are powered via some reasonably small AC-DC transformers which have changeable plugs. Plugs are supplied for European, US and UK sockets so you don't have to worry about getting a localised unit for your area which is a neat trick. The remote control has a charging stand so you can return it to its cradle when you're finished using it and it will stay charged all the time. You also get a set of reasonable length RCA leads to connect the receiver to your amplifier. Last, you get a fairly minimal manual which I've hardly used as all the best documentation is on-line:


Setting up really is very trivial, it's just a case of connecting all the wires, so power to the receiver and the remote charging stand, and a connection from the receiver to the amplifier. Once you've done this, just permit access by the two wireless devices (the receiver and the remote are both wi-fi) to your access point. Both devices DHCP and you're up and running with a squeezebox duet, nice and simple. This does, however, only get you Internet provided services if you want to get the full benefit of streaming your own music collection you need the Squeeze Centre software too, and hence I'm still waiting for my media server to be delivered before I get full functionality from my Squeezebox.

Gutted! A couple of weeks ago now I had a bad Friday, the train home from London where I'd been working with a customer all day was stupidly late and I had to change twice instead of going directly home. Then I get home and fire up my laptop to send the e-mail's I'd written during the day and the darned thing didn't work, argh! Seems in spite of working all day, my T41p had died on the trip home. After reporting the problem at work the following Monday it was decided the T41p needed a new motherboard and this wasn't economical to fix, so I was issued with a shiny new T61p a few days later.


I've been pleasantly surprised by my new laptop, I wasn't expecting great things since IBM sold the Thinkpad business to Lenovo but this thing is actually quite nice. I'll spare listing the full gory details to the technical specifications page. However, it has some nice additions over my previous laptop, namely built-in firewire (not that I'm likely to use it), built-in SD card reader (used that already), an extra USB port (always handy), a DVD writer, a hardware wireless off switch (presumably for use in planes), an enormous hard disk (compared to the T41p anyway), and a lovely 15.4" widescreen capable of 1920x1200 backed by a 256MB NVidia graphics card.

Unfortunately, it came pre-installed with Vista so that (along with the stupid Vista sticker next to the keyboard) were the first things to go. I've installed Redhat Enterprise Workstation 5.2 on it which may sound like an odd choice, but IBM have a layer of software designed to sit on top of Redhat to enable us to install things like Lotus Notes, Sametime, etc. This is known as the Open Client internally and works really nicely. Clearly, there are later and greater distributions I could use but on this issue I like to support IBM and the internal community of Linux desktop users so I choose to go with the officially provided solution.

I've been up and running for a week now with no problems so far, I've been able to do all the things I could do with my old laptop and all the things I need to be able to do in order to do my job. Of course, I make some modifications to the way things work to suit my tastes (such as running KDE instead of Gnome) but all these work well too which is a great reflection on the modular nature of all things involved with Linux. I hope I continue to be surprised and pleased with the machine, and I'm definitely surprised at the ease of transition between the two machines for me.

Question 4 has been out for a while now and it's taken me a while to blog the solution for a couple of reasons, I've not had much time to work out the solution recently, and this question seems a lot more difficult than the other three (for me at least).

Here's my question this time around:

Find the smallest number that can be expressed as
the sum of 25 consecutive prime numbers,
the sum of 99 consecutive prime numbers,
the sum of 189 consecutive prime numbers,
the sum of 467 consecutive prime numbers,
the sum of 535 consecutive prime numbers,
and is itself a prime number.

For example, 41 is the smallest prime number that can be expressed as
the sum of 3 consecutive primes (11 + 13 + 17 = 41) and
the sum of 6 consecutive primes (2 + 3 + 5 + 7 + 11 + 13 = 41).

First of all I thought I'd write a routine (once again in Perl) to generate prime numbers. I know I'm not entering a competition to find the worlds largest primes so chose to write an optimised solution rather than a super efficient one. The difference here is computational complexity v coding complexity. I chose the simpler code but less efficient solution rather than the more complicated code but efficient solutions offered by algorithms such as the Sieve of Eratosthenes.

sub primes {
  my $max = shift || 10;
  my @primes = ( 2, 3, 5, 7 );
  return @primes if ($max <= 9);
  my $loop = 9;
  while (scalar(@primes) < $max) {
    my $is_prime = 1;
    for (my $div = 3; $div < ($loop-1)/2; $div++) {
      $is_prime = 0 if ($loop % $div == 0);
    }
    push (@primes,$loop) if ($is_prime);
    $loop += 2;
  }
  return @primes;
}

Now I had a way of populating an array with prime numbers I thought about the solution a bit more carefully and decided it wasn't likely to be simple to calculate, however long the code I managed to write was. So, I decided to search around for lists of prime numbers and decided to use a list of the first million primes. I could, on reflection, just used my routine to generate 1 million primes and written them to a file instead of generating each time.

The solution I came up with is shown below. It starts by populating an array with the first million primes from the downloaded file. Then the sums of the required continuous number of primes are generated and stored in another array. At this early stage, the solution is now contained in this array (with the assumption the solution exists within the first one million primes of course) so it's just a case of searching the array to find it. In order to find the number, I numerically sort the list. Now it's a simple case of finding the first (and therefore lowest) prime in the new list that is repeated 5 times.

use strict;
use FileHandle;

sub sum_primes {
  my $amount = shift;
  my $start = 0;
  my @sums;

  while ($amount < scalar(@_)) {
    my $sum = 0;
    for (my $i = $start; $i < $amount; $i++) {
      $sum += $_[$i];
    }
    push(@sums,$sum);
    $start++;
    $amount++;
  }
  return @sums;
}

sub read_primes {
  my $filename = shift;
  my $fh = new FileHandle;
  $fh->open($filename) || die "$filename: $!\n";
  my @primes;
  push(@primes,split) while (<$fh>);
  $fh->close();
  return @primes;
}

sub is_prime {
  my $num = shift;
  foreach my $prime (@_) {
    return 1 if ($num == $prime);
  }
  return 0;
}

my @primes = read_primes("1000000.txt");
my @sum_list;
push(@sum_list, sum_primes(25,@primes));
push(@sum_list, sum_primes(99,@primes));
push(@sum_list, sum_primes(189,@primes));
push(@sum_list, sum_primes(467,@primes));
push(@sum_list, sum_primes(535,@primes));
@sum_list = sort(@sum_list);
my $prev = 0;
my $same = 0;
foreach my $num (@sum_list) {
  if ($num == $prev) {
    $same++;
    if ($same == 4) {
      print "Found $num, checking... ";
      if (is_prime($num,@primes)) {
        print "PRIME! :-)\n";;
        last;
      } else {
        print "not prime :-(\n";
        $same = 0;
      }
    }
  } else {
    $same = 0;
  }
  $prev = $num;
}

This code takes a few minutes to run. I'm sure it's not the smartest solution to the problem, there must be some maths I can use to calculate a solution. Instead, this approach turns the problem into a search solution but it works pretty well and identified the correct answer of 6990493 for my question.

Thanks to Ian Hughes for the picture on his flickr. Yesterday, at work, the Hursley Linux Special Interest Group ran a little trade show type event for a couple of hours after lunch. The idea was to provide a bit of away from your desk time for folks around the lab to see what we Linux geeks have been getting up to. Various people interested in using Linux inside and outside work came along to demo their gadgets.

The picture shows me showing off my old Linux audio centre. But, also at the event were the main organiser of the day Jon Levell (showing Fedora 9 and an eeepc), and Nick O'Leary (showing his N800 and various arduino gadgets), Gareth Jones (showing his accelerometer based USB rocket launcher and bluetooth tweetjects), Andy Stanford-Clark (showing his NSLU2 driven house, and an OLPC), Laura Cowen (showing an OLPC), Steve Godwin (showing MythTV), and Chris law (showing Amora).

I thought it was quite a nice little selection of Linux related stuff to look through for the masses of people turning up, plenty of other things we could have shown too of course. The afternoon seemed very much a success, generating some real interest in the various demo items and lots of interesting questions too. Thanks to everyone for taking part!