A few weeks ago I posted about a quick service I put together that compared textbook buyback prices from a few of the top websites.  I’ve been working on expanding that the past few weeks, and am now unveiling a site dedicated to it.

BookScouter.com is the most comprehensive comparison site for quickly searching for textbook sale prices. It currently scrapes prices from 21 other sites – which is all of them that I could find. The website is written in PHP using a custom framework that I’ve developed and use exclusively now. I found an excellent website called opensourcetemplates.org that has website templates available for free. Their ‘Nautilius’ theme is the one I chose for this site.

The backend of the site is written in Perl. It uses a pretty straightforward LWP to fetch the page, and some regular expressions to pull the price from the pages it obtains. Each site was custom coded, but I got it down to a pretty re-usable script where I just customize a few of the things, like the input variable name for the ISBN and the regex that contains the price. A few of the sites were more complicated than the others and required fetching a couple pages to obtain a session ID.

I’m pretty happy with the end result. Please try to look up a few books and see if you have anything of value sitting around. No registration or any personal information is ever required and it is completely free to use.

I read about the Quantum Random Bit Generator Service the other day on slashdot. The service is offered for free with a quick registration at https://random.irb.hr/ They provide the source code and Windows and Linux binaries to connect to the service and retrieve some random data.

Earlier that day I was marveling at the availability of a Perl module to interface with just about anything. I thought it seemed like a good opportunity to write one for this new service.   They provided some C source code, so I figured that I should be able to read through it well enough to understand what it was doing.

The interface that they provide is just a raw TCP connection.  You have to send some header information including your username and password as well as the number of bytes of data you are requesting.   It then sends back a bunch of random bits, and then I transform that into whatever type of numbers you want.

It ended up taking me entirely too long to implement, but I had dedicated enough time to it that I felt pretty committed.  I read through the provided C code, and did a bunch of tcpdumps to capture the traffic that their working program sent and made sure that mine matched it bit by bit.  Eventually I got it working.  I’ve packaged into a module that I’m calling Data::Random::QRBGS.  Now, it is simple to get some random data from the service like this:

  use Data::Random::QRBGS;

  $qrbgs = new Data::Random::QRBGS('username', 'password');

  @ints = $qrbgs->get(4);
  print "Got random integers: @intsn";

  @shorts = $qrbgs->get(2, 's');
  print "Got random shorts: @shortsn";

  $bytes = $qrbgs->getraw(1024);

I’ve created a page at https://www.brandonchecketts.com/qrbgs.php that contains a little documentation and a link to download it.

I’d like to see about getting the module made available through CPAN, but it is actually turning out to be quite complicate to do that. I’ve requested an account, and I guess that has to get approved manually. They instructions recommend joining the mailing list and discussing the module for a while before before actually submitting it. I’ll get around to that as I have time I guess.

I was reading the blog of a friend of a friend and came across a discussion about selling used books online. It sounded like there are a bunch of different sites that buy used books. Each of them allows you to put in an ISBN number to see what they are willing to buy it for.

To find the best price, you would have to browse all of these sites to see who was offering the most money. Sometimes the a book may sell for a dollar at one site, but nine dollars at another, so it is worth your time to check out all of the sites.

Sounds like a good candidate for automation to me. I am already doing a pretty similar, but more complicated, version of this with GamePriceWatcher.com. It didn’t take me much time to write some scripts to scrape prices from about eight of these sites. I also included the Perl WWW::Scraper::ISBN module to retrieve some of the details about the book and it has turned out pretty well.

I have it working now at https://avazio.com/sellbooks.php, and may move it over to its own domain if it seems like anybody is using it.

I spent quite a while today debugging a problem where call_user_func was not passing a parameter by reference. I was trying to pass an object into a function whose name is not known until run time.

Passing it by reference means that changes made to $var inside foo() are made to the actual variable instead of to a copy of the value (when passed by value).  However, for some reason, when calling a function with call_user_func(), it passes everything by value, regardless of how the function is defined.

function foo(&$var)
{
  $var++;
}

$bar = 1;
foo($bar);
echo $bar;    // outputs '2'

$function = 'foo';

call_user_func($function, $bar);
echo $bar;  // you'd expect this to output 3 now, but it still outputs 2

$function($bar);
echo $bar;  // outputs 3 now

As the sample code shows, the solution is to avoid the use of the call_user_func() function by using a variable function name. Thanks to Steve Hannah’s blog post at https://www.sjhannah.com/blog/?p=86 for helping me to solve this one.

I’m working on a site that will use the Paypal API for submitting merchant account transactions to them. I’d like to validate as much credit card information as possible before passing any information to a 3rd party, since are different kind of credit cards companies and options, so I’ve been reading to find out more about it. I came across the mod10 check that credit cards use and wrote a little PHP function to validate a card number

function sumdigits($number)
{
  $sum = 0;
  for($i = 0; $i <= strlen($number) - 1; $i++) {
    $sum += substr($number, $i, 1);
  }
  return $sum;
}

function mod10check($number)
{
  $sum_number = '';
  for($i = strlen($number) - 1; $i >= 0; $i--) {
    $thisdigit = substr($number, $i, 1);
    $sum_number .= ( $loop %2 == 0) ? $thisdigit : sumdigits($thisdigit * 2);
  }
  return sumdigits($sum_number) % 10 == 0 ? true : false;
}