PHPDNS 1.05 Released

The latest version of PHPDNS version 1.05 is now available for download.

This release version wraps up changes from versions 1.04 and 1.05; mainly a fsockopen timeout bug and a typo.

1.05 has been available on the PHPDNS SCM on github for a while and with no errors reported is now available as the general download.

For anyone interested the bleeding edge releases (and source code in an easily-forkable form) is found at: https://github.com/purplepixie/phpdns

PHPDNS Main site and downloads: http://www.purplepixie.org/phpdns/

Working With Big Numbers

Recently a friend asked me a some questions:

“97 raised to the power of 242 has equalled infinity on every calculator I’ve used, but it’s not infinite just very big. Why do they say infinity? And what is 97^242?”

The answer to the first part is easy; precision (and hence maximum values) are limited. Since this isn’t a basic intro to binary and computing we won’t go into it but just say in the good old days this would have resulted in the number simply wrapping. Modern devices and systems detect this overflow and show a special case result, usually Infinity/Inf or sometimes Not-a-Number/NaN.

Big number calculation in R

97^242 in R

96^242 in Matlab

97^242 in Matlab

Above are two common tools (R and Matlab) both running on 64-bit Linux and overflowing for 97^242. The difference in overflow can be seen in that the OSX Calculator can handle 97^71 but overflows at 96^72, whereas both Matlab and R will handle 97^166 but not 97^156.

Ok well Google has a calculator function so maybe we can just ask it for 97^242?

Google for 97^242

Google for 97^242

Alas, no. But maybe if we trick it with 97+242 to get it’s calculator up and then use that?

Google calculator

97^242 in Google Calc

Nope. So how do we go about trying to calculate 97^242?

There are approaches we can use to estimate it (one of the most promising being looking for differences between powers of 100 and 97 then extrapolating, maybe something for another blog post) but we want an exact answer. As shown by Matlab/R and common logic built-in types are just too small and will overflow.

The solution comes, as with so much in life, from GNU in the GNU Multiple Precision Arithmetic Library aka GNU MP Bignum.

Once installed (yum install gmp-devel on Fedora) it’s just a case of hacking together some C to calculate the result (note the below isn’t supposed to be efficient, more transparent):

Build this with: gcc bignum.c -lgmp -o bignum

And voila:

Answer: 6291579554172660514180168586029512181759771859911909633079235697774386086528343277488812886056338013920280508647975158853848809035553070842805751211101339655910548731303652360707362342079349547320620109301210997985503312350525910702941569606402987567610468491227904389508486082138580406254059512446817845870945561908178074689723504831108854735558785367285467641732532222094514773911007516550984383178257067496267923472962067697340265687768345831925754550553895144516227499602812609

Which for those who can’t be bothered to count digits is rounded as 6.29E+480.

Though of course after all this loading of libraries and writing of C it turns out that although Google couldn’t answer it, naturally Wolfram|Alpha could:

Wolfram Alpha Calculation

Wolfram|Alpha Calculates 97^242 with Ease!

PHPDNS SCM on GitHub.com

PHPDNS, the Purplepixie.org open source direct DNS query API for PHP, now has its development source code hosted on github.com.

Packaged releases (and release source code versions) will continue to be distributed directly from the PHPDNS web site.

The source code management (SCM) repository for development code can be found at: https://github.com/purplepixie/phpdns

All Purplepixie projects hosted on github can be found at: https://github.com/purplepixie

C/C++ CGI File Upload

A long time ago when I still had (some) hair and hadn’t bitten the PHP bullet I played around with C++ CGIs. Owing to a lack of then available HOW-TO docs I went on to write a (badly written and error-filled) CGI in C/C++ HOW-TO and also a CGI Variable Wrapper. The HOW-TO did what it said on the tin and the wrapper provided an easy API to read/write GET and POST variables as well as cookies.

Surprisingly both the HOW-TO and the wrapper are still in use and I get contacted form time to time with queries. The most common query regards file upload which the wrapper doesn’t support. To illustrate a simple file upload I cobbled together a quick and dirty C example which I’ve provided via email ever since.

So here, for general reference, is my demonstration C code. Please note this is very untested and unrobust, even dodgier than my usual fare. I keep meddling with the idea of finding time to do a proper job either of a standalone file upload API or integrating support into the CGI wrapper. All of this is really just for kicks though as there are better solutions available.

 

Forum Enabled for New Registrations

A while back I was forced to disable registrations to the Purplepixie Support Forum thanks to a massive quantity of spam, most of which was of a very unseemly nature (very very unseemly).

Following an update to the Purplepixie Forum phpBB and enabling of reCAPTCHA I’ve now re-enabled user registration.

Apologies to anyone who’s had difficulty getting support and had to end up emailing me directly, but I hope the new changes will allow the forum to operate from now on.

 

Variable Length Arguments in C++, Java, and PHP

Normally in software development we define methods with a given number of parameters (and their type in some languages). Quite often however we want to be able to deal with different numbers of arguments and there are two widely used approaches; different methods and default parameters.

Different methods relies on the concept that the call to function is matched not just on the name of the method but also the count (and type if applicable) of the parameters. So if we wanted a method that could accept one or two integers in C++ we could define two methods:

So if we called SomeFunction(1) the first would be used, SomeFunction(1,2) would use the second.

Default parameters allows us to define some of the parameters as optional and their default values if not passed so the definition:

Would accept one or two integer parameters. SomeMethod(1,2) would have a=1 and b=2 whereas SomeMethod(1) would use the default value and so b=0.

This is all very good and highly useful in a variety of situations but suppose you wanted to handle any number of parameters, from a very small set (or zero) to a large number. Using either of these techniques would require a lot of additional coding, creating a method for each length or the longest set of default parameters imaginable.

This is where the concept of variable length arguments for a method comes in; we want to be able to define a method and accept an arbitrary number of arguments which it can process (please note in most if not all cases the best option for this would be to pass something like a Vector in C++ or an array in PHP, but best practice is not the point of the exercise).

Let’s consider a problem.

We want to have a LineShape function. This function takes a series of Points (a simple class just containing an X and Y coordinate). In a proper system it would then start with the first point and draw a line to each consecutive one but for our example we just want it to print a list of the points it will draw to/from in order.

This could be two points (a single line) or a complex shape of an undetermined total number of points (again note the caveat above that a Vector/List/Array would be the best and safest way to do this in TRW).

So for our implementation we need:

  • A simple Point class
  • A method (LineShape) that takes an arbitrary number of Points and prints out the coordinates
  • Code to create a set of Points and pass them to LineShape

How to do this varies from language to language and, as you might assume, it’s hardest and most dangerous in C++ (because of it’s lack of type safety), slightly easier in Java and PHP (Java because of it’s high type safety and PHP because of it’s lack of any enforced typing).

Variable Length Arguments in C++

To implement in C++ we make use of the va_ functionality provided in stdarg.h. The function is defined as taking the number of parameters passed (int) and then the parameters themselves represented by “…”.

We read the parameter count and then iterate through reading each in turn with va_arg and specifying the type to be used. Note in C++ you must specify the number of parameters being passed when calling the method.

There you have it in C++ (well actually using C libraries); but don’t do it (see above).

Variable Length Arguments in Java

In Java it’s a lot easier as the functionality is built-in to the language. Additionally you don’t need to pass the number of parameters and also the type is determined for the entire set of parameters (in our case Point).

Note that in order for Point to be instantiated as non-static it must be in a seperate file (Point.java).

So our Point class is:

And the main Var.java contains:

So in Java we just need to declare a method with Type… name and then iterate through the array in a for loop.

Variable Length Arguments in PHP

PHP isn’t quite as built-in as Java (an actual language construct) but PHP natively provides functions to support variable length parameters to methods such as func_num_args (number of arguments) and func_get_args (arguments as an array).

 

PHP Dynamic Factories

Design patterns are common solutions to commonly-encountered problems in software development. Of these the most widely used are creational patterns – methods of creating objects, most notably the factory pattern. The factory pattern is used for the centralised instantiation of related class objects.

Let’s illustrate this with an example using the idea of shapes. We want to have an abstract base Shape class and then concrete classes derived from the base class of specific (or concrete) shapes.

We would, without any of this factory nonsense, instantiate the classes directly:

Using the standard factory method rather than instantiating directly we would create classes to handle the instantiation for us. First we define an abstract ShapeFactory class and then specific concrete factories to create individual shapes. So adding to our previous code:

So now, rather than creating instances directly we first create a concrete factory instance and then use that to create the concrete shape:

This is all very well and good (and design patterns in general are a very powerful and useful tool) but it fails to take advantage of PHP’s powers of runtime adaptability, the ability to change and update code behaviour and functionality during execution (at runtime).

For example having these factories provides us with a standard way to create shapes but how would we add new shapes easily. With the factory pattern we would still need code modification, new shapes require a new factory and for this factory to be explicitly used to create objects.

With the wonder that is PHP however we can be more flexible, more dynamic.

Consider the possibility we want to be able to create any type of shape from a factory.

As an example you could have a Shape Maker class which took a text string for the type and returned an appropriate object (this is known as a paramerized factory):

This would allow the creation of shapes as follows:

But we would really like to go further than this; we want to create a dynamic factory, one in which shapes are simply registered with a type and a associated class name, and then created by passing the type.

For this to work we would need the following:

  • A list (array) of registered types and their class names
  • A method to register new types
  • A method to create an object of the given type
  • A fall-back; what to do if a creation request is made for a type that doesn’t exist

To make this all even easier to use in our example the class will be an abstract class using static members and methods (though of course the same idea holds true for a non-abstract class using non-static members, it would just need to be instantiated first).

So, using the same shape code but replacing the factory code with the following:

We create a ShapeFactory class with the ability to register, check the existence of, and create shape classes. All that remains to do is to register the Square and Circle classes with type names (the lower case versions). Once this is done they can be created with:

Which creates objects of Square and Circle using the dynamic type identifiers.

So what is the advantage of this?

Well imagine now in our code we wish to add a new shape. This shape (Triangle) is contained in a file triangle.php which may or may not be included at runtime with an include_once. We may want to limit the inclusion of triangle to an add-on pack or for just specific users, so we have logic to decide if triangle should be included.

All triangle.php needs now to contain is:

And then if it is included the triangle type becomes automatically available from the ShapeFactory. If ShapeFactory had been extended (very easy) to return a list of possible shapes which was used to populate a UI component then triangle would now appear, could be selected, and used as a type identifier to create a concrete shape of the correct type (triangle) at runtime.

The idea of the dynamic factory can be taken much further with list provision or description fields for example and is with certain purplepixie.org products such as FreeDESK.

Another layer of abstraction could also easily be added; all the functionality of the ShapeFactory would be generic to any dynamic factory, just the list of types and objects being unique (if this was a non-static class these could be just different instances of a general DynamicFactory class). In the static example we create a DynamicFactory and then extend from it product-specific dynamic factories to be used to house the lists of specific types:

Some links to source files for download:

classic.php : Shows the classic factory pattern

dynamic.php : Shows the first dynamic iteration along with the maker class

dynamic2.php : Shows the finalised version