Your XSLT processor is Turing Complete

Just by change I stumpled upon a nice approach to proving that XSLT processors are Turing Complete. This approach by Unidex involves a XSLT definition that implements the Turing Machine and an example language of writing Turing machines in XML.

(Note that the page is there for quite some time so when it's down somewhere in future, use the WayBackMachine to access it)

If you want to try it, download the XSLT, download the example and put the two together with a simple snippet:

        static void Main(string[] args)
        {
            var transform = new XslCompiledTransform();
            transform.Load(XmlReader.Create(File.Open("turing.xslt", FileMode.Open)));

            StringBuilder sb = new StringBuilder();
            StringWriter sw = new StringWriter(sb);

            var p = new XsltArgumentList();
            p.AddParam("tape", "", "199");
            transform.Transform(XmlReader.Create(File.Open("turing.xml", FileMode.Open)), p, sw);

            Console.WriteLine(sb.ToString());

            Console.ReadLine();
        }

Running this yields

Step number = 1
Tape        = 199
Tape head     ^
State       = go_right
Next symbol = 1
Next state  = go_right
Movement    = right

Step number = 2
Tape        = 199
Tape head      ^
State       = go_right
Next symbol = 9
Next state  = go_right
Movement    = right

Step number = 3
Tape        = 199
Tape head       ^
State       = go_right
Next symbol = 9
Next state  = go_right
Movement    = right

Step number = 4
Tape        = 199
Tape head        ^
State       = go_right
Next symbol =
Next state  = increment
Movement    = left

Step number = 5
Tape        = 199
Tape head       ^
State       = increment
Next symbol = 0
Next state  = increment
Movement    = left

Step number = 6
Tape        = 190
Tape head      ^
State       = increment
Next symbol = 0
Next state  = increment
Movement    = left

Step number = 7
Tape        = 100
Tape head     ^
State       = increment
Next symbol = 2
Next state  = stop
Movement    = left

The Turing machine has halted.
Final state = stop
Final tape  =  200
Tape head     ^