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The pbrain Programming Language

Note: This is a rather old post that didn’t quite survive the transition from my old ASP.NET server to my new Linux-based server. I imported it as-is into WordPress so that it would be readable again.

This is an extension to the Brainf**k programming language that adds the ability to define and call procedures. If you’re wondering about the asterisks in the name of the programming language, it’s because the name is rather rude. That’s why I decided to use a name for my extension that a good bit tamer. After all, my Mom and my kids still read my site from time to time.

After making a rather simplistic interpreter in C++, I decided to go ahead and make a .NET compiler as well. The first one was in C++/CLI, the new extension to standard C++ available with Visual Studio 2005 and .NET 2.0. I wanted to make this available for .NET 1.1 as well, so I ported the compiler to C# (since I’m not a huge fan of Managed C++, the precursor to C++/CLI).

Thanks to rdragon on the Undernet #c++ channel and to Daniel Cristofani for their comments and input.

A Nice IDE

There’s a nice integrated development environment written by Tim Rohlfs that includes support for pbrain. I was delighted when he added support. Go download it, try it out, and let him know how much you like it.

This is the pbrain program referred to in the comment header of the interpreter source.

+([-])
+(-:< <[>>+< <-]>[>+< -]>)
+([-]>++++++++++[< ++++>-]< ++++++++>[-]++:.)
>+++>+++++>++:
>+++:

Another pbrain Program

This pbrain program initializes a memory location to 65, the ASCII value of the letter ‘A’. It then calls a function for subsequent memory locations to copy the previous location and add one to it. Once a few cells are initialized, it prints all the cells to standard output.

+([-]< [-]<[>+>+< <-]>>[< <+>>-])
+([-]>[-]+:< +)
>>+++++++++++++[< +++++>-]
 ++:
>++:
>++:
>++:
< <<<.>.>.>.>.

The .NET Compiler

Here is the C# source code for the .NET compiler, which compiles both pbrain and traditional Brainf**k code. It performs some simple optimizations as well:

  • A sequence of + and - instructions will be
    concatenated into one operation sequence, so that a sequence like
    ++++ will add 4 to the current memory location instead of
    adding 1 four times.
  • Likewise, a sequence of > and <
    instructions will be concatenated into one operation sequence, so that
    a sequence like >>>> will move the pointer forward four
    locations rather than 1 location at a time.
  • The null loop, [-], will be compiled to set the
    current memory location to zero, since that is the practical effect of
    the loop anyway. This will allow for efficiently zeroing out a memory
    location whether it holds the value 1 or 10001.
using System;
using System.Collections;
using System.IO;
using System.Reflection;
using System.Reflection.Emit;
using System.Text;
using System.Threading;



namespace ParksComputing.Pbrain
{
   /// 
   /// Compiler implements the pbrain compiler.
   /// 
   class Compiler
   {
      /// 
      /// The main entry point for the application.
      /// 
      static void Main(string[] args)
      {
         if (args.Length > 0)
         {
            String fileName = args[0];

            Compiler compiler;
            compiler = new Compiler(fileName);

            Type myType = compiler.Compile();
         }
      }

      private String fileName;
      private String asmName;
      private String asmFileName;

      private AssemblyBuilder myAsmBldr;

      private FieldBuilder mem;
      private FieldBuilder mp;
      private FieldBuilder tmp;
      private FieldBuilder vtbl;

      private TypeBuilder myTypeBldr;

      private MethodInfo readMI;
      private MethodInfo writeMI;
      private MethodInfo hashAddMI;
      private MethodInfo hashGetMI;

      private int methodCount;
      private int callCount;


      void Ldc(ILGenerator il, int count)
      {
         switch (count)
         {
         case 0:
            il.Emit(OpCodes.Ldc_I4_0);
            break;

         case 1:
            il.Emit(OpCodes.Ldc_I4_1);
            break;

         case 2:
            il.Emit(OpCodes.Ldc_I4_2);
            break;

         case 3:
            il.Emit(OpCodes.Ldc_I4_3);
            break;

         case 4:
            il.Emit(OpCodes.Ldc_I4_4);
            break;

         case 5:
            il.Emit(OpCodes.Ldc_I4_5);
            break;

         case 6:
            il.Emit(OpCodes.Ldc_I4_6);
            break;

         case 7:
            il.Emit(OpCodes.Ldc_I4_7);
            break;

         case 8:
            il.Emit(OpCodes.Ldc_I4_8);
            break;

         default:
            il.Emit(OpCodes.Ldc_I4, count);
            break;
         }
      }


      void Forward(ILGenerator il, int count)
      {
         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldc.i4 1	
         Ldc(il, count);

         //add
         il.Emit(OpCodes.Add);

         //stsfld int32 pbout.mp
         il.Emit(OpCodes.Stsfld, mp);
      }


      void Back(ILGenerator il, int count)
      {
         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldc.i4 1	
         Ldc(il, count);

         //sub
         il.Emit(OpCodes.Sub);

         //stsfld int32 pbout.mp
         il.Emit(OpCodes.Stsfld, mp);
      }


      void Plus(ILGenerator il, int count)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         //ldc.i4 1
         Ldc(il, count);

         //add
         il.Emit(OpCodes.Add);

         //stsfld int32 pbout.tmp
         il.Emit(OpCodes.Stsfld, tmp);

         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldsfld int32 pbout.tmp	
         il.Emit(OpCodes.Ldsfld, tmp);

         //stelem.i4
         il.Emit(OpCodes.Stelem_I4);
      }


      void Minus(ILGenerator il, int count)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         //ldc.i4 1	
         Ldc(il, count);

         //sub
         il.Emit(OpCodes.Sub);

         //stsfld int32 pbout.tmp
         il.Emit(OpCodes.Stsfld, tmp);

         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldsfld int32 pbout.tmp	
         il.Emit(OpCodes.Ldsfld, tmp);

         //stelem.i4
         il.Emit(OpCodes.Stelem_I4);
      }


      void Read(ILGenerator il)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //call void [mscorlib]System.Console.Write(char)
         il.EmitCall(OpCodes.Call, readMI, null);

         //stelem.i4
         il.Emit(OpCodes.Stelem_I4);
      }


      void Write(ILGenerator il)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         //call void [mscorlib]System.Console.Write(char)
         il.EmitCall(OpCodes.Call, writeMI, null);
      }


      void LoopBegin(ILGenerator il, Label endLabel)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         //brfalse loop_1_end
         il.Emit(OpCodes.Brfalse, endLabel);
      }


      void LoopEnd(ILGenerator il, Label beginLabel)
      {
         //br loop_1_start
         il.Emit(OpCodes.Br, beginLabel);
      }


      void Call(ILGenerator il)
      {
         //ldsfld object pbout.vtbl
         il.Emit(OpCodes.Ldsfld, vtbl);

         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         //box int32
         il.Emit(OpCodes.Box, typeof(int));

         //call instance object [mscorlib]System.Collections.Hashtable.get_Item(object)
         il.EmitCall(OpCodes.Call, hashGetMI, null);

         //calli void()
         il.EmitCalli(OpCodes.Calli, System.Runtime.InteropServices.CallingConvention.StdCall, null, null);
      }


      void Zero(ILGenerator il)
      {
         //ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld, mem);

         //ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld, mp);

         //ldc.i4.0
         il.Emit(OpCodes.Ldc_I4_0);

         //stelem.i4
         il.Emit(OpCodes.Stelem_I4);
      }


      Type Compile()
      {
         // .field private static int32 mp
         mp = myTypeBldr.DefineField("mp", typeof(int), FieldAttributes.Private | FieldAttributes.Static );

         // .field private static int32[] mem
         mem = myTypeBldr.DefineField("mem", typeof(Array), FieldAttributes.Private | FieldAttributes.Static );

         // .field private static int32 tmp
         tmp = myTypeBldr.DefineField("tmp", typeof(int), FieldAttributes.Private | FieldAttributes.Static );

         // .method private static int32 main() cil managed
         MethodBuilder mainBldr = myTypeBldr.DefineMethod(
            "main", 
            (MethodAttributes)(MethodAttributes.Private | MethodAttributes.Static), 
            typeof(int), 
            null 
            );

         ILGenerator il = mainBldr.GetILGenerator();



         // ldc.i4 30000
         il.Emit(OpCodes.Ldc_I4, 30000);
         // newarr [mscorlib]System.Int32
         il.Emit(OpCodes.Newarr, typeof(int));
         // stsfld int32[] pbout.mem
         il.Emit(OpCodes.Stsfld, mem);

         // ldc.i4 0
         il.Emit(OpCodes.Ldc_I4_0);
         // stsfld int32 pbout.mp
         il.Emit(OpCodes.Stsfld, mp);


         Parse(il);


         // ldsfld int32[] pbout.mem
         il.Emit(OpCodes.Ldsfld , mem);
         // ldsfld int32 pbout.mp
         il.Emit(OpCodes.Ldsfld , mp);
         // ldelem.i4 
         il.Emit(OpCodes.Ldelem_I4);

         // ret
         il.Emit(OpCodes.Ret);


         Type pboutType = myTypeBldr.CreateType();
         myAsmBldr.SetEntryPoint(mainBldr);
         myAsmBldr.Save(asmFileName);
         Console.WriteLine( "Assembly saved as '{0}'.", asmFileName );

         return pboutType;
      }


      void Parse(ILGenerator il)
      {
         using (FileStream fs = File.OpenRead(fileName))
         {
            char c;
            int n;

            Queue q = new Queue();

            while ((n = fs.ReadByte()) != -1)
            {
               c = (char)n;
               q.Enqueue(c);

               if (c == ':')
               {
                  ++callCount;
               }
            }

            if (callCount > 0)
            {
               // .field private static object vtbl
               vtbl = myTypeBldr.DefineField("vtbl", typeof(Object), FieldAttributes.Private | FieldAttributes.Static );

               //newobj instance void [mscorlib]System.Collections.Hashtable..ctor()
               Type hashtableType = typeof(System.Collections.Hashtable);
                  ConstructorInfo constructorInfo = hashtableType.GetConstructor(
                     (BindingFlags.Instance | BindingFlags.Public), 
                     null, 
                     CallingConventions.HasThis, 
                     System.Type.EmptyTypes, 
                     null
                     );
               il.Emit(OpCodes.Newobj, constructorInfo);
               //stsfld object pbout.vtbl
               il.Emit(OpCodes.Stsfld, vtbl);
            }

            Interpret(q, il);
         }
      }


      MethodBuilder Procedure(Queue q)
      {
         Type[] temp0 = {myTypeBldr};
         StringBuilder sb = new StringBuilder();
         sb.Append("pb_");
         sb.Append(methodCount);
         String name = sb.ToString();

         MethodBuilder procBldr = myTypeBldr.DefineMethod(
            name, 
            (MethodAttributes.Private | MethodAttributes.Static),
            null,
            System.Type.EmptyTypes
            );

         ILGenerator il = procBldr.GetILGenerator();

         Interpret(q, il);

         // ret
         il.Emit(OpCodes.Ret);

         return procBldr;
      }


      int CountDuplicates(Queue q, char c)
      {
         int count = 1;
         char inst = c;

         while (c == inst && q.Count > 0)
         {
            c = (char)q.Peek();

            if (c == inst)
            {
               c = (char)q.Dequeue();
               ++count;
            }
         }

         return count;
      }


      void Interpret(Queue q, ILGenerator il)
      {
         System.Collections.IEnumerator myEnumerator = q.GetEnumerator();

         char c;
         byte b;

         while (q.Count > 0)
         {
            c = (char)q.Dequeue();

            switch (c)
            {
            case '+':
               Plus(il, CountDuplicates(q, c));
               break;

            case '-':
               Minus(il, CountDuplicates(q, c));
               break;

            case '>':
               Forward(il, CountDuplicates(q, c));
               break;

            case '< ':
               Back(il, CountDuplicates(q, c));
               break;

            case ',':
               Read(il);
               break;

            case '.':
               Write(il);
               break;

            case '[':
            {
               if (q.Count > 0)
               {
                  Queue lq = new Queue();

                  int nest = 0;
                  int startPos = q.Count;
                  bool pair = false;
                  bool zero = false;
                  bool opt = true;

                  // Find the matching ]
                  while (q.Count > 0)
                  {
                     c = (char)q.Dequeue();

                     if (c == '[')
                     {
                        ++nest;
                     }
                     else if (c == ']')
                     {
                        if (nest > 0)
                        {
                           --nest;
                        }
                        else
                        {
                           pair = true;
                           break;
                        }
                     }
                        // Check for null loop, [-], which set the current cell
                        // to zero. There's no need to loop. Just store a zero
                        // and move on.
                     else if (opt && c == '-' && (startPos - q.Count) == 1)
                     {
                        opt = false;

                        // If the next character is the end of the loop...
                        if ((char)q.Peek() == ']')
                        {
                           // Eat the ] and stop the loop
                           c = (char)q.Dequeue();
                           zero = true;
                           break;
                        }
                     }

                     lq.Enqueue(c);
                  }

                  if (zero)
                  {
                     Zero(il);
                     break;
                  }

                  // If no matching ] is found in source block, report error.
                  if (q.Count != 0 && !pair)
                  {
                     // throw System.Exception();
                  }

                  Label beginLabel = il.DefineLabel();
                  Label endLabel = il.DefineLabel();

                  il.MarkLabel(beginLabel);
                  LoopBegin(il, endLabel);

                  Interpret(lq, il);
                  LoopEnd(il, beginLabel);
                  il.MarkLabel(endLabel);
               }
            }
               break;

            case '(':
            {
               // LoopBegin(il, endLabel);

               if (q.Count > 0)
               {
                  bool pair = false;

                  Queue lq = new Queue();

                  int nest = 0;

                  // Find the matching )
                  while (q.Count > 0)
                  {
                     c = (char)q.Dequeue();

                     if (c == '(')
                     {
                        ++nest;
                     }
                     else if (c == ')')
                     {
                        if (nest > 0)
                        {
                           --nest;
                        }
                        else
                        {
                           pair = true;
                           break;
                        }
                     }

                     lq.Enqueue(c);
                  }

                  // If no matching ) is found in source block, report error.
                  if (q.Count != 0 && !pair)
                  {
                     // throw 5;
                  }

                  MethodBuilder procBldr = Procedure(lq);

                  //ldsfld object pbout.vtbl
                  il.Emit(OpCodes.Ldsfld, vtbl);

                  //ldsfld int32[] pbout.mem
                  il.Emit(OpCodes.Ldsfld, mem);

                  //ldsfld int32 pbout.mp
                  il.Emit(OpCodes.Ldsfld, mp);

                  //ldelem.i4 
                  il.Emit(OpCodes.Ldelem_I4);

                  //box int32
                  il.Emit(OpCodes.Box, typeof(int));

                  //ldftn void pbout.pb_0()
                  il.Emit(OpCodes.Ldftn, procBldr);

                  //call instance void [mscorlib]System.Collections.Hashtable.Add(object,object)
                  il.EmitCall(OpCodes.Call, hashAddMI, null);
               }

               ++methodCount;
            }
               break;

            case ':':
               Call(il);
               break;

            default:
               break;
            }
         }
      }


      Compiler(String fileNameInit)
      {
         fileName = fileNameInit;
         methodCount = 0;
         callCount = 0;
         asmName = Path.GetFileNameWithoutExtension(fileName);
         asmFileName = Path.GetFileName(Path.ChangeExtension(fileName,  ".exe"));

         AssemblyName myAsmName = new AssemblyName();
         myAsmName.Name = asmName;

         myAsmBldr = Thread.GetDomain().DefineDynamicAssembly(myAsmName, AssemblyBuilderAccess.RunAndSave);

         Type[] temp1 = { typeof(Char) };
         writeMI = typeof(Console).GetMethod("Write", temp1);
         readMI = typeof(Console).GetMethod("Read");

         Type[] temp2 = { typeof(Object), typeof(Object) };
         hashAddMI = typeof(System.Collections.Hashtable).GetMethod("Add", temp2);

         Type[] temp3 = { typeof(Object)};
         hashGetMI = typeof(System.Collections.Hashtable).GetMethod("get_Item", temp3);

         // .class private auto ansi pbout extends [mscorlib]System.Object
         ModuleBuilder myModuleBldr = myAsmBldr.DefineDynamicModule(asmFileName, asmFileName);
         myTypeBldr = myModuleBldr.DefineType(asmName);
      }
   };
}

The Interpreter

Here is the C++ source code for the interpreter. If you want to compile it with g++ make sure you get version 3.4, which shouldn’t choke on the wchar_t bits.

I don’t claim that this is a particularly efficient interpreter. I thought of it, designed it, and wrote it between the hours of 9:45 PM and 6:15 AM, in a single session.

This interpreter will also work with traditional Brainf**k code.

/*
   Interpreter for the pbrain programming language (procedural Brainf**k)
   Copyright(C) Paul M. Parks
   All Rights Reserved.

   v1.4.3
   2004/07/15 12:10

   paul@parkscomputing.com

http://www.parkscomputing.com/

   The syntax is the same as traditional Brainf**k, with the following 
   symbols added:

   (
      Begin procedure

   )
      End procecure

   :
      Call procedure identified by the value at the current location

   
   Procedures are identified by numeric ID:

   +([-])

      Assuming the current location is zero, defines a procedure number 1 that 
      sets the current location to zero when called.

   ++(< <[>>+< <-]>[>+< -]>)

      Assuming the current location is zero, defines a procedure number 2 that 
      accepts two parameters. It adds parameter 1 and parameter 2 and places 
      the result in the location that was current when the procedure was 
      called, zeroing out parameters 1 and 2 in the process.

   +++([-]>++++++++++[< ++++>-]< ++++++++>[-]++:.)

      Assuming the current location is zero, defines a procedure 3 that prints 
      the ASCII equivalent of the numeral at the current location, between 0 
      and 9.

   +++>+++++>++:

      Calls procedure 2, passing in parameters 3 and 5.

   All of the above examples may be combined into the program below. Note that 
   the procedures are numbered 1, 2, and 3 because the current location is 
   incremented prior to each procedure definition.

      +([-])
      +(-:< <[>>+< <-]>[>+< -]>)
      +([-]>++++++++++[< ++++>-]< ++++++++>[-]++:.)
      >+++>+++++>++:
      >+++:

   An error condition is reported with a short diagnostic to stderr and an 
   error number returned from the executable. Errors reported by the 
   interpreter are as follows:

      1 - Out of memory
      2 - Unknown procedure
      3 - Memory address out of range
      4 - Cannot find matching ] for beginning [
      999 - Unknown exception

*/


#include 
#include 
#include 
#include 
#include 


#if defined(_MSC_VER)
#pragma warning(disable: 4571)
#endif

// Define the type contained in the memory array
#ifndef PBRAIN_MEM_TYPE
#define PBRAIN_MEM_TYPE int
#endif

// Define the character input/output type.
#ifndef PBRAIN_CHARACTER_TYPE
#define PBRAIN_CHARACTER_TYPE wchar_t
#endif

// Set the initial size of the memory array, if not defined externally.
#ifndef PBRAIN_INIT_MEM_SIZE
#define PBRAIN_INIT_MEM_SIZE 30000
#endif

// By default, use a dynamic array to store memory locations.
#ifndef PBRAIN_STATIC_MEMORY
typedef std::vector Mem;
Mem mem(PBRAIN_INIT_MEM_SIZE);
Mem::size_type mp = 0;
#else
PBRAIN_MEM_TYPE mem[PBRAIN_INIT_MEM_SIZE];
size_t mp = 0;
#endif


// Placeholder template class to be specialized below.
template struct io_types{};


// Define appropriate I/O and stream iterator types for working with byte 
// characters.
template<> struct io_types
{
   static std::istream& cin;
   static std::ostream& cout;
   typedef std::basic_ifstream > ifstream;
   typedef std::istream_iterator istream_iterator;
};

std::istream& io_types::cin = std::cin;
std::ostream& io_types::cout = std::cout;


// Define appropriate I/O and stream iterator types for working with wide 
// characters.
template<> struct io_types
{
   static std::wistream& cin;
   static std::wostream& cout;
   typedef std::basic_ifstream > ifstream;
   typedef std::istream_iterator istream_iterator;
};

std::wistream& io_types::cin = std::wcin;
std::wostream& io_types::cout = std::wcout;


// Useful type that chooses the appropriate typedefs for the character width
typedef io_types io;

// Type for storing a string of instructions; used for procedures and loops
typedef std::vector SourceBlock;

// Type for storing procedures indexed by number
typedef std::map > Procedures;


// Map of procedure IDs to procedures
Procedures procedures;


// Interpret a container of instructions
template void interpret(It ii, It eos)
{
   while (ii != eos)
   {
      switch (*ii)
      {
      case '+':
         ++mem[mp];         
         break;

      case '-':
         --mem[mp];
         break;

      case '>':
         ++mp;

#ifndef PBRAIN_STATIC_MEMORY
         // If memory is kept in a dynamic array, the array will grow as 
         // needed.
         try
         {
            if (mp == mem.size())
            {
               mem.resize(mem.size() * 2);
            }
         }
         catch (...)
         {
            // Ostensibly an out-of-memory condition.
            throw 1;
         }
#else
         // Static memory cannot grow, so throw when limit reached
         if (mp == sizeof(mem) / sizeof(PBRAIN_MEM_TYPE))
         {
            throw 1;
         }
#endif

         break;

      case '< ':
         --mp;

         // Throw out-of-range error if cell location is decremented below 0
         if (static_cast(mp) < 0)
         {
            throw 3;
         }

         break;

      case '.':
         io::cout.put(static_cast(mem[mp]));
         break;

      case ',':
         mem[mp] = static_cast(io::cin.get());
         break;

      case '[':
         // Move to first instruction in the loop
         ++ii;

         {
            int nest = 0;
            It begin = ii;

            // Find the matching ]
            while (ii != eos)
            {
               if (*ii == '[')
               {
                  ++nest;
               }
               else if (*ii == ']')
               {
                  if (nest)
                  {
                     --nest;
                  }
                  else
                  {
                     break;
                  }
               }

               ++ii;
            }

            // If no matching ] is found in source block, report error.
            if (ii == eos)
            {
               throw 4;
            }

            // At this point the iterator will point at the matching ] 
            // character, which is one instruction past the end of the range 
            // of instructions to be processed in a loop.
            loop(begin, ii);
         }

         break;

      case '(':
         ++ii;

         {
            SourceBlock sourceBlock;

            while (ii != eos && *ii != ')')
            {
               sourceBlock.push_back(*ii);
               ++ii;
            }

            procedures.insert(std::make_pair(mem[mp], sourceBlock));
         }

         break;

      case ':':
         {
            // Look up the source block that matches the value at the current 
            // location. If found, execute it.
            Procedures::iterator i = procedures.find(mem[mp]);

            if (i != procedures.end())
            {
               interpret(i->second.begin(), i->second.end());
            }
            else
            {
               throw 2;
            }
         }
         break;

      default:
         break;
      }

      ++ii;
   }
}


template void loop(It ii, It eos)
{
   // Interpret instructions until the value in the current memory location 
   // is zero
   while (mem[mp])
   {
      interpret(ii, eos);
   }
}


template void parse(C& c)
{
   io::istream_iterator ii(c);
   io::istream_iterator eos;

   SourceBlock sourceBlock;

   // Copy instructions from the input stream to a source block.
   while (ii != eos)
   {
      sourceBlock.push_back(*ii);
      ++ii;
   }

   // Execute the instructions in the source block
   interpret(sourceBlock.begin(), sourceBlock.end());
}


int main(int argc, char** argv)
try
{
   // Read from a file if a filename is provided as an argument.
   if (argc > 1)
   {
      io::ifstream source(argv[1]);

      if (source.is_open())
      {
         parse(source);
      }
   }
   // Otherwise interpret code from stdin
   else
   {
      parse(io::cin);
   }
}
catch(int e)
{
   std::cerr < < "Error " << e << ", cell " << unsigned int(mp) << "\n";
   exit(e);
}
catch(...)
{
   std::cerr << "Error " << 999 << ", cell " << unsigned int(mp) << "\n";
   exit(999);
}

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