It's just past midnight and I'm tired. https://upload.wikimedia.org/wikipedia/en/7/76/Busy_Beaver_1.JPG

busy_beaver.cpp

#include <iostream>
#include <vector>

// https://upload.wikimedia.org/wikipedia/en/7/76/Busy_Beaver_1.JPG

class Turing
{
public:
  std::vector<bool> tape; // our tape, with an alphabet of {0,1}
  int state; // the machine's current state

  Turing::Turing()
  {
    // initialising the tape the lazy way
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);
    this->tape.push_back(false);

    this->state = 3; // starting state
  }

  int compute(int i)
  {
    if(i < this->tape.size()) // bounds check
    {
    if(this->state == 0) // halt
      {
        return 0;
      }
      else if(this->state == 1) // state A
      {
        if(this->tape[i] == false)
        {
          this->tape[i] = true;
          this->state = 2;
          compute(++i); // move right
        }
        else if(this->tape[i] == true)
        {
          this->state = 3;
          compute(--i); // move left
        }
      }
      else if(this->state == 2) // state B
      {
        if(this->tape[i] == false)
        {
          this->tape[i] = true;
          this->state = 1;
          compute(--i);
        }
        else if(this->tape[i] == true)
        {
          this->state = 2;
          compute(++i);
        }
      }
      else if(this->state == 3) // state C
      {
        if(this->tape[i] == false)
        {
          this->tape[i] = true;
          this->state = 2;
          compute(--i);
        }
        else if(this->tape[i] == true)
        {
          this->state = 0;
          compute(i);
        }
      }
    }
  }
};

int main(void)
{
  int i = 0;
  Turing t;

  t.compute(0); // go!

  // print tape after operations performed
  for(i=0;i<t.tape.size();i++)
  {
    std::cout << t.tape[i] << std::endl;
  }
  
  return 0;
}