# include <cstdlib>
# include <iostream>
# include <ctime>

using namespace std;

int main ( )
//
//  RANDOM_WALK_2D puts a fly at the origin of a box whose walls are
//  20 units away, and a spider at a random position.  The spider and
//  the fly randomly walk around the box.  The fly "wins" by stepping on
//  the spider.  The spider wins by stepping on the fly.  The fly escapes
//  by reaching one of the walls.
//
{
  int dist, step, x, x2, y, y2;
  int random_int ( int a, int b );

  srand ( time ( 0 ) );

  step = 0;
  x = 0;
  y = 0;

  x2 = random_int ( -10, 10 );
  y2 = random_int ( -10, 10 );
  dist = abs ( x - x2 ) + abs ( y - y2 );
//
//  Output initial position for GNUPLOT.
//
  cout << "  " << step 
       << "  " << x << "  " << y 
       << "  " << x2 << "  " << y2 
       << "  " << dist << "\n";

  while ( true )
  {
    step = step + 1;

    x = x + random_int ( -1, +1 );
    y = y + random_int ( -1, +1 );
//
//  Did we touch the spider?
//
    if ( x == x2 && y == y2 )
    {
      cerr << "Caught the spider on step " << step << "\n";
      break;
    }
//
//  Did we reach the wall?
//
    if ( x == -20 || x == +20 || y == -20 || y == +20 )
    {
      cerr << "Reached safety on step " << step << "\n";
      break;
    }
//
//  The spider moves.
//
    x2 = x2 + random_int ( -1, +1 );
    y2 = y2 + random_int ( -1, +1 );
//
//  The spider doesn't leave the box.
//
    if ( x2 < -20 )
    {
      x2 = -20;
    }
    if ( 20 < x2 )
    {
      x2 = 20;
    }
    if ( y2 < -20 )
    {
      y2 = -20;
    }
    if ( 20 < y2 )
    {
      y2 = 20;
    }
//
//  Did the spider eat the fly?
//
    if ( x == x2 && y == y2 )
    {
      cerr << "The spider caught the fly on step " << step << "\n";
      break;
    }
//
//  Compute distance.
//  The ABS() function is used for the absolute value of ints.
//
    dist = abs ( x - x2 ) + abs ( y - y2 );
//
//  Output current status for GNUPLOT.
//
    cout << "  " << step 
         << "  " << x << "  " << y 
         << "  " << x2 << "  " << y2 
         << "  " << dist << "\n";
  }
  return 0;
}
int random_int ( int a, int b )
//
// RANDOM_INT returns a random int between a and b.
//
{
  int range;
  int value;
//
//  If we want integers between A and B, there are actually
//  B - A + 1 values.
//
  range = b - a + 1;

  value = a + rand ( ) % range;

  return value;
}