using System;

// undegrad, grad, nostudent will implement student.

public interface student
{
   object accept(svisitor v);
}

public interface svisitor
{
   object visit(undergrad x);
   object visit(grad x);
   object visit(nostudent x);  // default student is nostudent
}

public class stubase  // base class for many student classes:
{
  public readonly string name;
  private double gpa;
  public double GPA  // example of a "property"
  {
     get { return gpa; }
     set { double v=value; if (v>=0 && v<=4.0) gpa=v; } // value is keyword
  }
  public student next; // pointer to next student in list
  public stubase(string n, double g, student nx)  // convenience
  { name=n;  GPA=g; next=nx; }

  // ok to put the following in the base class?
  // public object accept(svisitor v) { return v.visit(this); }
  // NO because the type of *this* will be fixed as stubase.
}// stubase

public class undergrad: stubase, student
{
   public string major;
   public undergrad(string n, double g, student nx): base(n,g,nx)
   { string[] Majors = {"cs","cse","math","physics","chem","art","history"};
     major = Majors[ (new Random()).Next(0,Majors.Length) ];
   }

  public object accept(svisitor v) { return v.visit(this); }
}

public class grad: stubase,student
{
   public string topic; // thesis topic   
   public grad(string n, string t, student nx):base(n,0,nx)
   {
      topic =t;  GPA = (new Random()).NextDouble() * 4.0;
   }

  public object accept(svisitor v) { return v.visit(this); }
}//grad

public class nostudent : student // just a tag
{
  public object accept(svisitor v) { return v.visit(this); }
}

////////////// visitors
// gpa averager : computes undergrad/grad gpa separately and combined.
// returns overall average
public class gpaaverager : svisitor
{
   double ugpa=0, ggpa=0;
   int ucount=0, gcount=0;
   public object visit(undergrad x) 
   { ugpa += x.GPA; ucount++; 
     return x.next.accept(this); 
   }
   public object visit(grad x) 
   {
     ggpa += x.GPA; gcount++;
     return x.next.accept(this); 
   }
   public object visit(nostudent x)
   {
      double uave = ugpa/ucount;
      double gave = ggpa/gcount;
      Console.WriteLine("Undergrad average gpa = "+uave);
      Console.WriteLine("Grad average gpa = "+gave);
      double uratio = (ucount*1.0)/(gcount+ucount);
      double ave = uave*uratio + gave*(1-uratio); // overall ave
      ucount = 0; gcount = 0; ugpa = 0; ggpa = 0;  // reset
      return ave;  // doubles are also objects (no need to use Double)
   }
}// gpaaverager

// probation auditor: finds students on academic probation!
// uses loop instead of recursion
public class auditor : svisitor
{
   protected bool stop = false;
   protected student current;
   public auditor(student start)
   {  
      current = start;
   }

   public void run()  // now the visitor is stateful
   {
      stop = false;   // reset if needed
      while (!stop) current.accept(this);
   }

   public object visit(undergrad x)
   {
     if (x.GPA<2.0) Console.WriteLine(x.name+" is on academic probation");
     current = x.next; return null;
   }
   public object visit(grad x)
   {
     if (x.GPA<3.0) Console.WriteLine(x.name+" is on academic probation");
     current = x.next;  return null; 
   }  
   public object visit(nostudent x)  { stop = true; return null; }
}

/*
Compile into stu13.dll with csc /t:library stu13.cs
*/