// file:    SequenceGenerator.java
// author:  Robert Keller
// purpose: Illustrate Enumeration "on-the-fly"

import java.util.*;

/**
 * A SequenceGenerator is an Enumeration based on 3 functions:
 * output is applied to current to create the next element
 * update is applied to current to produce the next value of current
 * stopper is applied to current to know when to stop
 * current is initialized in the constructor
 */

class SequenceGenerator implements Enumeration
{
Object current;		// the current state of the generator
Function1 output;	// the output function:  current ==> result
Function1 update;	// the update function:  current ==> current
Function1 stopper;	// the stopper function: current ==> Boolean

/** Construct a SequenceGenerator. */

SequenceGenerator(Object initial, 
                  Function1 _output, 
                  Function1 _update, 
                  Function1 _stopper)
  {
  current = initial;
  output  = _output;
  update  = _update;
  stopper = _stopper;
  }


/** Produce the next element in the sequence. */

public Object nextElement()
  {
  Object result = output.apply(current);
  current = update.apply(current);
  return result;
  }


/** Indicate whether there are more elements in the sequence. */

public boolean hasMoreElements()
  {
  return !((Boolean)stopper.apply(current)).booleanValue();
  }


public static void main(String arg[])
  {
  Enumeration E = 
    new SequenceGenerator(new Integer(0),
                          new Outputter(),
                          new Updater(),
                          new Stopper(100));

  while( E.hasMoreElements() )
    {
    System.out.println(E.nextElement());
    }
  }
}


/** a sample output function object */

class Outputter implements Function1
{
/** Apply this function to an argument to get the output value. */

public Object apply(Object arg)
  {
  long value = ((Number)arg).longValue();
  return new Long(value *  value);
  }
}


/** a sample update function object */

class Updater implements Function1
{
/** Apply this function to an argument to get the next value of current. */

public Object apply(Object arg)
  {
  long value = ((Number)arg).longValue();
  return new Long(value + 2);
  }
}


/** a sample stopper function object */

class Stopper implements Function1
{
long limit;

/** Create a stopper function based on a limit */

Stopper(long _limit)
  {
  limit = _limit;
  }

/** Apply this function to an argument to determine whether to stop. */

public Object apply(Object arg)
  {
  long value = ((Number)arg).longValue();
  return new Boolean(value > limit);
  }
}