/*
 * Sequence.java
 *
 * R. Perry, July 1998
 */

package sequence;

import java.security.*;
import java.io.*;

/**
 * Generation and comparisons of a sequence of pseudo-random long values.
 *
 * @author  Rick Perry
 * @version Mon Jul 20 12:44:24 EDT 1998
 */

public class Sequence
{
 /**
  * The SecureRandom seed.
  */
  private byte[] seed;

 /**
  * The SecureRandom instance.
  */
  private SecureRandom rand;

 /**
  * Current sequence number.
  */
  private long current;

 /**
  * Constructs a sequence using the specified seed
  * to initialize the SecureRandom instance.
  *
  * @param seed array of 20 bytes.
  * @exception IllegalArgumentException
  *             if the seed argument is null or its length is not 20.
  */
  public Sequence( byte[] seed)
  {
    if( seed == null)
      throw new IllegalArgumentException( "bad seed = null");

    if( seed.length != 20)
      throw new IllegalArgumentException( "bad seed length (" + seed.length + ") != 20");

    this.seed = seed;
    rand = new SecureRandom( seed);
    current = rand.nextLong();
  }

 /**
  * Constructs a sequence using SecureRandom.getSeed()
  * to initialize the SecureRandom instance.
  */
  public Sequence()
  {
    this( SecureRandom.getSeed(20));	// this call takes some time
  }

 /**
  * Returns the seed used to initialize the sequence.
  *
  * @return the 20-byte seed array.
  */
  public byte[] getSeed()
  {
    return seed;
  }

 /**
  * Returns the next pseudo-random sequence number.
  *
  * @return pseudo-random sequence number.
  */
  public long next()
  {
    long r = current;

    current = rand.nextLong();

    return r;
  }

 /**
  * Compares the specified argument with the current sequence
  * number.  If they are equal, updates the current sequence
  * number to the next value.
  *
  * @param c sequence number to compare.
  * @return true if the argument equals the current sequence number,
  * otherwise false.
  */
  public boolean compare( long c)
  {
    if( c != current)
      return false;

    current = rand.nextLong();

    return true;
  }

 /**
  * Simple test program.
  */
  public static void main (String[] args)
  {
//    Sequence t;
//
//    t = new Sequence(null);
//
//    byte[] j = ("abc").getBytes();
//    t = new Sequence(j);

    Sequence s = new Sequence();
    byte[] seed = s.getSeed();
    long[] a = new long[10];

    System.out.println( "\nUsing random seed");
    for( int i = 0; i < 10; ++i) {
      a[i] = s.next();
      System.out.println( a[i]);
    }

    s = new Sequence( seed);

    System.out.println( "\nUsing previous seed");
    for( int i = 0; i < 10; ++i)
      System.out.println( s.next());

    s = new Sequence( seed);

    System.out.println( "\nCheck compare...");
    for( int i = 0; i < 10; ++i)
      System.out.println( s.compare(a[i]));
  }
}