src/repl/BlockingPrologControl.java

package com.googlecode.prolog_cafe.repl;

import com.googlecode.prolog_cafe.exceptions.PrologException;
import com.googlecode.prolog_cafe.lang.Predicate;
import com.googlecode.prolog_cafe.lang.PrologControl;
import com.googlecode.prolog_cafe.lang.Term;

/**
 * Prolog thread.<br>
 * The <code>BlockingPrologControl</code> class is an implementation of
 * <em>Prolog Box Control Flow Model</em>.<br>
 * This <code>BlockingPrologControl</code> provides methods
 * for both sequential and parallel execution.
 *
 * <pre>
 * // An example of sequential execution
 * // calls a goal <code>father(abraham, X)</code> and get all solutions.
 * PrologControl p = new PrologControl();
 * Predicate code = new PRED_father_2();
 * Term a1 = SymbolTerm.makeSymbol("abraham");
 * Term a2 = new VariableTerm();
 * Term[] args = {a1, a2};
 * p.setPredicate(code, args);
 * for (boolean r = p.call(); r; r = p.redo()) {
 *     System.out.println(a2.toString());
 * }
 * </pre>
 *
 * <pre>
 * // To get only one solution.
 * PrologControl p = new PrologControl();
 * Predicate code = new PRED_father_2();
 * Term a1 = SymbolTerm.makeSymbol("abraham");
 * Term a2 = new VariableTerm();
 * Term[] args = {a1, a2};
 * if (p.execute(code, args))
 *     System.out.println(a2.toString());
 * else
 *     System.out.println("fail");
 * </pre>
 *
 * <pre>
 * // An example of parallel execution
 * // calls <code>queens(4,X)</code> and <code>queens(8,Y)</code> in parallel.
 * // Usage:
 * //   % plcafe -cp queens.jar T
 * //
 * import com.googlecode.prolog_cafe.lang.*;
 * public class T {
 *     public static void main(String args[]) {
 *     long t = System.currentTimeMillis();
 *     boolean r1 = true;
 *     boolean r2 = true;
 *     Term a1[] = {new IntegerTerm(4), new VariableTerm()};
 *     Term a2[] = {new IntegerTerm(8), new VariableTerm()};
 *
 *     PrologControl e1 = new PrologControl();
 *     PrologControl e2 = new PrologControl();
 *     Term v1 = new VariableTerm();
 *     Term v2 = new VariableTerm();
 *     e1.setPredicate(new PRED_queens_2(), a1);
 *     e2.setPredicate(new PRED_queens_2(), a2);
 *     System.out.println("Start");
 *     e1.start();
 *     e2.start();
 *     while (r1 || r2) {
 *      try {
 *      Thread.sleep(10);
 *      } catch (InterruptedException e) {}
 *      if (r1 && e1.ready()) {
 *      r1 = e1.next();
 *      if (r1) {
 *          System.out.println("Success1 = " + a1[1]);
 *          e1.cont();
 *      } else {
 *          System.out.println("Fail1");
 *      }
 *      } else if (r2 && e2.ready()) {
 *      r2 = e2.next();
 *      if (r2) {
 *          System.out.println("Success2 = " + a2[1]);
 *          e2.cont();
 *      } else {
 *          System.out.println("Fail2");
 *      }
 *      } else {
 *      System.out.println("Waiting");
 *      try {
 *          Thread.sleep(100);
 *      } catch (InterruptedException e) {}
 *      }
 *  }
 *  System.out.println("End");
 *  long t1 = System.currentTimeMillis();
 *  long t2 = t1 - t;
 *  System.out.println("time = " + t2 + "msec.");
 *    }
 * }
 * </pre>
 *
 * @author Shawn Pearce (sop@google.com)
 */
public class BlockingPrologControl
    extends PrologControl
    implements Runnable {

  /** A volatile instance variable holding a thread. */
  private volatile Thread thread;

  /** A flag that indicates whether the result of goal is <code>true</code> or <code>false</code>. */
  private boolean result;

  /** A flag that indicates whether the result of goal is ready or not. */
  private boolean resultReady;

  /** Constructs a new <code>BlockingPrologControl</code>. */
  public BlockingPrologControl() {
    engine.init();
  }

  /**
   * Returns <code>true</code> if the system succeeds to find a first solution
   * of the given goal, <code>false</code> otherwise.<br>
   *
   * This method is useful to find only one solution.<br>
   *
   * This method first initilizes the Prolog engine by invoking <code>engine.init()</code>,
   * allocates a new <code>Thread</code> object, and start the execution of the given goal.
   * And then it stops the thread and returns <code>true</code>
   * if the goal succeeds, <code>false</code> otherwise.
   * @see #run
   */
  public synchronized boolean execute(String pkg, String functor, Term... args) {
    return execute(getPrologClassLoader().predicate(pkg, functor, args));
  }

  /**
   * Returns <code>true</code> if the system succeeds to find a first solution
   * of the given goal, <code>false</code> otherwise.<br>
   *
   * This method is useful to find only one solution.<br>
   *
   * This method first initilizes the Prolog engine by invoking <code>engine.init()</code>,
   * allocates a new <code>Thread</code> object, and start the execution of the given goal.
   * And then it stops the thread and returns <code>true</code>
   * if the goal succeeds, <code>false</code> otherwise.
   * @see #run
   */
  public synchronized boolean execute(Predicate p) {
  setPredicate(p);
  thread = new Thread(this);
  thread.setName("Prolog-" + p.toString());
  thread.start(); // execute run() in new thread.
  try {
      wait();     // wait caller's thread.
  } catch (InterruptedException e) {}
  stop();
  return result;
  }

  /**
   * Returns <code>true</code> if the system succeeds to find a first solution
   * of the goal, <code>false</code> otherwise.<br>
   *
   * This method first invokes the <code>start()</code> method that
   * initilizes the Prolog engine, allocates a new <code>Thread</code> object,
   * and start the execution.
   * And then it returns the <code>boolean</code> whose value is <code>next()</code>.
   * @see #start
   * @see #next
   */
  public synchronized boolean call() {
  start();
  return next();
  }

  /**
   * Returns <code>true</code> if the system succeeds to find a next solution
   * of the goal, <code>false</code> otherwise.<br>
   *
   * This method first invokes the <code>cont()</code> method that
   * sets the <code>resultReady</code> to <code>false</code>
   * and wakes up all threads that are waiting on this object's monitor.
   * And then it returns the <code>boolean</code> whose value is <code>next()</code>.
   * @see #cont
   * @see #next
   */
  public synchronized boolean redo() {
  cont();
  return next();
  }

  /**
   * Is invoked when the system succeeds to find a solution.<br>
   *
   * This method is invoked from the initial continuation goal
   * (a <code>Success</code> object).<br>
   *
   * This method first sets the <code>resultReady</code> and <code>result</code> to <code>true</code>.
   * And then it wakes up all threads that are waiting by <code>notifyAll()</code>.
   * Finally, while the <code>thread</code> is not <code>null</code> and
   * the <code>resultReady</code> is <code>true</code>,
   * it waits until another thread invokes the <code>notify()</code> method
   * or the <code>notifyAll()</code> method for this object.
   * @see #resultReady
   * @see #result
   * @see #thread
   */
  @Override
  protected synchronized void success() {
  resultReady = true;
  result = true;
  notifyAll();
  while (thread != null && resultReady) {
      try {
      wait();
      } catch (InterruptedException e) {}
  }
  }

  /**
   * Is invoked after failure of all trials.<br>
   *
   * This method is invoked from the <code>run</code> method.<br>
   *
   * This method first sets the <code>resultReady</code> and <code>result</code>
   * to <code>true</code> and <code>false</code> respectively.
   * And then it wakes up all threads that are waiting by <code>notifyAll()</code>.
   * Finally, while the <code>thread</code> is not <code>null</code> and
   * the <code>resultReady</code> is <code>true</code>,
   * it waits until another thread invokes the <code>notify()</code> method
   * or the <code>notifyAll()</code> method for this object.
   * @see #resultReady
   * @see #result
   * @see #thread
   */
  @Override
  protected synchronized void fail() {
  resultReady = true;
  result = false;
  notifyAll();
  while (thread != null && resultReady) {
      try {
      wait();
      } catch (InterruptedException e) {}
  }
  }

  /** @return true if the engine is no longer supposed to execute. */
  @Override
  public boolean isEngineStopped() {
    return thread == null;
  }

  /** Waits for this thread to die. */
  public synchronized void join() {
  while (thread != null && ! resultReady) {
      try {
      wait();
      } catch (InterruptedException e) {}
  }
  stop();
  }

  /**
   * Forces the thread to stop.<br>
   *
   * This method first sets the <code>resultReady</code> and <code>thread</code>
   * to <code>false</code> and <code>null</code> respectively.
   * And then it wakes up all threads that are waiting by <code>notifyAll()</code>.
   * @see #resultReady
   * @see #thread
   */
  public synchronized void stop() {
  resultReady = false;
  thread = null;
  notifyAll();
  }

  /**
   * Forces the thread to start the execution.<br>
   *
   * This method initilizes the Prolog engine by invoking <code>engine.init()</code>,
   * allocates a new <code>Thread</code> object, and start the execution.
   * The Java Virtual Machine calls the <code>run</code> method of this thread.
   * @see #run
   */
  public synchronized void start() {
  resultReady = false;
  thread = new Thread(this);
  thread.start();
  }

  /**
   * Forces the thread to continue the execution.<br>
   *
   * This method sets the <code>resultReady</code> to <code>false</code>,
   * and then wakes up all threads that are waiting by <code>notifyAll()</code>.
   * @see #resultReady
   */
  public synchronized void cont() {
  resultReady = false;
  notifyAll();
  }

  /**
   * Returns <code>true</code> if the result of goal is ready,
   * <code>false</code> otherwise.
   * @return a <code>boolean</code> whose value is <code>resultReady</code>.
   * @see #resultReady
   */
  public synchronized boolean ready() {
  return resultReady;
  }

  /**
   * Returns <code>true</code> if the result of goal is ready and true, otherwise <code>false</code>.
   * @return a <code>boolean</code> whose value is <code>(ready() &amp;&amp; result)</code>.
   * @see #ready
   * @see #result
   */
  public synchronized boolean in_success() {
  return ready() && result;
  }

  /**
   * Returns <code>true</code> if the result of goal is ready and false, otherwise <code>false</code>.
   * @return a <code>boolean</code> whose value is <code>(ready() &amp;&amp; !result)</code>.
   * @see #ready
   * @see #result
   */
  public synchronized boolean in_failure() {
  return ready() && ! result;
  }

  /**
   * Wait until the system finds a next solution,
   * and then returns the result as <code>boolean</code>.<br>
   *
   * This method first waits until another thread invokes the <code>notify()</code>
   * method or the <code>notifyAll()</code> method for this object,
   * while the <code>thread</code> is not <code>null</code> and
   * the <code>resultReady</code> is <code>false</code>.
   * And then invokes the <code>stop()</code> if the <code>result</code> is <code>false</code>.
   * Finally, returns the <code>result</code>.
   * @see #resultReady
   * @see #result
   * @see #thread
   */
  public synchronized boolean next() {
  while (thread != null && ! resultReady) {
      try {
      wait();
      } catch (InterruptedException e) {}
  }
  if (! result) {
      stop();
  }
  return result;
  }

  /**
   * Executes the goal.<br>
   *
   * Every time finding a solution, the <code>success</code> method is invoked.
   * And then the <code>fail</code> method is invoked after failure of all trials.
   * Finally, the <code>stop</code> method is invoked at the end of this <code>run</code>.
   * @see #success
   * @see #fail
   * @see #stop
   */
  @Override
  public void run() {
    try {
      executePredicate();
  } catch (PrologException e){
      System.out.println(e.toString());
  } catch (Exception e){
      printStackTrace(e);
  }
  stop();
  }
}