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  /*
   * Copyright 1999-2004 The Apache Software Foundation.
   *
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
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  /*
   * $Id: CoroutineManager.java,v 1.2.4.1 2005/09/15 08:14:58 suresh_emailid Exp $
   */
  package jaxp.sun.org.apache.xml.internal.dtm.ref;
  
  import java.util.BitSet;
  
  import jaxp.sun.org.apache.xml.internal.res.XMLErrorResources;
  import jaxp.sun.org.apache.xml.internal.res.XMLMessages;
  
  
  /**
   * <p>Support the coroutine design pattern.</p>
   *
   * <p>A coroutine set is a very simple cooperative non-preemptive
   * multitasking model, where the switch from one task to another is
   * performed via an explicit request. Coroutines interact according to
   * the following rules:</p>
   *
   * <ul>
   * <li>One coroutine in the set has control, which it retains until it
   * either exits or resumes another coroutine.</li>
   * <li>A coroutine is activated when it is resumed by some other coroutine
   * for the first time.</li>
   * <li>An active coroutine that gives up control by resuming another in
   * the set retains its context -- including call stack and local variables
   * -- so that if/when it is resumed, it will proceed from the point at which
   * it last gave up control.</li>
   * </ul>
   *
   * <p>Coroutines can be thought of as falling somewhere between pipes and
   * subroutines. Like call/return, there is an explicit flow of control
   * from one coroutine to another. Like pipes, neither coroutine is
   * actually "in charge", and neither must exit in order to transfer
   * control to the other. </p>
   *
   * <p>One classic application of coroutines is in compilers, where both
   * the parser and the lexer are maintaining complex state
   * information. The parser resumes the lexer to process incoming
   * characters into lexical tokens, and the lexer resumes the parser
   * when it has reached a point at which it has a reliably interpreted
   * set of tokens available for semantic processing. Structuring this
   * as call-and-return would require saving and restoring a
   * considerable amount of state each time. Structuring it as two tasks
   * connected by a queue may involve higher overhead (in systems which
   * can optimize the coroutine metaphor), isn't necessarily as clear in
   * intent, may have trouble handling cases where data flows in both
   * directions, and may not handle some of the more complex cases where
   * more than two coroutines are involved.</p>
   *
   * <p>Most coroutine systems also provide a way to pass data between the
   * source and target of a resume operation; this is sometimes referred
   * to as "yielding" a value.  Others rely on the fact that, since only
   * one member of a coroutine set is running at a time and does not
   * lose control until it chooses to do so, data structures may be
   * directly shared between them with only minimal precautions.</p>
   *
   * <p>"Note: This should not be taken to mean that producer/consumer
   * problems should be always be done with coroutines." Queueing is
   * often a better solution when only two threads of execution are
   * involved and full two-way handshaking is not required. It's a bit
   * difficult to find short pedagogical examples that require
   * coroutines for a clear solution.</p>
   *
   * <p>The fact that only one of a group of coroutines is running at a
   * time, and the control transfer between them is explicit, simplifies
   * their possible interactions, and in some implementations permits
   * them to be implemented more efficiently than general multitasking.
   * In some situations, coroutines can be compiled out entirely;
   * in others, they may only require a few instructions more than a
   * simple function call.</p>
   *
   * <p>This version is built on top of standard Java threading, since
   * that's all we have available right now. It's been encapsulated for
   * code clarity and possible future optimization.</p>
   *
   * <p>(Two possible approaches: wait-notify based and queue-based. Some
   * folks think that a one-item queue is a cleaner solution because it's
   * more abstract -- but since coroutine _is_ an abstraction I'm not really
   * worried about that; folks should be able to switch this code without
   * concern.)</p>
   *
   * <p>%TBD% THIS SHOULD BE AN INTERFACE, to facilitate building other
   * implementations... perhaps including a true coroutine system
   * someday, rather than controlled threading. Arguably Coroutine
   * itself should be an interface much like Runnable, but I think that
   * can be built on top of this.</p>
   * */
  public class CoroutineManager
  {
    /** "Is this coroutine ID number already in use" lookup table.
     * Currently implemented as a bitset as a compromise between
     * compactness and speed of access, but obviously other solutions
     * could be applied.
     * */
    BitSet m_activeIDs=new BitSet();
  
    /** Limit on the coroutine ID numbers accepted. I didn't want the
     * in-use table to grow without bound. If we switch to a more efficient
     * sparse-array mechanism, it may be possible to raise or eliminate
     * this boundary.
     * @xsl.usage internal
     */
    static final int m_unreasonableId=1024;
  
    /** Internal field used to hold the data being explicitly passed
     * from one coroutine to another during a co_resume() operation.
     * (Of course implicit data sharing may also occur; one of the reasons
     * for using coroutines is that you're guaranteed that none of the
     * other coroutines in your set are using shared structures at the time
     * you access them.)
     *
     * %REVIEW% It's been proposed that we be able to pass types of data
     * other than Object -- more specific object types, or
     * lighter-weight primitives.  This would seem to create a potential
     * explosion of "pass x recieve y back" methods (or require
     * fracturing resume into two calls, resume-other and
     * wait-to-be-resumed), and the weight issue could be managed by
     * reusing a mutable buffer object to contain the primitive
     * (remember that only one coroutine runs at a time, so once the
     * buffer's set it won't be walked on). Typechecking objects is
     * interesting from a code-robustness point of view, but it's
     * unclear whether it makes sense to encapsulate that in the
     * coroutine code or let the callers do it, since it depends on RTTI
     * either way. Restricting the parameters to objects implementing a
     * specific CoroutineParameter interface does _not_ seem to be a net
     * win; applications can do so if they want via front-end code, but
     * there seem to be too many use cases involving passing an existing
     * object type that you may not have the freedom to alter and may
     * not want to spend time wrapping another object around.
     * */
    Object m_yield=null;
  
    // Expose???
    final static int NOBODY=-1;
    final static int ANYBODY=-1;
  
    /** Internal field used to confirm that the coroutine now waking up is
     * in fact the one we intended to resume. Some such selection mechanism
     * is needed when more that two coroutines are operating within the same
     * group.
     */
    int m_nextCoroutine=NOBODY;
  
    /** <p>Each coroutine in the set managed by a single
     * CoroutineManager is identified by a small positive integer. This
     * brings up the question of how to manage those integers to avoid
     * reuse... since if two coroutines use the same ID number, resuming
     * that ID could resume either. I can see arguments for either
     * allowing applications to select their own numbers (they may want
     * to declare mnemonics via manefest constants) or generating
     * numbers on demand.  This routine's intended to support both
     * approaches.</p>
     *
     * <p>%REVIEW% We could use an object as the identifier. Not sure
     * it's a net gain, though it would allow the thread to be its own
     * ID. Ponder.</p>
     *
     * @param coroutineID  If >=0, requests that we reserve this number.
     * If <0, requests that we find, reserve, and return an available ID
     * number.
     *
     * @return If >=0, the ID number to be used by this coroutine. If <0,
     * an error occurred -- the ID requested was already in use, or we
     * couldn't assign one without going over the "unreasonable value" mark
     * */
    public synchronized int co_joinCoroutineSet(int coroutineID)
    {
      if(coroutineID>=0)
        {
          if(coroutineID>=m_unreasonableId || m_activeIDs.get(coroutineID))
            return -1;
        }
      else
        {
          // What I want is "Find first clear bit". That doesn't exist.
          // JDK1.2 added "find last set bit", but that doesn't help now.
          coroutineID=0;
          while(coroutineID<m_unreasonableId)
            {
              if(m_activeIDs.get(coroutineID))
                ++coroutineID;
              else
                break;
            }
          if(coroutineID>=m_unreasonableId)
            return -1;
        }
  
      m_activeIDs.set(coroutineID);
      return coroutineID;
    }
  
    /** In the standard coroutine architecture, coroutines are
     * identified by their method names and are launched and run up to
     * their first yield by simply resuming them; its's presumed that
     * this recognizes the not-already-running case and does the right
     * thing. We seem to need a way to achieve that same threadsafe
     * run-up...  eg, start the coroutine with a wait.
     *
     * %TBD% whether this makes any sense...
     *
     * @param thisCoroutine the identifier of this coroutine, so we can
     * recognize when we are being resumed.
     * @exception java.lang.NoSuchMethodException if thisCoroutine isn't
     * a registered member of this group. %REVIEW% whether this is the
     * best choice.
     * */
    public synchronized Object co_entry_pause(int thisCoroutine) throws java.lang.NoSuchMethodException
    {
      if(!m_activeIDs.get(thisCoroutine))
        throw new java.lang.NoSuchMethodException();
  
      while(m_nextCoroutine != thisCoroutine)
        {
          try
            {
              wait();
            }
          catch(java.lang.InterruptedException e)
            {
              // %TBD% -- Declare? Encapsulate? Ignore? Or
              // dance widdershins about the instruction cache?
            }
        }
  
      return m_yield;
    }
  
    /** Transfer control to another coroutine which has already been started and
     * is waiting on this CoroutineManager. We won't return from this call
     * until that routine has relinquished control.
     *
     * %TBD% What should we do if toCoroutine isn't registered? Exception?
     *
     * @param arg_object A value to be passed to the other coroutine.
     * @param thisCoroutine Integer identifier for this coroutine. This is the
     * ID we watch for to see if we're the ones being resumed.
     * @param toCoroutine  Integer identifier for the coroutine we wish to
     * invoke.
     * @exception java.lang.NoSuchMethodException if toCoroutine isn't a
     * registered member of this group. %REVIEW% whether this is the best choice.
     * */
    public synchronized Object co_resume(Object arg_object,int thisCoroutine,int toCoroutine) throws java.lang.NoSuchMethodException
    {
      if(!m_activeIDs.get(toCoroutine))
        throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_NOT_AVAIL, new Object[]{Integer.toString(toCoroutine)})); //"Coroutine not available, id="+toCoroutine);
  
      // We expect these values to be overwritten during the notify()/wait()
      // periods, as other coroutines in this set get their opportunity to run.
      m_yield=arg_object;
      m_nextCoroutine=toCoroutine;
  
      notify();
      while(m_nextCoroutine != thisCoroutine || m_nextCoroutine==ANYBODY || m_nextCoroutine==NOBODY)
        {
          try
            {
              // System.out.println("waiting...");
              wait();
            }
          catch(java.lang.InterruptedException e)
            {
              // %TBD% -- Declare? Encapsulate? Ignore? Or
              // dance deasil about the program counter?
            }
        }
  
      if(m_nextCoroutine==NOBODY)
        {
          // Pass it along
          co_exit(thisCoroutine);
          // And inform this coroutine that its partners are Going Away
          // %REVIEW% Should this throw/return something more useful?
          throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_CO_EXIT, null)); //"CoroutineManager recieved co_exit() request");
        }
  
      return m_yield;
    }
  
    /** Terminate this entire set of coroutines. The others will be
     * deregistered and have exceptions thrown at them. Note that this
     * is intended as a panic-shutdown operation; under normal
     * circumstances a coroutine should always end with co_exit_to() in
     * order to politely inform at least one of its partners that it is
     * going away.
     *
     * %TBD% This may need significantly more work.
     *
     * %TBD% Should this just be co_exit_to(,,CoroutineManager.PANIC)?
     *
     * @param thisCoroutine Integer identifier for the coroutine requesting exit.
     * */
    public synchronized void co_exit(int thisCoroutine)
    {
      m_activeIDs.clear(thisCoroutine);
      m_nextCoroutine=NOBODY; // %REVIEW%
      notify();
    }
  
    /** Make the ID available for reuse and terminate this coroutine,
     * transferring control to the specified coroutine. Note that this
     * returns immediately rather than waiting for any further coroutine
     * traffic, so the thread can proceed with other shutdown activities.
     *
     * @param arg_object    A value to be passed to the other coroutine.
     * @param thisCoroutine Integer identifier for the coroutine leaving the set.
     * @param toCoroutine   Integer identifier for the coroutine we wish to
     * invoke.
     * @exception java.lang.NoSuchMethodException if toCoroutine isn't a
     * registered member of this group. %REVIEW% whether this is the best choice.
     * */
    public synchronized void co_exit_to(Object arg_object,int thisCoroutine,int toCoroutine) throws java.lang.NoSuchMethodException
    {
      if(!m_activeIDs.get(toCoroutine))
        throw new java.lang.NoSuchMethodException(XMLMessages.createXMLMessage(XMLErrorResources.ER_COROUTINE_NOT_AVAIL, new Object[]{Integer.toString(toCoroutine)})); //"Coroutine not available, id="+toCoroutine);
  
      // We expect these values to be overwritten during the notify()/wait()
      // periods, as other coroutines in this set get their opportunity to run.
      m_yield=arg_object;
      m_nextCoroutine=toCoroutine;
  
      m_activeIDs.clear(thisCoroutine);
  
      notify();
    }
  }
  