/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements. See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership. The ASF licenses this file
 * to you 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.
 */
/*
 * $Id: DTMStringPool.java 468653 2006-10-28 07:07:05Z minchau $
 */

package org.apache.xml.dtm.ref;

import java.util.Vector;

import org.apache.xml.utils.IntVector;

DTMStringPool is an "interning" mechanism for strings. It will
create a stable 1:1 mapping between a set of string values and a set of
integer index values, so the integers can be used to reliably and
uniquely identify (and when necessary retrieve) the strings.
Design Priorities:

String-to-index lookup speed is critical.
Index-to-String lookup speed is slightly less so.
Threadsafety is not guaranteed at this level.
Enforce that in the application if needed.
Storage efficiency is an issue but not a huge one.
It is expected that string pools won't exceed about 2000 entries.


Implementation detail: A standard Hashtable is relatively
inefficient when looking up primitive int values, especially when
we're already maintaining an int-to-string vector.  So I'm
maintaining a simple hash chain within this class.
NOTE: There is nothing in the code that has a real dependency upon
String. It would work with any object type that implements reliable
.hashCode() and .equals() operations. The API enforces Strings because
it's safer that way, but this could trivially be turned into a general
ObjectPool if one was needed.
Status: Passed basic test in main().
/** <p>DTMStringPool is an "interning" mechanism for strings. It will
 * create a stable 1:1 mapping between a set of string values and a set of
 * integer index values, so the integers can be used to reliably and
 * uniquely identify (and when necessary retrieve) the strings.</p>
 *
 * <p>Design Priorities:
 * <ul>
 * <li>String-to-index lookup speed is critical.</li>
 * <li>Index-to-String lookup speed is slightly less so.</li>
 * <li>Threadsafety is not guaranteed at this level.
 * Enforce that in the application if needed.</li>
 * <li>Storage efficiency is an issue but not a huge one.
 * It is expected that string pools won't exceed about 2000 entries.</li>
 * </ul>
 * </p>
 *
 * <p>Implementation detail: A standard Hashtable is relatively
 * inefficient when looking up primitive int values, especially when
 * we're already maintaining an int-to-string vector.  So I'm
 * maintaining a simple hash chain within this class.</p>
 *
 * <p>NOTE: There is nothing in the code that has a real dependency upon
 * String. It would work with any object type that implements reliable
 * .hashCode() and .equals() operations. The API enforces Strings because
 * it's safer that way, but this could trivially be turned into a general
 * ObjectPool if one was needed.</p>
 *
 * <p>Status: Passed basic test in main().</p>
 * */
public class DTMStringPool
{
  Vector m_intToString;
  static final int HASHPRIME=101;
  int[] m_hashStart=new int[HASHPRIME];
  IntVector m_hashChain;
  public static final int NULL=-1;

  
Create a DTMStringPool using the given chain size
Params:
chainSize – The size of the hash chain vector/**
   * Create a DTMStringPool using the given chain size
   * 
   * @param chainSize The size of the hash chain vector
   */
  public DTMStringPool(int chainSize)
    {
      m_intToString=new Vector();
      m_hashChain=new IntVector(chainSize);
      removeAllElements();
      
      // -sb Add this to force empty strings to be index 0.
      stringToIndex("");
    }
  
  public DTMStringPool()
    {
      this(512);	
    }
    
  public void removeAllElements()
    {
      m_intToString.removeAllElements();
      for(int i=0;i<HASHPRIME;++i)
        m_hashStart[i]=NULL;
      m_hashChain.removeAllElements();
    }

  
Throws:
ArrayIndexOutOfBoundsException – 
 if index doesn't map to a string.
Returns:
string whose value is uniquely identified by this integer index./** @return string whose value is uniquely identified by this integer index.
   * @throws java.lang.ArrayIndexOutOfBoundsException
   *  if index doesn't map to a string.
   * */ 
  public String indexToString(int i)
    throws java.lang.ArrayIndexOutOfBoundsException
    {
      if(i==NULL) return null;
      return (String) m_intToString.elementAt(i);
    }

  
Returns:
integer index uniquely identifying the value of this string./** @return integer index uniquely identifying the value of this string. */ 
  public int stringToIndex(String s)
    {
      if(s==null) return NULL;
      
      int hashslot=s.hashCode()%HASHPRIME;
      if(hashslot<0) hashslot=-hashslot;

      // Is it one we already know?
      int hashlast=m_hashStart[hashslot];
      int hashcandidate=hashlast;
      while(hashcandidate!=NULL)
        {
          if(m_intToString.elementAt(hashcandidate).equals(s))
            return hashcandidate;

          hashlast=hashcandidate;
          hashcandidate=m_hashChain.elementAt(hashcandidate);
        }
      
      // New value. Add to tables.
      int newIndex=m_intToString.size();
      m_intToString.addElement(s);

      m_hashChain.addElement(NULL);	// Initialize to no-following-same-hash
      if(hashlast==NULL)  // First for this hash
        m_hashStart[hashslot]=newIndex;
      else // Link from previous with same hash
        m_hashChain.setElementAt(newIndex,hashlast);

      return newIndex;
    }

  
Command-line unit test driver. This test relies on the fact that
this version of the pool assigns indices consecutively, starting
from zero, as new unique strings are encountered.
/** Command-line unit test driver. This test relies on the fact that
   * this version of the pool assigns indices consecutively, starting
   * from zero, as new unique strings are encountered.
   */
  public static void main(String[] args)
  {
    String[] word={
      "Zero","One","Two","Three","Four","Five",
      "Six","Seven","Eight","Nine","Ten",
      "Eleven","Twelve","Thirteen","Fourteen","Fifteen",
      "Sixteen","Seventeen","Eighteen","Nineteen","Twenty",
      "Twenty-One","Twenty-Two","Twenty-Three","Twenty-Four",
      "Twenty-Five","Twenty-Six","Twenty-Seven","Twenty-Eight",
      "Twenty-Nine","Thirty","Thirty-One","Thirty-Two",
      "Thirty-Three","Thirty-Four","Thirty-Five","Thirty-Six",
      "Thirty-Seven","Thirty-Eight","Thirty-Nine"};

    DTMStringPool pool=new DTMStringPool();

    System.out.println("If no complaints are printed below, we passed initial test.");

    for(int pass=0;pass<=1;++pass)
      {
        int i;

        for(i=0;i<word.length;++i)
          {
            int j=pool.stringToIndex(word[i]);
            if(j!=i)
              System.out.println("\tMismatch populating pool: assigned "+
                                 j+" for create "+i);
          }

        for(i=0;i<word.length;++i)
          {
            int j=pool.stringToIndex(word[i]);
            if(j!=i)
              System.out.println("\tMismatch in stringToIndex: returned "+
                                 j+" for lookup "+i);
          }

        for(i=0;i<word.length;++i)
          {
            String w=pool.indexToString(i);
            if(!word[i].equals(w))
              System.out.println("\tMismatch in indexToString: returned"+
                                 w+" for lookup "+i);
          }
        
        pool.removeAllElements();
        
        System.out.println("\nPass "+pass+" complete\n");
      } // end pass loop
  }
}