code: /*
   * 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.
   */
  /*
   * $Id$
   */
  
  package org.apache.xml.dtm.ref;
  
  import java.util.Vector;
  
  import org.apache.xml.utils.IntVector;
  
  /** <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
     * 
     * @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();
      }
  
    /** @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);
      }
  
    /** @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.
     */
    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
    }
  }
  