faststringbuffer.java

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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.
 */
/*
 * $Id: FastStringBuffer.java,v 1.2.4.1 2005/09/15 08:15:44 suresh_emailid Exp $
 */
package com.sun.org.apache.xml.internal.utils;

/**
 * Bare-bones, unsafe, fast string buffer. No thread-safety, no
 * parameter range checking, exposed fields. Note that in typical
 * applications, thread-safety of a StringBuffer is a somewhat
 * dubious concept in any case.
 * <p>
 * Note that Stree and DTM used a single FastStringBuffer as a string pool,
 * by recording start and length indices within this single buffer. This
 * minimizes heap overhead, but of course requires more work when retrieving
 * the data.
 * <p>
 * FastStringBuffer operates as a "chunked buffer". Doing so
 * reduces the need to recopy existing information when an append
 * exceeds the space available; we just allocate another chunk and
 * flow across to it. (The array of chunks may need to grow,
 * admittedly, but that's a much smaller object.) Some excess
 * recopying may arise when we extract Strings which cross chunk
 * boundaries; larger chunks make that less frequent.
 * <p>
 * The size values are parameterized, to allow tuning this code. In
 * theory, Result Tree Fragments might want to be tuned differently 
 * from the main document's text. 
 * <p>
 * %REVIEW% An experiment in self-tuning is
 * included in the code (using nested FastStringBuffers to achieve
 * variation in chunk sizes), but this implementation has proven to
 * be problematic when data may be being copied from the FSB into itself.
 * We should either re-architect that to make this safe (if possible)
 * or remove that code and clean up for performance/maintainability reasons.
 * <p>
 */
public class FastStringBuffer
{
  // If nonzero, forces the inial chunk size.
  /**/static final int DEBUG_FORCE_INIT_BITS=0;
  
  	// %BUG% %REVIEW% *****PROBLEM SUSPECTED: If data from an FSB is being copied
  	// back into the same FSB (variable set from previous variable, for example) 
  	// and blocksize changes in mid-copy... there's risk of severe malfunction in 
  	// the read process, due to how the resizing code re-jiggers storage. Arggh. 
  	// If we want to retain the variable-size-block feature, we need to reconsider 
  	// that issue. For now, I have forced us into fixed-size mode.
    static final boolean DEBUG_FORCE_FIXED_CHUNKSIZE=true;

	/** Manifest constant: Suppress leading whitespace.
	 * This should be used when normalize-to-SAX is called for the first chunk of a
	 * multi-chunk output, or one following unsuppressed whitespace in a previous
	 * chunk.
	 * @see #sendNormalizedSAXcharacters(org.xml.sax.ContentHandler,int,int)
	 */
	public static final int SUPPRESS_LEADING_WS=0x01;
	
	/** Manifest constant: Suppress trailing whitespace.
	 * This should be used when normalize-to-SAX is called for the last chunk of a
	 * multi-chunk output; it may have to be or'ed with SUPPRESS_LEADING_WS.
	 */
	public static final int SUPPRESS_TRAILING_WS=0x02;
	
	/** Manifest constant: Suppress both leading and trailing whitespace.
	 * This should be used when normalize-to-SAX is called for a complete string.
	 * (I'm not wild about the name of this one. Ideas welcome.)
	 * @see #sendNormalizedSAXcharacters(org.xml.sax.ContentHandler,int,int)
	 */
	public static final int SUPPRESS_BOTH
		= SUPPRESS_LEADING_WS | SUPPRESS_TRAILING_WS;

	/** Manifest constant: Carry trailing whitespace of one chunk as leading 
	 * whitespace of the next chunk. Used internally; I don't see any reason
	 * to make it public right now.
	 */
	private static final int CARRY_WS=0x04;

	/**
   * Field m_chunkBits sets our chunking strategy, by saying how many
   * bits of index can be used within a single chunk before flowing over
   * to the next chunk. For example, if m_chunkbits is set to 15, each
   * chunk can contain up to 2^15 (32K) characters  
   */
  int m_chunkBits = 15;

  /**
   * Field m_maxChunkBits affects our chunk-growth strategy, by saying what
   * the largest permissible chunk size is in this particular FastStringBuffer
   * hierarchy. 
   */
  int m_maxChunkBits = 15;

  /**
   * Field m_rechunkBits affects our chunk-growth strategy, by saying how
   * many chunks should be allocated at one size before we encapsulate them
   * into the first chunk of the next size up. For example, if m_rechunkBits
   * is set to 3, then after 8 chunks at a given size we will rebundle
   * them as the first element of a FastStringBuffer using a chunk size
   * 8 times larger (chunkBits shifted left three bits).
   */
  int m_rebundleBits = 2;

  /**
   * Field m_chunkSize establishes the maximum size of one chunk of the array
   * as 2**chunkbits characters.
   * (Which may also be the minimum size if we aren't tuning for storage) 
   */
  int m_chunkSize;  // =1<<(m_chunkBits-1);

  /**
   * Field m_chunkMask is m_chunkSize-1 -- in other words, m_chunkBits
   * worth of low-order '1' bits, useful for shift-and-mask addressing
   * within the chunks. 
   */
  int m_chunkMask;  // =m_chunkSize-1;

  /**
   * Field m_array holds the string buffer's text contents, using an
   * array-of-arrays. Note that this array, and the arrays it contains, may be
   * reallocated when necessary in order to allow the buffer to grow;
   * references to them should be considered to be invalidated after any
   * append. However, the only time these arrays are directly exposed
   * is in the sendSAXcharacters call.
   */
  char[][] m_array;

  /**
   * Field m_lastChunk is an index into m_array[], pointing to the last
   * chunk of the Chunked Array currently in use. Note that additional
   * chunks may actually be allocated, eg if the FastStringBuffer had
   * previously been truncated or if someone issued an ensureSpace request.
   * <p>
   * The insertion point for append operations is addressed by the combination
   * of m_lastChunk and m_firstFree.
   */
  int m_lastChunk = 0;

  /**
   * Field m_firstFree is an index into m_array[m_lastChunk][], pointing to
   * the first character in the Chunked Array which is not part of the
   * FastStringBuffer's current content. Since m_array[][] is zero-based,
   * the length of that content can be calculated as
   * (m_lastChunk<<m_chunkBits) + m_firstFree 
   */
  int m_firstFree = 0;

  /**
   * Field m_innerFSB, when non-null, is a FastStringBuffer whose total
   * length equals m_chunkSize, and which replaces m_array[0]. This allows
   * building a hierarchy of FastStringBuffers, where early appends use
   * a smaller chunkSize (for less wasted memory overhead) but later
   * ones use a larger chunkSize (for less heap activity overhead).
   */
  FastStringBuffer m_innerFSB = null;

  /**
   * Construct a FastStringBuffer, with allocation policy as per parameters.
   * <p>
   * For coding convenience, I've expressed both allocation sizes in terms of
   * a number of bits. That's needed for the final size of a chunk,
   * to permit fast and efficient shift-and-mask addressing. It's less critical
   * for the inital size, and may be reconsidered.
   * <p>
   * An alternative would be to accept integer sizes and round to powers of two;
   * that really doesn't seem to buy us much, if anything.
   *
   * @param initChunkBits Length in characters of the initial allocation
   * of a chunk, expressed in log-base-2. (That is, 10 means allocate 1024
   * characters.) Later chunks will use larger allocation units, to trade off
   * allocation speed of large document against storage efficiency of small
   * ones.
   * @param maxChunkBits Number of character-offset bits that should be used for
   * addressing within a chunk. Maximum length of a chunk is 2^chunkBits
   * characters.
   * @param rebundleBits Number of character-offset bits that addressing should
   * advance before we attempt to take a step from initChunkBits to maxChunkBits
   */
  public FastStringBuffer(int initChunkBits, int maxChunkBits,
                          int rebundleBits)
  {
    if(DEBUG_FORCE_INIT_BITS!=0) initChunkBits=DEBUG_FORCE_INIT_BITS;
    
    // %REVIEW%
    // Should this force to larger value, or smaller? Smaller less efficient, but if
    // someone requested variable mode it's because they care about storage space.
    // On the other hand, given the other changes I'm making, odds are that we should
    // adopt the larger size. Dither, dither, dither... This is just stopgap workaround
    // anyway; we need a permanant solution.
    //
    if(DEBUG_FORCE_FIXED_CHUNKSIZE) maxChunkBits=initChunkBits;
    //if(DEBUG_FORCE_FIXED_CHUNKSIZE) initChunkBits=maxChunkBits;

    m_array = new char[16][];

    // Don't bite off more than we're prepared to swallow!
    if (initChunkBits > maxChunkBits)
      initChunkBits = maxChunkBits;

    m_chunkBits = initChunkBits;
    m_maxChunkBits = maxChunkBits;
    m_rebundleBits = rebundleBits;
    m_chunkSize = 1 << (initChunkBits);
    m_chunkMask = m_chunkSize - 1;
    m_array[0] = new char[m_chunkSize];
  }

  /**
   * Construct a FastStringBuffer, using a default rebundleBits value.
   *
   * NEEDSDOC @param initChunkBits
   * NEEDSDOC @param maxChunkBits
   */
  public FastStringBuffer(int initChunkBits, int maxChunkBits)
  {
    this(initChunkBits, maxChunkBits, 2);
  }

  /**
   * Construct a FastStringBuffer, using default maxChunkBits and
   * rebundleBits values.
   * <p>
   * ISSUE: Should this call assert initial size, or fixed size?
   * Now configured as initial, with a default for fixed.
   *
   * NEEDSDOC @param initChunkBits
   */
  public FastStringBuffer(int initChunkBits)
  {
    this(initChunkBits, 15, 2);
  }

  /**
   * Construct a FastStringBuffer, using a default allocation policy.
   */
  public FastStringBuffer()
  {

    // 10 bits is 1K. 15 bits is 32K. Remember that these are character
    // counts, so actual memory allocation unit is doubled for UTF-16 chars.
    //
    // For reference: In the original FastStringBuffer, we simply
    // overallocated by blocksize (default 1KB) on each buffer-growth.
    this(10, 15, 2);
  }

  /**
   * Get the length of the list. Synonym for length().
   *
   * @return the number of characters in the FastStringBuffer's content.
   */
  public final int size()
  {
    return (m_lastChunk << m_chunkBits) + m_firstFree;
  }

  /**
   * Get the length of the list. Synonym for size().
   *
   * @return the number of characters in the FastStringBuffer's content.
   */
  public final int length()
  {
    return (m_lastChunk << m_chunkBits) + m_firstFree;
  }

  /**
   * Discard the content of the FastStringBuffer, and most of the memory
   * that was allocated by it, restoring the initial state. Note that this
   * may eventually be different from setLength(0), which see.
   */
  public final void reset()
  {

    m_lastChunk = 0;
    m_firstFree = 0;

    // Recover the original chunk size
    FastStringBuffer innermost = this;

    while (innermost.m_innerFSB != null)
    {
      innermost = innermost.m_innerFSB;
    }

    m_chunkBits = innermost.m_chunkBits;
    m_chunkSize = innermost.m_chunkSize;
    m_chunkMask = innermost.m_chunkMask;

    // Discard the hierarchy
    m_innerFSB = null;
    m_array = new char[16][0];
    m_array[0] = new char[m_chunkSize];
  }

  /**
   * Directly set how much of the FastStringBuffer's storage is to be
   * considered part of its content. This is a fast but hazardous
   * operation. It is not protected against negative values, or values
   * greater than the amount of storage currently available... and even
   * if additional storage does exist, its contents are unpredictable.
   * The only safe use for our setLength() is to truncate the FastStringBuffer
   * to a shorter string.
   *
   * @param l New length. If l<0 or l>=getLength(), this operation will
   * not report an error but future operations will almost certainly fail.
   */
  public final void setLength(int l)
  {
    m_lastChunk = l >>> m_chunkBits;

    if (m_lastChunk == 0 && m_innerFSB != null)
    {
      // Replace this FSB with the appropriate inner FSB, truncated
      m_innerFSB.setLength(l, this);
    }
    else
    {
      m_firstFree = l & m_chunkMask;
      
	  // There's an edge case if l is an exact multiple of m_chunkBits, which risks leaving
	  // us pointing at the start of a chunk which has not yet been allocated. Rather than 
	  // pay the cost of dealing with that in the append loops (more scattered and more
	  // inner-loop), we correct it here by moving to the safe side of that
	  // line -- as we would have left the indexes had we appended up to that point.
      if(m_firstFree==0 && m_lastChunk>0)
      {
      	--m_lastChunk;
      	m_firstFree=m_chunkSize;
      }
    }
  }

  /**
   * Subroutine for the public setLength() method. Deals with the fact
   * that truncation may require restoring one of the innerFSBs
   *
   * NEEDSDOC @param l
   * NEEDSDOC @param rootFSB
   */
  private final void setLength(int l, FastStringBuffer rootFSB)
  {

    m_lastChunk = l >>> m_chunkBits;

    if (m_lastChunk == 0 && m_innerFSB != null)
    {
      m_innerFSB.setLength(l, rootFSB);
    }
    else
    {

      // Undo encapsulation -- pop the innerFSB data back up to root.
      // Inefficient, but attempts to keep the code simple.
      rootFSB.m_chunkBits = m_chunkBits;
      rootFSB.m_maxChunkBits = m_maxChunkBits;
      rootFSB.m_rebundleBits = m_rebundleBits;
      rootFSB.m_chunkSize = m_chunkSize;
      rootFSB.m_chunkMask = m_chunkMask;
      rootFSB.m_array = m_array;
      rootFSB.m_innerFSB = m_innerFSB;
      rootFSB.m_lastChunk = m_lastChunk;

      // Finally, truncate this sucker.
      rootFSB.m_firstFree = l & m_chunkMask;
    }
  }

  /**
   * Note that this operation has been somewhat deoptimized by the shift to a
   * chunked array, as there is no factory method to produce a String object
   * directly from an array of arrays and hence a double copy is needed.
   * By using ensureCapacity we hope to minimize the heap overhead of building
   * the intermediate StringBuffer.
   * <p>
   * (It really is a pity that Java didn't design String as a final subclass
   * of MutableString, rather than having StringBuffer be a separate hierarchy.
   * We'd avoid a <strong>lot</strong> of double-buffering.)
   *
   * @return the contents of the FastStringBuffer as a standard Java string.
   */
  public final String toString()
  {

    int length = (m_lastChunk << m_chunkBits) + m_firstFree;

    return getString(new StringBuffer(length), 0, 0, length).toString();
  }

  /**
   * Append a single character onto the FastStringBuffer, growing the
   * storage if necessary.
   * <p>
   * NOTE THAT after calling append(), previously obtained
   * references to m_array[][] may no longer be valid....
   * though in fact they should be in this instance.
   *
   * @param value character to be appended.
   */
  public final void append(char value)
  {
    
    char[] chunk;

    // We may have preallocated chunks. If so, all but last should
    // be at full size.
    boolean lastchunk = (m_lastChunk + 1 == m_array.length);

    if (m_firstFree < m_chunkSize)  // Simplified test single-character-fits
      chunk = m_array[m_lastChunk];
    else
    {

      // Extend array?
      int i = m_array.length;

      if (m_lastChunk + 1 == i)
      {