*/
  private void writeHeader() throws IOException
  {
    logger.debug("num extensions blocks for header:  " + numExtensionBlocks);
    // Build up the header array
    byte[] headerBlock = new byte[BIG_BLOCK_SIZE];
    byte[] extensionBlockData = new byte[BIG_BLOCK_SIZE * numExtensionBlocks];

    // Copy in the identifier
    System.arraycopy(IDENTIFIER, 0, headerBlock, 0, IDENTIFIER.length);

    // Copy in some magic values - no idea what they mean
    headerBlock[0x18] = 0x3e;
    headerBlock[0x1a] = 0x3;
    headerBlock[0x1c] = (byte) 0xfe;
    headerBlock[0x1d] = (byte) 0xff;
    headerBlock[0x1e] = 0x9;
    headerBlock[0x20] = 0x6;
    headerBlock[0x39] = 0x10;

    // Set the number of BBD blocks
    IntegerHelper.getFourBytes(numBigBlockDepotBlocks, 
                               headerBlock, 
                               NUM_BIG_BLOCK_DEPOT_BLOCKS_POS);  

    // Set the small block depot chain to -2 ie. no small block chain
    IntegerHelper.getFourBytes(-2,
                               headerBlock,
                               SMALL_BLOCK_DEPOT_BLOCK_POS);

    // Set the extension block 
    IntegerHelper.getFourBytes(extensionBlock,
                               headerBlock,
                               EXTENSION_BLOCK_POS);

    // Set the number of extension blocks to be the number of BBD blocks - 1
    IntegerHelper.getFourBytes(numExtensionBlocks,
                               headerBlock, 
                               NUM_EXTENSION_BLOCK_POS);
    
    // Set the root start block
    IntegerHelper.getFourBytes(rootStartBlock,
                               headerBlock,
                               ROOT_START_BLOCK_POS);

    // Set the block numbers for the BBD.  Set the BBD running 
    // after the excel data and summary information
    int pos = BIG_BLOCK_DEPOT_BLOCKS_POS;

    // See how many blocks fit into the header
    int blocksToWrite = Math.min(numBigBlockDepotBlocks, 
                                 (BIG_BLOCK_SIZE - 
                                  BIG_BLOCK_DEPOT_BLOCKS_POS)/4);
    int extensionBlock = 0;
    int blocksWritten = 0;

    for (int i = 0 ; i < blocksToWrite; i++)
    {
      IntegerHelper.getFourBytes(bbdStartBlock + i, 
                                 headerBlock,
                                 pos);
      pos += 4;
      blocksWritten++;
    }
    
    // Pad out the rest of the header with blanks
    for (int i = pos; i < BIG_BLOCK_SIZE; i++)
    {
      headerBlock[i] = (byte) 0xff;
    }

    out.write(headerBlock);

    // Write out the extension blocks
    pos = 0;

    for (int extBlock = 0; extBlock < numExtensionBlocks; extBlock++)
    {
      blocksToWrite = Math.min(numBigBlockDepotBlocks - blocksWritten, 
                               BIG_BLOCK_SIZE/4 -1);

      for(int j = 0 ; j < blocksToWrite; j++)
      {
        IntegerHelper.getFourBytes(bbdStartBlock + blocksWritten + j, 
                                   extensionBlockData,
                                   pos);
        pos += 4;
      }

      blocksWritten += blocksToWrite;

      // Indicate the next block, or the termination of the chain
      int nextBlock = (blocksWritten == numBigBlockDepotBlocks) ? 
                              -2 : extBlock+1 ;
      IntegerHelper.getFourBytes(nextBlock, extensionBlockData, pos);
      pos +=4;
    }

    if (numExtensionBlocks > 0)
    {
      // Pad out the rest of the extension block with blanks
      for (int i = pos; i < extensionBlockData.length; i++)
      {
        extensionBlockData[i] = (byte) 0xff;
      }

      out.write(extensionBlockData);
    }
  }

  /**
   * Checks that the data can fit into the current BBD block.  If not,
   * then it moves on to the next block
   * 
   * @exception IOException 
   */
  private void checkBbdPos() throws IOException
  {
    if (bbdPos >= BIG_BLOCK_SIZE)
    {
      // Write out the extension block.  This will simply be the next block
      out.write(bigBlockDepot);
      
      // Create a new block
      bigBlockDepot = new byte[BIG_BLOCK_SIZE];
      bbdPos = 0;
    }
  }

  /**
   * Writes out the big block chain
   *
   * @param startBlock the starting block of the big block chain
   * @param numBlocks the number of blocks in the chain
   * @exception IOException
   */
  private void writeBlockChain(int startBlock, int numBlocks) 
    throws IOException
  {
    int blocksToWrite = numBlocks - 1;
    int blockNumber   = startBlock + 1;
    
    while (blocksToWrite > 0)
    {
      int bbdBlocks = Math.min(blocksToWrite, (BIG_BLOCK_SIZE - bbdPos)/4);

      for (int i = 0 ; i < bbdBlocks; i++)
      {
        IntegerHelper.getFourBytes(blockNumber, bigBlockDepot, bbdPos);
        bbdPos +=4 ;
        blockNumber++;
      }
      
      blocksToWrite -= bbdBlocks;
      checkBbdPos();
    }

    // Write the end of the block chain 
    IntegerHelper.getFourBytes(-2, bigBlockDepot, bbdPos);
    bbdPos += 4;
    checkBbdPos();
  }

  /**
   * Writes the block chains for the additional property sets
   *
   * @exception IOException
   */
  private void writeAdditionalPropertySetBlockChains() throws IOException
  {
    if (additionalPropertySets == null)
    {
      return;
    }

    int blockNumber = excelDataStartBlock + excelDataBlocks + 16;

    int numBlocks2 = rootEntryPropertySet.data.length >= SMALL_BLOCK_THRESHOLD ?
      getBigBlocksRequired(rootEntryPropertySet.data.length) :
      SMALL_BLOCK_THRESHOLD / BIG_BLOCK_SIZE;

    String psname2 = rootEntryPropertySet.propertyStorage.name;
    logger.debug("writing big block chain for  " + psname2 + " block " + blockNumber + " numBlocks " + numBlocks2);
    writeBlockChain(blockNumber, numBlocks2);
    blockNumber += numBlocks2;
    
    for (Iterator i = additionalPropertySets.iterator(); i.hasNext() ; )
    {
      ReadPropertyStorage rps = (ReadPropertyStorage) i.next();

      int numBlocks = rps.data.length >= SMALL_BLOCK_THRESHOLD ?
        getBigBlocksRequired(rps.data.length) :
        SMALL_BLOCK_THRESHOLD / BIG_BLOCK_SIZE;

      String psname = rps.propertyStorage.name;
      logger.debug("writing big block chain for  " + psname + " block " + blockNumber + " numBlocks " + numBlocks);
      writeBlockChain(blockNumber, numBlocks);
      blockNumber += numBlocks;
    }
  }
  
  /**
   * Writes out the Big Block Depot
   * 
   * @exception IOException 
   */
  private void writeBigBlockDepot() throws IOException
  {
    // This is after the excel data, the summary information, the
    // big block property sets and the small block depot
    bigBlockDepot = new byte[BIG_BLOCK_SIZE];
    bbdPos = 0;

    // Write out the extension blocks, indicating them as special blocks
    for (int i = 0 ; i < numExtensionBlocks; i++)
    {
      IntegerHelper.getFourBytes(-3, bigBlockDepot, bbdPos);
      bbdPos += 4;
      checkBbdPos();
    }

    writeBlockChain(excelDataStartBlock, excelDataBlocks);
    
    // The excel data has been written.  Now write out the rest of it

    // Write the block chain for the summary information
    int summaryInfoBlock = excelDataStartBlock + 
      excelDataBlocks + 
      additionalPropertyBlocks;

    for (int i = summaryInfoBlock; i < summaryInfoBlock + 7; i++)
    {
      IntegerHelper.getFourBytes(i + 1, bigBlockDepot, bbdPos);
      bbdPos +=4 ;
      checkBbdPos();
    } 

    // Write the end of the block chain for the summary info block
    IntegerHelper.getFourBytes(-2, bigBlockDepot, bbdPos);
    bbdPos += 4;
    checkBbdPos();

    // Write the block chain for the document summary information
    for (int i = summaryInfoBlock + 8; i < summaryInfoBlock + 15; i++)
    {
      IntegerHelper.getFourBytes(i + 1, bigBlockDepot, bbdPos);
      bbdPos +=4 ;
      checkBbdPos();
    } 

    // Write the end of the block chain for the document summary
    IntegerHelper.getFourBytes(-2, bigBlockDepot, bbdPos);
    bbdPos += 4;
    checkBbdPos();

    // Write out the block chain for the copied property sets, if present
    writeAdditionalPropertySetBlockChains();

    // The Big Block Depot immediately follows the document summary.  Denote 
    // these as a special block
    for (int i = 0; i < numBigBlockDepotBlocks; i++)
    {
      IntegerHelper.getFourBytes(-3, bigBlockDepot, bbdPos);
      bbdPos += 4;
      checkBbdPos();
    }

    // Write the root entry
    writeBlockChain(rootStartBlock, numRootEntryBlocks);

    // Pad out the remainder of the block
    if (bbdPos != 0)
    {
      for (int i = bbdPos; i < BIG_BLOCK_SIZE; i++)
      {
        bigBlockDepot[i] = (byte) 0xff;
      }
      out.write(bigBlockDepot);
    }
  }

  /**
   * Calculates the number of big blocks required to store data of the 
   * specified length
   *
   * @param length the length of the data
   * @return the number of big blocks required to store the data
   */
  private int getBigBlocksRequired(int length)
  {
    int blocks = length / BIG_BLOCK_SIZE;
    
    return (length % BIG_BLOCK_SIZE > 0 )? blocks + 1 : blocks;
  }

  /**
   * Calculates the number of small blocks required to store data of the 
   * specified length
   *
   * @param length the length of the data
   * @return the number of small blocks required to store the data
   */
  private int getSmallBlocksRequired(int length)
  {
    int blocks = length / SMALL_BLOCK_SIZE;
    
    return (length % SMALL_BLOCK_SIZE > 0 )? blocks + 1 : blocks;
  }

  /**
   * Writes out the property sets
   * 
   * @exception IOException 
   */
  private void writePropertySets() throws IOException
  {
    byte[] propertySetStorage = new byte[BIG_BLOCK_SIZE * numRootEntryBlocks];

    int pos = 0;
    int[] mappings = null;

    // Build up the mappings array
    if (additionalPropertySets != null)
    {
      mappings = new int[numPropertySets];
      
      // Map the standard ones to the first four
      for (int i = 0 ; i < STANDARD_PROPERTY_SETS.length ; i++)
      {
        ReadPropertyStorage rps = (ReadPropertyStorage) 
          readPropertySets.get(STANDARD_PROPERTY_SETS[i]);
        mappings[rps.number] = i;
      }

      // Now go through the original ones
      int newMapping = STANDARD_PROPERTY_SETS.length;
      for (Iterator i = additionalPropertySets.iterator(); i.hasNext(); )
      {
        ReadPropertyStorage rps = (ReadPropertyStorage) i.next();
        mappings[rps.number] = newMapping;
        newMapping++;
      }
    }

    int dir = 0;
    int previous = 0;
    int next = 0;

    // Set the root entry property set
    PropertyStorage ps = new PropertyStorage(ROOT_ENTRY_NAME);
    ps.setType(5);
    ps.setStartBlock(-2);
    ps.setSize(0);
    ps.setPrevious(-1);
    ps.setNext(-1);
    ps.setColour(0);

    dir = 2;
    if (additionalPropertySets != null)
    {
      ReadPropertyStorage rps = (ReadPropertyStorage) 
                            readPropertySets.get(ROOT_ENTRY_NAME);
      dir = mappings[rps.propertyStorage.directory];
    }
    ps.setDirectory(dir);

    System.arraycopy(ps.data, 0, 
                     propertySetStorage, pos, 
                     PROPERTY_STORAGE_BLOCK_SIZE);
    pos += PROPERTY_STORAGE_BLOCK_SIZE;


    // Set the workbook property set
    ps = new PropertyStorage(WORKBOOK_NAME);
    ps.setType(2);
    ps.setStartBlock(excelDataStartBlock);
      // start the excel data after immediately after this block
    ps.setSize(requiredSize);
      // alway use a big block stream - none of that messing around
      // with small blocks
    ps.setColour(1);
    
    previous = 3;

    if (additionalPropertySets != null)
    {
      ReadPropertyStorage rps = (ReadPropertyStorage) 
        readPropertySets.get(WORKBOOK_NAME);
      previous = mappings[rps.propertyStorage.previous];
    }

    ps.setPrevious(previous);
    ps.setNext(-1);
    ps.setDirectory(-1);
    ps.setColour(1);

    System.arraycopy(ps.data, 0, 
                     propertySetStorage, pos, 
                     PROPERTY_STORAGE_BLOCK_SIZE);
    pos += PROPERTY_STORAGE_BLOCK_SIZE;

    // Set the summary information
    ps = new PropertyStorage(SUMMARY_INFORMATION_NAME);
    ps.setType(2);
    ps.setStartBlock(excelDataStartBlock + excelDataBlocks);
    ps.setSize(SMALL_BLOCK_THRESHOLD);
    ps.setColour(1);

    previous = 1;
    next = 3;

    if (additionalPropertySets != null)
    {
      ReadPropertyStorage rps = (ReadPropertyStorage) 
                            readPropertySets.get(SUMMARY_INFORMATION_NAME);

      previous = rps.propertyStorage.previous != - 1 ?
        mappings[rps.propertyStorage.previous] : -1 ;
      next = rps.propertyStorage.next != - 1 ?
        mappings[rps.propertyStorage.next] : -1 ;
    }

    ps.setPrevious(previous);
    ps.setNext(next);
    ps.setDirectory(-1);
    ps.setColour(1);

    System.arraycopy(ps.data, 0, 
                     propertySetStorage, pos, 
                     PROPERTY_STORAGE_BLOCK_SIZE);
    pos += PROPERTY_STORAGE_BLOCK_SIZE;

    // Set the document summary information
    ps = new PropertyStorage(DOCUMENT_SUMMARY_INFORMATION_NAME);
    ps.setType(2);
    ps.setStartBlock(excelDataStartBlock + excelDataBlocks + 8);
    ps.setSize(SMALL_BLOCK_THRESHOLD);
    ps.setPrevious(-1);
    ps.setNext(-1);
    ps.setDirectory(-1);
    ps.setColour(1);

    System.arraycopy(ps.data, 0, 
                     propertySetStorage, pos, 
                     PROPERTY_STORAGE_BLOCK_SIZE);
    pos += PROPERTY_STORAGE_BLOCK_SIZE;



    // Write out the additional property sets
    if (additionalPropertySets == null)
    {
      out.write(propertySetStorage);
      return;
    }

    int bigBlock = excelDataStartBlock + excelDataBlocks + 16 +
      18;

    for (Iterator i = additionalPropertySets.iterator() ; i.hasNext(); )
    {
      ReadPropertyStorage rps = (ReadPropertyStorage) i.next();
 
      ps = new PropertyStorage(rps.propertyStorage.name);
      ps.setType(rps.propertyStorage.type);
      ps.setStartBlock(bigBlock);
      ps.setSize(Math.max(rps.propertyStorage.size, SMALL_BLOCK_THRESHOLD));

      previous = rps.propertyStorage.previous != -1 ? 
        mappings[rps.propertyStorage.previous] : -1;
      next = rps.propertyStorage.next != -1 ? 
        mappings[rps.propertyStorage.next] : -1;
      dir = rps.propertyStorage.directory != -1 ? 
        mappings[rps.propertyStorage.directory] : -1;

      ps.setPrevious(previous);
      ps.setNext(next);
      ps.setDirectory(dir);
      ps.setColour(1);

      System.arraycopy(ps.data, 0, 
                       propertySetStorage, pos, 
                       PROPERTY_STORAGE_BLOCK_SIZE);
      pos += PROPERTY_STORAGE_BLOCK_SIZE;

      
      if (rps.data.length >= SMALL_BLOCK_THRESHOLD)
      {
        bigBlock += getBigBlocksRequired(rps.data.length);
      }
      else
      {
        bigBlock += SMALL_BLOCK_THRESHOLD / BIG_BLOCK_SIZE;
      }
    }
    
    out.write(propertySetStorage);
  }
}