path: root/src/java/com/healthmarketscience/jackcess/IndexData.java

/*
Copyright (c) 2005 Health Market Science, Inc.

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307
USA

You can contact Health Market Science at info@healthmarketscience.com
or at the following address:

Health Market Science
2700 Horizon Drive
Suite 200
King of Prussia, PA 19406
*/

package com.healthmarketscience.jackcess;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;
import java.util.Map;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import static com.healthmarketscience.jackcess.IndexCodes.*;
import static com.healthmarketscience.jackcess.ByteUtil.ByteStream;


/**
 * Access table index data.  This is the actual data which backs a logical
 * Index, where one or more logical indexes can be backed by the same index
 * data.
 * 
 * @author Tim McCune
 */
public abstract class IndexData {
  
  protected static final Log LOG = LogFactory.getLog(Index.class);

  /** special entry which is less than any other entry */
  public static final Entry FIRST_ENTRY =
    createSpecialEntry(RowId.FIRST_ROW_ID);
  
  /** special entry which is greater than any other entry */
  public static final Entry LAST_ENTRY =
    createSpecialEntry(RowId.LAST_ROW_ID);

  /** special object which will always be greater than any other value, when
      searching for an index entry range in a multi-value index */
  public static final Object MAX_VALUE = new Object();

  /** special object which will always be greater than any other value, when
      searching for an index entry range in a multi-value index */
  public static final Object MIN_VALUE = new Object();
  
  protected static final int INVALID_INDEX_PAGE_NUMBER = 0;          
  
  /** Max number of columns in an index */
  static final int MAX_COLUMNS = 10;
  
  protected static final byte[] EMPTY_PREFIX = new byte[0];

  private static final short COLUMN_UNUSED = -1;

  static final byte ASCENDING_COLUMN_FLAG = (byte)0x01;

  static final byte UNIQUE_INDEX_FLAG = (byte)0x01;
  static final byte IGNORE_NULLS_INDEX_FLAG = (byte)0x02;
  static final byte SPECIAL_INDEX_FLAG = (byte)0x08; // set on MSysACEs and MSysAccessObjects indexes, purpose unknown
  static final byte UNKNOWN_INDEX_FLAG = (byte)0x80; // always seems to be set on indexes in access 2000+

  private static final int MAGIC_INDEX_NUMBER = 1923;

  private static final ByteOrder ENTRY_BYTE_ORDER = ByteOrder.BIG_ENDIAN;
  
  /** type attributes for Entries which simplify comparisons */
  public enum EntryType {
    /** comparable type indicating this Entry should always compare less than
        valid RowIds */
    ALWAYS_FIRST,
    /** comparable type indicating this Entry should always compare less than
        other valid entries with equal entryBytes */
    FIRST_VALID,
    /** comparable type indicating this RowId should always compare
        normally */
    NORMAL,
    /** comparable type indicating this Entry should always compare greater
        than other valid entries with equal entryBytes */
    LAST_VALID,
    /** comparable type indicating this Entry should always compare greater
        than valid RowIds */
    ALWAYS_LAST;
  }
  
  static final Comparator<byte[]> BYTE_CODE_COMPARATOR =
    new Comparator<byte[]>() {
      public int compare(byte[] left, byte[] right) {
        if(left == right) {
          return 0;
        }
        if(left == null) {
          return -1;
        }
        if(right == null) {
          return 1;
        }

        int len = Math.min(left.length, right.length);
        int pos = 0;
        while((pos < len) && (left[pos] == right[pos])) {
          ++pos;
        }
        if(pos < len) {
          return ((ByteUtil.asUnsignedByte(left[pos]) <
                   ByteUtil.asUnsignedByte(right[pos])) ? -1 : 1);
        }
        return ((left.length < right.length) ? -1 :
                ((left.length > right.length) ? 1 : 0));
      }
    };
        
  
  /** owning table */
  private final Table _table;
  /** 0-based index data number */
  private final int _number;
  /** Page number of the root index data */
  private int _rootPageNumber;
  /** offset within the tableDefinition buffer of the uniqueEntryCount for
      this index */
  private final int _uniqueEntryCountOffset;
  /** The number of unique entries which have been added to this index.  note,
      however, that it is never decremented, only incremented (as observed in
      Access). */
  private int _uniqueEntryCount;
  /** List of columns and flags */
  private final List<ColumnDescriptor> _columns =
    new ArrayList<ColumnDescriptor>();
  /** the logical indexes which this index data backs */
  private final List<Index> _indexes = new ArrayList<Index>();
  /** flags for this index */
  private byte _indexFlags;
  /** Usage map of pages that this index owns */
  private UsageMap _ownedPages;
  /** <code>true</code> if the index entries have been initialized,
      <code>false</code> otherwise */
  private boolean _initialized;
  /** modification count for the table, keeps cursors up-to-date */
  private int _modCount;
  /** temp buffer used to read/write the index pages */
  private final TempBufferHolder _indexBufferH =
    TempBufferHolder.newHolder(TempBufferHolder.Type.SOFT, true);
  /** temp buffer used to create index entries */
  private ByteStream _entryBuffer;
  /** max size for all the entries written to a given index data page */
  private final int _maxPageEntrySize;
  /** whether or not this index data is backing a primary key logical index */
  private boolean _primaryKey;
  /** FIXME, for SimpleIndex, we can't write multi-page indexes or indexes using the entry compression scheme */
  private boolean _readOnly;
  
  protected IndexData(Table table, int number, int uniqueEntryCount,
                      int uniqueEntryCountOffset)
  {
    _table  = table;
    _number = number;
    _uniqueEntryCount = uniqueEntryCount;
    _uniqueEntryCountOffset = uniqueEntryCountOffset;
    _maxPageEntrySize = calcMaxPageEntrySize(_table.getFormat());
  }

  /**
   * Creates an IndexData appropriate for the given table, using information
   * from the given table definition buffer.
   */
  public static IndexData create(Table table, ByteBuffer tableBuffer,
                                 int number, JetFormat format)
    throws IOException
  {
    int uniqueEntryCountOffset =
      (format.OFFSET_INDEX_DEF_BLOCK +
       (number * format.SIZE_INDEX_DEFINITION) + 4);
    int uniqueEntryCount = tableBuffer.getInt(uniqueEntryCountOffset);

    return(table.doUseBigIndex() ?
           new BigIndexData(table, number, uniqueEntryCount,
                            uniqueEntryCountOffset) :
           new SimpleIndexData(table, number, uniqueEntryCount, 
                               uniqueEntryCountOffset));
  }

  public Table getTable() {
    return _table;
  }
  
  public JetFormat getFormat() {
    return getTable().getFormat();
  }

  public PageChannel getPageChannel() {
    return getTable().getPageChannel();
  }

  /**
   * @return the "main" logical index which is backed by this data.
   */
  public Index getPrimaryIndex() {
    return _indexes.get(0);
  }

  /**
   * @return All of the Indexes backed by this data (unmodifiable List)
   */
  public List<Index> getIndexes() {
    return Collections.unmodifiableList(_indexes);
  }

  /**
   * Adds a logical index which this data is backing.
   */
  void addIndex(Index index) {

    // we keep foreign key indexes at the back of the list.  this way the
    // primary index will be a non-foreign key index (if any)
    if(index.isForeignKey()) {
      _indexes.add(index);
    } else {
      int pos = _indexes.size();
      while(pos > 0) {
        if(!_indexes.get(pos - 1).isForeignKey()) {
          break;
        }
        --pos;
      }
      _indexes.add(pos, index);

      // also, keep track of whether or not this is a primary key index
      _primaryKey |= index.isPrimaryKey();
    }
  }

  public byte getIndexFlags() {
    return _indexFlags;
  }

  public int getIndexDataNumber() {
    return _number;
  }
  
  public int getUniqueEntryCount() {
    return _uniqueEntryCount;
  }

  public int getUniqueEntryCountOffset() {
    return _uniqueEntryCountOffset;
  }

  protected boolean isBackingPrimaryKey() {
    return _primaryKey;
  }

  /**
   * Whether or not {@code null} values are actually recorded in the index.
   */
  public boolean shouldIgnoreNulls() {
    return((_indexFlags & IGNORE_NULLS_INDEX_FLAG) != 0);
  }

  /**
   * Whether or not index entries must be unique.
   * <p>
   * Some notes about uniqueness:
   * <ul>
   * <li>Access does not seem to consider multiple {@code null} entries
   *     invalid for a unique index</li>
   * <li>text indexes collapse case, and Access seems to compare <b>only</b>
   *     the index entry bytes, therefore two strings which differ only in
   *     case <i>will violate</i> the unique constraint</li>
   * </ul>
   */
  public boolean isUnique() {
    return(isBackingPrimaryKey() || ((_indexFlags & UNIQUE_INDEX_FLAG) != 0));
  }
  
  /**
   * Returns the Columns for this index (unmodifiable)
   */
  public List<ColumnDescriptor> getColumns() {
    return Collections.unmodifiableList(_columns);
  }

  /**
   * Whether or not the complete index state has been read.
   */
  public boolean isInitialized() {
    return _initialized;
  }

  protected int getRootPageNumber() {
    return _rootPageNumber;
  }

  protected void setReadOnly() {
    _readOnly = true;
  }

  protected boolean isReadOnly() {
    return _readOnly;
  }

  protected int getMaxPageEntrySize() {
    return _maxPageEntrySize;
  }

  /**
   * Returns the number of database pages owned by this index data.
   */
  public int getOwnedPageCount() {
    return _ownedPages.getPageCount();
  }
  
  void addOwnedPage(int pageNumber) throws IOException {
    _ownedPages.addPageNumber(pageNumber);
  }

  /**
   * Returns the number of index entries in the index.  Only called by unit
   * tests.
   * <p>
   * Forces index initialization.
   */
  protected int getEntryCount()
    throws IOException
  {
    initialize();
    EntryCursor cursor = cursor();
    Entry endEntry = cursor.getLastEntry();
    int count = 0;
    while(!endEntry.equals(cursor.getNextEntry())) {
      ++count;
    }
    return count;
  }
  
  /**
   * Forces initialization of this index (actual parsing of index pages).
   * normally, the index will not be initialized until the entries are
   * actually needed.
   */
  public void initialize() throws IOException {
    if(!_initialized) {
      readIndexEntries();
      _initialized = true;
    }
  }

  /**
   * Writes the current index state to the database.
   * <p>
   * Forces index initialization.
   */
  public void update() throws IOException
  {
    // make sure we've parsed the entries
    initialize();
    
    if(_readOnly) {
      throw new UnsupportedOperationException(
          "FIXME cannot write indexes of this type yet, see Database javadoc for info on enabling large index support");
    }
    updateImpl();
  }

  /**
   * Read the rest of the index info from a tableBuffer
   * @param tableBuffer table definition buffer to read from initial info
   * @param availableColumns Columns that this index may use
   */
  public void read(ByteBuffer tableBuffer, List<Column> availableColumns)
    throws IOException
  {
    ByteUtil.forward(tableBuffer, getFormat().SKIP_BEFORE_INDEX); //Forward past Unknown

    for (int i = 0; i < MAX_COLUMNS; i++) {
      short columnNumber = tableBuffer.getShort();
      byte colFlags = tableBuffer.get();
      if (columnNumber != COLUMN_UNUSED) {
        // find the desired column by column number (which is not necessarily
        // the same as the column index)
        Column idxCol = null;
        for(Column col : availableColumns) {
          if(col.getColumnNumber() == columnNumber) {
            idxCol = col;
            break;
          }
        }
        if(idxCol == null) {
          throw new IOException("Could not find column with number "
                                + columnNumber + " for index");
        }
        _columns.add(newColumnDescriptor(idxCol, colFlags));
      }
    }

    int umapRowNum = tableBuffer.get();
    int umapPageNum = ByteUtil.get3ByteInt(tableBuffer);
    _ownedPages = UsageMap.read(getTable().getDatabase(), umapPageNum,
                                umapRowNum, false);
    
    _rootPageNumber = tableBuffer.getInt();

    ByteUtil.forward(tableBuffer, getFormat().SKIP_BEFORE_INDEX_FLAGS); //Forward past Unknown
    _indexFlags = tableBuffer.get();
    ByteUtil.forward(tableBuffer, getFormat().SKIP_AFTER_INDEX_FLAGS); //Forward past other stuff
  }

  /**
   * Writes the index row count definitions into a table definition buffer.
   * @param buffer Buffer to write to
   * @param indexes List of IndexBuilders to write definitions for
   */
  protected static void writeRowCountDefinitions(
      ByteBuffer buffer, int indexCount, JetFormat format)
  {
    // index row counts (empty data)
    ByteUtil.forward(buffer, (indexCount * format.SIZE_INDEX_DEFINITION));
  }

  /**
   * Writes the index definitions into a table definition buffer.
   * @param buffer Buffer to write to
   * @param indexes List of IndexBuilders to write definitions for
   */
  protected static void writeDefinitions(
      ByteBuffer buffer, List<Column> columns, List<IndexBuilder> indexes,
      int tdefPageNumber, PageChannel pageChannel, JetFormat format)
    throws IOException
  {
    ByteBuffer rootPageBuffer = pageChannel.createPageBuffer();
    writeDataPage(rootPageBuffer, SimpleIndexData.NEW_ROOT_DATA_PAGE, 
                  tdefPageNumber, format);

    for(IndexBuilder idx : indexes) {
      buffer.putInt(MAGIC_INDEX_NUMBER); // seemingly constant magic value

      // write column information (always MAX_COLUMNS entries)
      List<IndexBuilder.Column> idxColumns = idx.getColumns();
      for(int i = 0; i < MAX_COLUMNS; ++i) {

        short columnNumber = COLUMN_UNUSED;
        byte flags = 0;

        if(i < idxColumns.size()) {

          // determine column info
          IndexBuilder.Column idxCol = idxColumns.get(i);
          flags = idxCol.getFlags();

          // find actual table column number
          for(Column col : columns) {
            if(col.getName().equalsIgnoreCase(idxCol.getName())) {
              columnNumber = col.getColumnNumber();
              break;
            }
          }
          if(columnNumber == COLUMN_UNUSED) {
            // should never happen as this is validated before
            throw new IllegalArgumentException(
                "Column with name " + idxCol.getName() + " not found");
          }
        }
         
        buffer.putShort(columnNumber); // table column number
        buffer.put(flags); // column flags (e.g. ordering)
      }

      buffer.put(idx.getUmapRowNumber()); // umap row
      ByteUtil.put3ByteInt(buffer, idx.getUmapPageNumber()); // umap page

      // write empty root index page
      pageChannel.writePage(rootPageBuffer, idx.getRootPageNumber());

      buffer.putInt(idx.getRootPageNumber());
      buffer.putInt(0); // unknown
      buffer.put(idx.getFlags()); // index flags (unique, etc.)
      ByteUtil.forward(buffer, 5); // unknown
    }
  }

  /**
   * Adds a row to this index
   * <p>
   * Forces index initialization.
   * 
   * @param row Row to add
   * @param rowId rowId of the row to be added
   */
  public void addRow(Object[] row, RowId rowId)
    throws IOException
  {
    int nullCount = countNullValues(row);
    boolean isNullEntry = (nullCount == _columns.size());
    if(shouldIgnoreNulls() && isNullEntry) {
      // nothing to do
      return;
    }
    if(isBackingPrimaryKey() && (nullCount > 0)) {
      throw new IOException("Null value found in row " + Arrays.asList(row)
                            + " for primary key index " + this);
    }
    
    // make sure we've parsed the entries
    initialize();

    Entry newEntry = new Entry(createEntryBytes(row), rowId);
    if(addEntry(newEntry, isNullEntry, row)) {
      ++_modCount;
    } else {
      LOG.warn("Added duplicate index entry " + newEntry + " for row: " +
               Arrays.asList(row));
    }
  }

  /**
   * Adds an entry to the correct index dataPage, maintaining the order.
   */
  private boolean addEntry(Entry newEntry, boolean isNullEntry, Object[] row)
    throws IOException
  {
    DataPage dataPage = findDataPage(newEntry);
    int idx = dataPage.findEntry(newEntry);
    if(idx < 0) {
      // this is a new entry
      idx = missingIndexToInsertionPoint(idx);

      Position newPos = new Position(dataPage, idx, newEntry, true);
      Position nextPos = getNextPosition(newPos);
      Position prevPos = getPreviousPosition(newPos);
      
      // determine if the addition of this entry would break the uniqueness
      // constraint.  See isUnique() for some notes about uniqueness as
      // defined by Access.
      boolean isDupeEntry =
        (((nextPos != null) &&
          newEntry.equalsEntryBytes(nextPos.getEntry())) ||
          ((prevPos != null) &&
           newEntry.equalsEntryBytes(prevPos.getEntry())));
      if(isUnique() && !isNullEntry && isDupeEntry)
      {
        throw new IOException(
            "New row " + Arrays.asList(row) +
            " violates uniqueness constraint for index " + this);
      }

      if(!isDupeEntry) {
        ++_uniqueEntryCount;
      }

      dataPage.addEntry(idx, newEntry);
      return true;
    }
    return false;
  }
  
  /**
   * Removes a row from this index
   * <p>
   * Forces index initialization.
   * 
   * @param row Row to remove
   * @param rowId rowId of the row to be removed
   */
  public void deleteRow(Object[] row, RowId rowId)
    throws IOException
  {
    int nullCount = countNullValues(row);
    if(shouldIgnoreNulls() && (nullCount == _columns.size())) {
      // nothing to do
      return;
    }
    
    // make sure we've parsed the entries
    initialize();

    Entry oldEntry = new Entry(createEntryBytes(row), rowId);
    if(removeEntry(oldEntry)) {
      ++_modCount;
    } else {
      LOG.warn("Failed removing index entry " + oldEntry + " for row: " +
               Arrays.asList(row));
    }
  }

  /**
   * Removes an entry from the relevant index dataPage, maintaining the order.
   * Will search by RowId if entry is not found (in case a partial entry was
   * provided).
   */
  private boolean removeEntry(Entry oldEntry)
    throws IOException
  {
    DataPage dataPage = findDataPage(oldEntry);
    int idx = dataPage.findEntry(oldEntry);
    boolean doRemove = false;
    if(idx < 0) {
      // the caller may have only read some of the row data, if this is the
      // case, just search for the page/row numbers
      // FIXME, we could force caller to get relevant values?
      EntryCursor cursor = cursor();
      Position tmpPos = null;
      Position endPos = cursor._lastPos;
      while(!endPos.equals(
                tmpPos = cursor.getAnotherPosition(Cursor.MOVE_FORWARD))) {
        if(tmpPos.getEntry().getRowId().equals(oldEntry.getRowId())) {
          dataPage = tmpPos.getDataPage();
          idx = tmpPos.getIndex();
          doRemove = true;
          break;
        }
      }
    } else {
      doRemove = true;
    }

    if(doRemove) {
      // found it!
      dataPage.removeEntry(idx);
    }
    
    return doRemove;
  }
      
  /**
   * Gets a new cursor for this index.
   * <p>
   * Forces index initialization.
   */
  public EntryCursor cursor()
    throws IOException
  {
    return cursor(null, true, null, true);
  }
  
  /**
   * Gets a new cursor for this index, narrowed to the range defined by the
   * given startRow and endRow.
   * <p>
   * Forces index initialization.
   * 
   * @param startRow the first row of data for the cursor, or {@code null} for
   *                 the first entry
   * @param startInclusive whether or not startRow is inclusive or exclusive
   * @param endRow the last row of data for the cursor, or {@code null} for
   *               the last entry
   * @param endInclusive whether or not endRow is inclusive or exclusive
   */
  public EntryCursor cursor(Object[] startRow,
                            boolean startInclusive,
                            Object[] endRow,
                            boolean endInclusive)
    throws IOException
  {
    initialize();
    Entry startEntry = FIRST_ENTRY;
    byte[] startEntryBytes = null;
    if(startRow != null) {
      startEntryBytes = createEntryBytes(startRow);
      startEntry = new Entry(startEntryBytes,
                             (startInclusive ?
                              RowId.FIRST_ROW_ID : RowId.LAST_ROW_ID));
    }
    Entry endEntry = LAST_ENTRY;
    if(endRow != null) {
      // reuse startEntryBytes if startRow and endRow are same array.  this is
      // common for "lookup" code
      byte[] endEntryBytes = ((startRow == endRow) ?
                              startEntryBytes :
                              createEntryBytes(endRow));
      endEntry = new Entry(endEntryBytes,
                           (endInclusive ?
                            RowId.LAST_ROW_ID : RowId.FIRST_ROW_ID));
    }
    return new EntryCursor(findEntryPosition(startEntry),
                           findEntryPosition(endEntry));
  }

  private Position findEntryPosition(Entry entry)
    throws IOException
  {
    DataPage dataPage = findDataPage(entry);
    int idx = dataPage.findEntry(entry);
    boolean between = false;
    if(idx < 0) {
      // given entry was not found exactly.  our current position is now
      // really between two indexes, but we cannot support that as an integer
      // value, so we set a flag instead
      idx = missingIndexToInsertionPoint(idx);
      between = true;
    }
    return new Position(dataPage, idx, entry, between);
  }

  private Position getNextPosition(Position curPos)
    throws IOException
  {
    // get the next index (between-ness is handled internally)
    int nextIdx = curPos.getNextIndex();
    Position nextPos = null;
    if(nextIdx < curPos.getDataPage().getEntries().size()) {
      nextPos = new Position(curPos.getDataPage(), nextIdx);
    } else {
      int nextPageNumber = curPos.getDataPage().getNextPageNumber();
      DataPage nextDataPage = null;
      while(nextPageNumber != INVALID_INDEX_PAGE_NUMBER) {
        DataPage dp = getDataPage(nextPageNumber);
        if(!dp.isEmpty()) {
          nextDataPage = dp;
          break;
        }
        nextPageNumber = dp.getNextPageNumber();
      }
      if(nextDataPage != null) {
        nextPos = new Position(nextDataPage, 0);
      }
    }
    return nextPos;
  }

  /**
   * Returns the Position before the given one, or {@code null} if none.
   */
  private Position getPreviousPosition(Position curPos)
    throws IOException
  {
    // get the previous index (between-ness is handled internally)
    int prevIdx = curPos.getPrevIndex();
    Position prevPos = null;
    if(prevIdx >= 0) {
      prevPos = new Position(curPos.getDataPage(), prevIdx);
    } else {
      int prevPageNumber = curPos.getDataPage().getPrevPageNumber();
      DataPage prevDataPage = null;
      while(prevPageNumber != INVALID_INDEX_PAGE_NUMBER) {
        DataPage dp = getDataPage(prevPageNumber);
        if(!dp.isEmpty()) {
          prevDataPage = dp;
          break;
        }
        prevPageNumber = dp.getPrevPageNumber();
      }
      if(prevDataPage != null) {
        prevPos = new Position(prevDataPage,
                               (prevDataPage.getEntries().size() - 1));
      }
    }
    return prevPos;
  }

  /**
   * Returns the valid insertion point for an index indicating a missing
   * entry.
   */
  protected static int missingIndexToInsertionPoint(int idx) {
    return -(idx + 1);
  }

  /**
   * Constructs an array of values appropriate for this index from the given
   * column values, expected to match the columns for this index.
   * @return the appropriate sparse array of data
   * @throws IllegalArgumentException if the wrong number of values are
   *         provided
   */
  public Object[] constructIndexRowFromEntry(Object... values)
  {
    if(values.length != _columns.size()) {
      throw new IllegalArgumentException(
          "Wrong number of column values given " + values.length +
          ", expected " + _columns.size());
    }
    int valIdx = 0;
    Object[] idxRow = new Object[getTable().getColumnCount()];
    for(ColumnDescriptor col : _columns) {
      idxRow[col.getColumnIndex()] = values[valIdx++];
    }
    return idxRow;
  }
    
  /**
   * Constructs an array of values appropriate for this index from the given
   * column value.
   * @return the appropriate sparse array of data or {@code null} if not all
   *         columns for this index were provided
   */
  public Object[] constructIndexRow(String colName, Object value)
  {
    return constructIndexRow(Collections.singletonMap(colName, value));
  }
  
  /**
   * Constructs an array of values appropriate for this index from the given
   * column values.
   * @return the appropriate sparse array of data or {@code null} if not all
   *         columns for this index were provided
   */
  public Object[] constructIndexRow(Map<String,Object> row)
  {
    for(ColumnDescriptor col : _columns) {
      if(!row.containsKey(col.getName())) {
        return null;
      }
    }

    Object[] idxRow = new Object[getTable().getColumnCount()];
    for(ColumnDescriptor col : _columns) {
      idxRow[col.getColumnIndex()] = row.get(col.getName());
    }
    return idxRow;
  }  

  @Override
  public String toString() {
    StringBuilder rtn = new StringBuilder();
    rtn.append("\n\tData number: ").append(_number);
    rtn.append("\n\tPage number: ").append(_rootPageNumber);
    rtn.append("\n\tIs Backing Primary Key: ").append(isBackingPrimaryKey());
    rtn.append("\n\tIs Unique: ").append(isUnique());
    rtn.append("\n\tIgnore Nulls: ").append(shouldIgnoreNulls());
    rtn.append("\n\tColumns: ").append(_columns);
    rtn.append("\n\tInitialized: ").append(_initialized);
    if(_initialized) {
      try {
        rtn.append("\n\tEntryCount: ").append(getEntryCount());
      } catch(IOException e) {
        throw new RuntimeException(e);
      }
    }
    return rtn.toString();
  }
  
  /**
   * Write the given index page out to a buffer
   */
  protected void writeDataPage(DataPage dataPage)
    throws IOException
  {
    if(dataPage.getCompressedEntrySize() > _maxPageEntrySize) {
      if(this instanceof SimpleIndexData) {
        throw new UnsupportedOperationException(
            "FIXME cannot write large index yet, see Database javadoc for info on enabling large index support");
      }
      throw new IllegalStateException("data page is too large");
    }
    
    ByteBuffer buffer = _indexBufferH.getPageBuffer(getPageChannel());

    writeDataPage(buffer, dataPage, getTable().getTableDefPageNumber(),
                  getFormat());

    getPageChannel().writePage(buffer, dataPage.getPageNumber());
  }

  /**
   * Writes the data page info to the given buffer.
   */
  protected static void writeDataPage(ByteBuffer buffer, DataPage dataPage,
                                      int tdefPageNumber, JetFormat format)
    throws IOException
  {
    buffer.put(dataPage.isLeaf() ?
               PageTypes.INDEX_LEAF :
               PageTypes.INDEX_NODE );  //Page type
    buffer.put((byte) 0x01);  //Unknown
    buffer.putShort((short) 0); //Free space
    buffer.putInt(tdefPageNumber);

    buffer.putInt(0); //Unknown
    buffer.putInt(dataPage.getPrevPageNumber()); //Prev page
    buffer.putInt(dataPage.getNextPageNumber()); //Next page
    buffer.putInt(dataPage.getChildTailPageNumber()); //ChildTail page

    byte[] entryPrefix = dataPage.getEntryPrefix();
    buffer.putShort((short) entryPrefix.length); // entry prefix byte count
    buffer.put((byte) 0); //Unknown

    byte[] entryMask = new byte[format.SIZE_INDEX_ENTRY_MASK];
    // first entry includes the prefix
    int totalSize = entryPrefix.length;
    for(Entry entry : dataPage.getEntries()) {
      totalSize += (entry.size() - entryPrefix.length);
      int idx = totalSize / 8;
      entryMask[idx] |= (1 << (totalSize % 8));
    }
    buffer.put(entryMask);

    // first entry includes the prefix
    buffer.put(entryPrefix);
    
    for(Entry entry : dataPage.getEntries()) {
      entry.write(buffer, entryPrefix);
    }

    // update free space
    buffer.putShort(2, (short) (format.PAGE_SIZE - buffer.position()));
  }

  /**
   * Reads an index page, populating the correct collection based on the page
   * type (node or leaf).
   */
  protected void readDataPage(DataPage dataPage)
    throws IOException
  {
    ByteBuffer buffer = _indexBufferH.getPageBuffer(getPageChannel());
    getPageChannel().readPage(buffer, dataPage.getPageNumber());

    boolean isLeaf = isLeafPage(buffer);
    dataPage.setLeaf(isLeaf);

    // note, "header" data is in LITTLE_ENDIAN format, entry data is in
    // BIG_ENDIAN format
    int entryPrefixLength = ByteUtil.getUnsignedShort(
        buffer, getFormat().OFFSET_INDEX_COMPRESSED_BYTE_COUNT);
    int entryMaskLength = getFormat().SIZE_INDEX_ENTRY_MASK;
    int entryMaskPos = getFormat().OFFSET_INDEX_ENTRY_MASK;
    int entryPos = entryMaskPos + entryMaskLength;
    int lastStart = 0;
    int totalEntrySize = 0;
    byte[] entryPrefix = null;
    List<Entry> entries = new ArrayList<Entry>();
    TempBufferHolder tmpEntryBufferH =
      TempBufferHolder.newHolder(TempBufferHolder.Type.HARD, true,
                                 ENTRY_BYTE_ORDER);

    Entry prevEntry = FIRST_ENTRY;
    for (int i = 0; i < entryMaskLength; i++) {
      byte entryMask = buffer.get(entryMaskPos + i);
      for (int j = 0; j < 8; j++) {
        if ((entryMask & (1 << j)) != 0) {
          int length = (i * 8) + j - lastStart;
          buffer.position(entryPos + lastStart);

          // determine if we can read straight from the index page (if no
          // entryPrefix).  otherwise, create temp buf with complete entry.
          ByteBuffer curEntryBuffer = buffer;
          int curEntryLen = length;
          if(entryPrefix != null) {
            curEntryBuffer = getTempEntryBuffer(
                buffer, length, entryPrefix, tmpEntryBufferH);
            curEntryLen += entryPrefix.length;
          }
          totalEntrySize += curEntryLen;

          Entry entry = newEntry(curEntryBuffer, curEntryLen, isLeaf);
          if(prevEntry.compareTo(entry) >= 0) {
            throw new IOException("Unexpected order in index entries, " +
                                  prevEntry + " >= " + entry);
          }
          
          entries.add(entry);

          if((entries.size() == 1) && (entryPrefixLength > 0)) {
            // read any shared entry prefix
            entryPrefix = new byte[entryPrefixLength];
            buffer.position(entryPos + lastStart);
            buffer.get(entryPrefix);
          }

          lastStart += length;
          prevEntry = entry;
        }
      }
    }

    dataPage.setEntryPrefix(entryPrefix != null ? entryPrefix : EMPTY_PREFIX);
    dataPage.setEntries(entries);
    dataPage.setTotalEntrySize(totalEntrySize);
    
    int prevPageNumber = buffer.getInt(getFormat().OFFSET_PREV_INDEX_PAGE);
    int nextPageNumber = buffer.getInt(getFormat().OFFSET_NEXT_INDEX_PAGE);
    int childTailPageNumber =
      buffer.getInt(getFormat().OFFSET_CHILD_TAIL_INDEX_PAGE);

    dataPage.setPrevPageNumber(prevPageNumber);
    dataPage.setNextPageNumber(nextPageNumber);
    dataPage.setChildTailPageNumber(childTailPageNumber);
  }

  /**
   * Returns a new Entry of the correct type for the given data and page type.
   */
  private Entry newEntry(ByteBuffer buffer, int entryLength, boolean isLeaf)
    throws IOException
  {
    if(isLeaf) {
      return new Entry(buffer, entryLength);
    }
    return new NodeEntry(buffer, entryLength);
  }

  /**
   * Returns an entry buffer containing the relevant data for an entry given
   * the valuePrefix.
   */
  private ByteBuffer getTempEntryBuffer(
      ByteBuffer indexPage, int entryLen, byte[] valuePrefix,
      TempBufferHolder tmpEntryBufferH)
  {
    ByteBuffer tmpEntryBuffer = tmpEntryBufferH.getBuffer(
        getPageChannel(), valuePrefix.length + entryLen);

    // combine valuePrefix and rest of entry from indexPage, then prep for
    // reading
    tmpEntryBuffer.put(valuePrefix);
    tmpEntryBuffer.put(indexPage.array(), indexPage.position(), entryLen);
    tmpEntryBuffer.flip();
    
    return tmpEntryBuffer;
  }
    
  /**
   * Determines if the given index page is a leaf or node page.
   */
  private static boolean isLeafPage(ByteBuffer buffer)
    throws IOException
  {
    byte pageType = buffer.get(0);
    if(pageType == PageTypes.INDEX_LEAF) {
      return true;
    } else if(pageType == PageTypes.INDEX_NODE) {
      return false;
    }
    throw new IOException("Unexpected page type " + pageType);
  }
  
  /**
   * Determines the number of {@code null} values for this index from the
   * given row.
   */
  private int countNullValues(Object[] values)
  {
    if(values == null) {
      return _columns.size();
    }
    
    // annoyingly, the values array could come from different sources, one
    // of which will make it a different size than the other.  we need to
    // handle both situations.
    int nullCount = 0;
    for(ColumnDescriptor col : _columns) {
      Object value = values[col.getColumnIndex()];
      if(col.isNullValue(value)) {
        ++nullCount;
      }
    }
    
    return nullCount;
  }

  /**
   * Creates the entry bytes for a row of values.
   */
  private byte[] createEntryBytes(Object[] values) throws IOException
  {
    if(values == null) {
      return null;
    }
    
    if(_entryBuffer == null) {
      _entryBuffer = new ByteStream();
    }
    _entryBuffer.reset();
    
    for(ColumnDescriptor col : _columns) {
      Object value = values[col.getColumnIndex()];
      if(Column.isRawData(value)) {
        // ignore it, we could not parse it
        continue;
      }

      if(value == MIN_VALUE) {
        // null is the "least" value
        _entryBuffer.write(getNullEntryFlag(col.isAscending()));
        continue;
      }
      if(value == MAX_VALUE) {
        // the opposite null is the "greatest" value
        _entryBuffer.write(getNullEntryFlag(!col.isAscending()));
        continue;
      }

      col.writeValue(value, _entryBuffer);
    }
    
    return _entryBuffer.toByteArray();
  }  
  
  /**
   * Writes the current index state to the database.  Index has already been
   * initialized.
   */
  protected abstract void updateImpl() throws IOException;
  
  /**
   * Reads the actual index entries.
   */
  protected abstract void readIndexEntries()
    throws IOException;

  /**
   * Finds the data page for the given entry.
   */
  protected abstract DataPage findDataPage(Entry entry)
    throws IOException;
  
  /**
   * Gets the data page for the pageNumber.
   */
  protected abstract DataPage getDataPage(int pageNumber)
    throws IOException;
  
  /**
   * Flips the first bit in the byte at the given index.
   */
  private static byte[] flipFirstBitInByte(byte[] value, int index)
  {
    value[index] = (byte)(value[index] ^ 0x80);

    return value;
  }

  /**
   * Flips all the bits in the byte array.
   */
  private static byte[] flipBytes(byte[] value) {
    return flipBytes(value, 0, value.length);
  }

  /**
   * Flips the bits in the specified bytes in the byte array.
   */
  static byte[] flipBytes(byte[] value, int offset, int length) {
    for(int i = offset; i < (offset + length); ++i) {
      value[i] = (byte)(~value[i]);
    } 
    return value;
  }

  /**
   * Writes the value of the given column type to a byte array and returns it.
   */
  private static byte[] encodeNumberColumnValue(Object value, Column column)
    throws IOException
  {
    // always write in big endian order
    return column.write(value, 0, ENTRY_BYTE_ORDER).array();
  }    

  /**
   * Creates one of the special index entries.
   */
  private static Entry createSpecialEntry(RowId rowId) {
    return new Entry((byte[])null, rowId);
  }

  /**
   * Constructs a ColumnDescriptor of the relevant type for the given Column.
   */
  private ColumnDescriptor newColumnDescriptor(Column col, byte flags)
    throws IOException
  {
    switch(col.getType()) {
    case TEXT:
    case MEMO:
      Column.SortOrder sortOrder = col.getTextSortOrder();
      if(Column.GENERAL_LEGACY_SORT_ORDER.equals(sortOrder)) {
        return new GenLegTextColumnDescriptor(col, flags);
      }
      if(Column.GENERAL_SORT_ORDER.equals(sortOrder)) {
        return new GenTextColumnDescriptor(col, flags);
      }
      // unsupported sort order
      LOG.warn("Unsupported collating sort order " + sortOrder + 
               " for text index, making read-only");
      setReadOnly();
      return new ReadOnlyColumnDescriptor(col, flags);
    case INT:
    case LONG:
    case MONEY:
    case COMPLEX_TYPE:
      return new IntegerColumnDescriptor(col, flags);
    case FLOAT:
    case DOUBLE:
    case SHORT_DATE_TIME:
      return new FloatingPointColumnDescriptor(col, flags);
    case NUMERIC:
      return (col.getFormat().LEGACY_NUMERIC_INDEXES ?
              new LegacyFixedPointColumnDescriptor(col, flags) :
              new FixedPointColumnDescriptor(col, flags));
    case BYTE:
      return new ByteColumnDescriptor(col, flags);
    case BOOLEAN:
      return new BooleanColumnDescriptor(col, flags);
    case GUID:
      return new GuidColumnDescriptor(col, flags);

    default:
      // FIXME we can't modify this index at this point in time
      LOG.warn("Unsupported data type " + col.getType() + 
               " for index, making read-only");
      setReadOnly();
      return new ReadOnlyColumnDescriptor(col, flags);
    }
  }

  /**
   * Returns the EntryType based on the given entry info.
   */
  private static EntryType determineEntryType(byte[] entryBytes, RowId rowId)
  {
    if(entryBytes != null) {
      return ((rowId.getType() == RowId.Type.NORMAL) ?
              EntryType.NORMAL :
              ((rowId.getType() == RowId.Type.ALWAYS_FIRST) ?
               EntryType.FIRST_VALID : EntryType.LAST_VALID));
    } else if(!rowId.isValid()) {
      // this is a "special" entry (first/last)
      return ((rowId.getType() == RowId.Type.ALWAYS_FIRST) ?
              EntryType.ALWAYS_FIRST : EntryType.ALWAYS_LAST);
    }
    throw new IllegalArgumentException("Values was null for valid entry");
  }

  /**
   * Returns the maximum amount of entry data which can be encoded on any
   * index page.
   */
  private static int calcMaxPageEntrySize(JetFormat format)
  {
    // the max data we can fit on a page is the min of the space on the page
    // vs the number of bytes which can be encoded in the entry mask
    int pageDataSize = (format.PAGE_SIZE -
                        (format.OFFSET_INDEX_ENTRY_MASK +
                         format.SIZE_INDEX_ENTRY_MASK));
    int entryMaskSize = (format.SIZE_INDEX_ENTRY_MASK * 8);
    return Math.min(pageDataSize, entryMaskSize);
  }
  
  /**
   * Information about the columns in an index.  Also encodes new index
   * values.
   */
  public static abstract class ColumnDescriptor
  {
    private final Column _column;
    private final byte _flags;

    private ColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      _column = column;
      _flags = flags;
    }

    public Column getColumn() {
      return _column;
    }

    public byte getFlags() {
      return _flags;
    }

    public boolean isAscending() {
      return((getFlags() & ASCENDING_COLUMN_FLAG) != 0);
    }
    
    public int getColumnIndex() {
      return getColumn().getColumnIndex();
    }
    
    public String getName() {
      return getColumn().getName();
    }

    protected boolean isNullValue(Object value) {
      return (value == null);
    }
    
    protected final void writeValue(Object value, ByteStream bout)
      throws IOException
    {
      if(isNullValue(value)) {
        // write null value
        bout.write(getNullEntryFlag(isAscending()));
        return;
      }
      
      // write the start flag
      bout.write(getStartEntryFlag(isAscending()));
      // write the rest of the value
      writeNonNullValue(value, bout);
    }

    protected abstract void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException; 
    
    @Override
    public String toString() {
      return "ColumnDescriptor " + getColumn() + "\nflags: " + getFlags();
    }
  }

  /**
   * ColumnDescriptor for integer based columns.
   */
  private static final class IntegerColumnDescriptor extends ColumnDescriptor
  {
    private IntegerColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      byte[] valueBytes = encodeNumberColumnValue(value, getColumn());
      
      // bit twiddling rules:
      // - isAsc  => flipFirstBit
      // - !isAsc => flipFirstBit, flipBytes
      
      flipFirstBitInByte(valueBytes, 0);
      if(!isAscending()) {
        flipBytes(valueBytes);
      }
      
      bout.write(valueBytes);
    }    
  }
  
  /**
   * ColumnDescriptor for floating point based columns.
   */
  private static final class FloatingPointColumnDescriptor
    extends ColumnDescriptor
  {
    private FloatingPointColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      byte[] valueBytes = encodeNumberColumnValue(value, getColumn());
      
      // determine if the number is negative by testing if the first bit is
      // set
      boolean isNegative = ((valueBytes[0] & 0x80) != 0);

      // bit twiddling rules:
      // isAsc && !isNeg => flipFirstBit
      // isAsc && isNeg => flipBytes
      // !isAsc && !isNeg => flipFirstBit, flipBytes
      // !isAsc && isNeg => nothing
      
      if(!isNegative) {
        flipFirstBitInByte(valueBytes, 0);
      }
      if(isNegative == isAscending()) {
        flipBytes(valueBytes);
      }
      
      bout.write(valueBytes);
    }    
  }
  
  /**
   * ColumnDescriptor for fixed point based columns (legacy sort order).
   */
  private static class LegacyFixedPointColumnDescriptor
    extends ColumnDescriptor
  {
    private LegacyFixedPointColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }

    protected void handleNegationAndOrder(boolean isNegative,
                                          byte[] valueBytes)
    {
      if(isNegative == isAscending()) {
        flipBytes(valueBytes);
      }

      // reverse the sign byte (after any previous byte flipping)
      valueBytes[0] = (isNegative ? (byte)0x00 : (byte)0xFF);      
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      byte[] valueBytes = encodeNumberColumnValue(value, getColumn());
      
      // determine if the number is negative by testing if the first bit is
      // set
      boolean isNegative = ((valueBytes[0] & 0x80) != 0);

      // bit twiddling rules:
      // isAsc && !isNeg => setReverseSignByte             => FF 00 00 ...
      // isAsc && isNeg => flipBytes, setReverseSignByte   => 00 FF FF ...
      // !isAsc && !isNeg => flipBytes, setReverseSignByte => FF FF FF ...
      // !isAsc && isNeg => setReverseSignByte             => 00 00 00 ...
      
      // v2007 bit twiddling rules (old ordering was a bug, MS kb 837148):
      // isAsc && !isNeg => setSignByte 0xFF            => FF 00 00 ...
      // isAsc && isNeg => setSignByte 0xFF, flipBytes  => 00 FF FF ...
      // !isAsc && !isNeg => setSignByte 0xFF           => FF 00 00 ...
      // !isAsc && isNeg => setSignByte 0xFF, flipBytes => 00 FF FF ...
      handleNegationAndOrder(isNegative, valueBytes);

      bout.write(valueBytes);
    }    
  }
  
  /**
   * ColumnDescriptor for new-style fixed point based columns.
   */
  private static final class FixedPointColumnDescriptor
    extends LegacyFixedPointColumnDescriptor
  {
    private FixedPointColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void handleNegationAndOrder(boolean isNegative,
                                          byte[] valueBytes)
    {
      // see notes above in FixedPointColumnDescriptor for bit twiddling rules

      // reverse the sign byte (before any byte flipping)
      valueBytes[0] = (byte)0xFF;

      if(isNegative == isAscending()) {
        flipBytes(valueBytes);
      }
    }    
  }
  
  /**
   * ColumnDescriptor for byte based columns.
   */
  private static final class ByteColumnDescriptor extends ColumnDescriptor
  {
    private ByteColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      byte[] valueBytes = encodeNumberColumnValue(value, getColumn());
      
      // bit twiddling rules:
      // - isAsc  => nothing
      // - !isAsc => flipBytes
      if(!isAscending()) {
        flipBytes(valueBytes);
      }
      
      bout.write(valueBytes);
    }    
  }
  
  /**
   * ColumnDescriptor for boolean columns.
   */
  private static final class BooleanColumnDescriptor extends ColumnDescriptor
  {
    private BooleanColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }

    @Override
    protected boolean isNullValue(Object value) {
      // null values are handled as booleans
      return false;
    }
    
    @Override
    protected void writeNonNullValue(Object value, ByteStream bout)
      throws IOException
    {
      bout.write(
          Column.toBooleanValue(value) ?
          (isAscending() ? ASC_BOOLEAN_TRUE : DESC_BOOLEAN_TRUE) :
          (isAscending() ? ASC_BOOLEAN_FALSE : DESC_BOOLEAN_FALSE));
    }
  }
  
  /**
   * ColumnDescriptor for "general legacy" sort order text based columns.
   */
  private static final class GenLegTextColumnDescriptor 
    extends ColumnDescriptor
  {
    private GenLegTextColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      GeneralLegacyIndexCodes.GEN_LEG_INSTANCE.writeNonNullIndexTextValue(
          value, bout, isAscending());
    }    
  }

  /**
   * ColumnDescriptor for "general" sort order (2010+) text based columns.
   */
  private static final class GenTextColumnDescriptor extends ColumnDescriptor
  {
    private GenTextColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      GeneralIndexCodes.GEN_INSTANCE.writeNonNullIndexTextValue(
          value, bout, isAscending());
    }    
  }

  /**
   * ColumnDescriptor for guid columns.
   */
  private static final class GuidColumnDescriptor extends ColumnDescriptor
  {
    private GuidColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }
    
    @Override
    protected void writeNonNullValue(
        Object value, ByteStream bout)
      throws IOException
    {
      byte[] valueBytes = encodeNumberColumnValue(value, getColumn());

      // index format <8-bytes> 0x09 <8-bytes> 0x08
      
      // bit twiddling rules:
      // - isAsc  => nothing
      // - !isAsc => flipBytes, _but keep 09 unflipped_!      
      if(!isAscending()) {
        flipBytes(valueBytes);
      }
      
      bout.write(valueBytes, 0, 8);
      bout.write(MID_GUID);
      bout.write(valueBytes, 8, 8);
      bout.write(isAscending() ? ASC_END_GUID : DESC_END_GUID);
    }
  }
  

  /**
   * ColumnDescriptor for columns which we cannot currently write.
   */
  private static final class ReadOnlyColumnDescriptor extends ColumnDescriptor
  {
    private ReadOnlyColumnDescriptor(Column column, byte flags)
      throws IOException
    {
      super(column, flags);
    }

    @Override
    protected void writeNonNullValue(Object value, ByteStream bout)
      throws IOException
    {
      throw new UnsupportedOperationException("should not be called");
    }
  }
    
  /**
   * A single leaf entry in an index (points to a single row)
   */
  public static class Entry implements Comparable<Entry>
  {
    /** page/row on which this row is stored */
    private final RowId _rowId;
    /** the entry value */
    private final byte[] _entryBytes;
    /** comparable type for the entry */
    private final EntryType _type;
    
    /**
     * Create a new entry
     * @param entryBytes encoded bytes for this index entry
     * @param rowId rowId in which the row is stored
     * @param type the type of the entry
     */
    private Entry(byte[] entryBytes, RowId rowId, EntryType type) {
      _rowId = rowId;
      _entryBytes = entryBytes;
      _type = type;
    }
    
    /**
     * Create a new entry
     * @param entryBytes encoded bytes for this index entry
     * @param rowId rowId in which the row is stored
     */
    private Entry(byte[] entryBytes, RowId rowId)
    {
      this(entryBytes, rowId, determineEntryType(entryBytes, rowId));
    }

    /**
     * Read an existing entry in from a buffer
     */
    private Entry(ByteBuffer buffer, int entryLen)
      throws IOException
    {
      this(buffer, entryLen, 0);
    }
    
    /**
     * Read an existing entry in from a buffer
     */
    private Entry(ByteBuffer buffer, int entryLen, int extraTrailingLen)
      throws IOException
    {
      // we need 4 trailing bytes for the rowId, plus whatever the caller
      // wants
      int colEntryLen = entryLen - (4 + extraTrailingLen);

      // read the entry bytes
      _entryBytes = new byte[colEntryLen];
      buffer.get(_entryBytes);

      // read the rowId
      int page = ByteUtil.get3ByteInt(buffer, ENTRY_BYTE_ORDER);
      int row = ByteUtil.getUnsignedByte(buffer);
      
      _rowId = new RowId(page, row);
      _type = EntryType.NORMAL;
    }
    
    public RowId getRowId() {
      return _rowId;
    }

    public EntryType getType() {
      return _type;
    }

    public Integer getSubPageNumber() {
      throw new UnsupportedOperationException();
    }
    
    public boolean isLeafEntry() {
      return true;
    }
    
    public boolean isValid() {
      return(_entryBytes != null);
    }

    protected final byte[] getEntryBytes() {
      return _entryBytes;
    }
    
    /**
     * Size of this entry in the db.
     */
    protected int size() {
      // need 4 trailing bytes for the rowId
      return _entryBytes.length + 4;
    }
    
    /**
     * Write this entry into a buffer
     */
    protected void write(ByteBuffer buffer,
                         byte[] prefix)
      throws IOException
    {
      if(prefix.length <= _entryBytes.length) {
        
        // write entry bytes, not including prefix
        buffer.put(_entryBytes, prefix.length,
                   (_entryBytes.length - prefix.length));
        ByteUtil.put3ByteInt(buffer, getRowId().getPageNumber(),
                             ENTRY_BYTE_ORDER);
        
      } else if(prefix.length <= (_entryBytes.length + 3)) {
        
        // the prefix includes part of the page number, write to temp buffer
        // and copy last bytes to output buffer
        ByteBuffer tmp = ByteBuffer.allocate(3);
        ByteUtil.put3ByteInt(tmp, getRowId().getPageNumber(),
                             ENTRY_BYTE_ORDER);
        tmp.flip();
        tmp.position(prefix.length - _entryBytes.length);
        buffer.put(tmp);
        
      } else {
        
        // since the row number would never be the same if the page number is
        // the same, nothing past the page number should ever be included in
        // the prefix.
        // FIXME, this could happen if page has only one row...
        throw new IllegalStateException("prefix should never be this long");
      }
      
      buffer.put((byte)getRowId().getRowNumber());
    }

    protected final String entryBytesToString() {
      return (isValid() ? ", Bytes = " + ByteUtil.toHexString(
                  ByteBuffer.wrap(_entryBytes), _entryBytes.length) :
              "");
    }
    
    @Override
    public String toString() {
      return "RowId = " + _rowId + entryBytesToString() + "\n";
    }

    @Override
    public int hashCode() {
      return _rowId.hashCode();
    }

    @Override
    public boolean equals(Object o) {
      return((this == o) ||
             ((o != null) && (getClass() == o.getClass()) &&
              (compareTo((Entry)o) == 0)));
    }

    /**
     * @return {@code true} iff the entryBytes are equal between this
     *         Entry and the given Entry
     */
    public boolean equalsEntryBytes(Entry o) {
      return(BYTE_CODE_COMPARATOR.compare(_entryBytes, o._entryBytes) == 0);
    }
    
    public int compareTo(Entry other) {
      if (this == other) {
        return 0;
      }

      if(isValid() && other.isValid()) {

        // comparing two valid entries.  first, compare by actual byte values
        int entryCmp = BYTE_CODE_COMPARATOR.compare(
            _entryBytes, other._entryBytes);
        if(entryCmp != 0) {
          return entryCmp;
        }

      } else {

        // if the entries are of mixed validity (or both invalid), we defer
        // next to the EntryType
        int typeCmp = _type.compareTo(other._type);
        if(typeCmp != 0) {
          return typeCmp;
        }
      }
      
      // at this point we let the RowId decide the final result
      return _rowId.compareTo(other.getRowId());
    }

    /**
     * Returns a copy of this entry as a node Entry with the given
     * subPageNumber.
     */
    protected Entry asNodeEntry(Integer subPageNumber) {
      return new NodeEntry(_entryBytes, _rowId, _type, subPageNumber);
    }
    
  }

  /**
   * A single node entry in an index (points to a sub-page in the index)
   */
  private static final class NodeEntry extends Entry {

    /** index page number of the page to which this node entry refers */
    private final Integer _subPageNumber;

    /**
     * Create a new node entry
     * @param entryBytes encoded bytes for this index entry
     * @param rowId rowId in which the row is stored
     * @param type the type of the entry
     * @param subPageNumber the sub-page to which this node entry refers
     */
    private NodeEntry(byte[] entryBytes, RowId rowId, EntryType type,
                      Integer subPageNumber) {
      super(entryBytes, rowId, type);
      _subPageNumber = subPageNumber;
    }
    
    /**
     * Read an existing node entry in from a buffer
     */
    private NodeEntry(ByteBuffer buffer, int entryLen)
      throws IOException
    {
      // we need 4 trailing bytes for the sub-page number
      super(buffer, entryLen, 4);

      _subPageNumber = ByteUtil.getInt(buffer, ENTRY_BYTE_ORDER);
    }

    @Override
    public Integer getSubPageNumber() {
      return _subPageNumber;
    }

    @Override
    public boolean isLeafEntry() {
      return false;
    }
    
    @Override
    protected int size() {
      // need 4 trailing bytes for the sub-page number
      return super.size() + 4;
    }
    
    @Override
    protected void write(ByteBuffer buffer, byte[] prefix) throws IOException {
      super.write(buffer, prefix);
      ByteUtil.putInt(buffer, _subPageNumber, ENTRY_BYTE_ORDER);
    }
    
    @Override
    public boolean equals(Object o) {
      return((this == o) ||
             ((o != null) && (getClass() == o.getClass()) &&
              (compareTo((Entry)o) == 0) &&
              (getSubPageNumber().equals(((Entry)o).getSubPageNumber()))));
    }

    @Override
    public String toString() {
      return ("Node RowId = " + getRowId() +
              ", SubPage = " + _subPageNumber + entryBytesToString() + "\n");
    }
        
  }

  /**
   * Utility class to traverse the entries in the Index.  Remains valid in the
   * face of index entry modifications.
   */
  public final class EntryCursor
  {
    /** handler for moving the page cursor forward */
    private final DirHandler _forwardDirHandler = new ForwardDirHandler();
    /** handler for moving the page cursor backward */
    private final DirHandler _reverseDirHandler = new ReverseDirHandler();
    /** the first (exclusive) row id for this cursor */
    private Position _firstPos;
    /** the last (exclusive) row id for this cursor */
    private Position _lastPos;
    /** the current entry */
    private Position _curPos;
    /** the previous entry */
    private Position _prevPos;
    /** the last read modification count on the Index.  we track this so that
        the cursor can detect updates to the index while traversing and act
        accordingly */
    private int _lastModCount;

    private EntryCursor(Position firstPos, Position lastPos)
    {
      _firstPos = firstPos;
      _lastPos = lastPos;
      _lastModCount = getIndexModCount();
      reset();
    }

    /**
     * Returns the DirHandler for the given direction
     */
    private DirHandler getDirHandler(boolean moveForward) {
      return (moveForward ? _forwardDirHandler : _reverseDirHandler);
    }

    public IndexData getIndexData() {
      return IndexData.this;
    }

    private int getIndexModCount() {
      return IndexData.this._modCount;
    }
    
    /**
     * Returns the first entry (exclusive) as defined by this cursor.
     */
    public Entry getFirstEntry() {
      return _firstPos.getEntry();
    }
  
    /**
     * Returns the last entry (exclusive) as defined by this cursor.
     */
    public Entry getLastEntry() {
      return _lastPos.getEntry();
    }

    /**
     * Returns {@code true} if this cursor is up-to-date with respect to its
     * index.
     */
    public boolean isUpToDate() {
      return(getIndexModCount() == _lastModCount);
    }
        
    public void reset() {
      beforeFirst();
    }

    public void beforeFirst() {
      reset(Cursor.MOVE_FORWARD);
    }

    public void afterLast() {
      reset(Cursor.MOVE_REVERSE);
    }

    protected void reset(boolean moveForward)
    {
      _curPos = getDirHandler(moveForward).getBeginningPosition();
      _prevPos = _curPos;
    }

    /**
     * Repositions the cursor so that the next row will be the first entry
     * >= the given row.
     */
    public void beforeEntry(Object[] row)
      throws IOException
    {
      restorePosition(
          new Entry(IndexData.this.createEntryBytes(row), RowId.FIRST_ROW_ID));
    }
    
    /**
     * Repositions the cursor so that the previous row will be the first
     * entry <= the given row.
     */
    public void afterEntry(Object[] row)
      throws IOException
    {
      restorePosition(
          new Entry(IndexData.this.createEntryBytes(row), RowId.LAST_ROW_ID));
    }
    
    /**
     * @return valid entry if there was a next entry,
     *         {@code #getLastEntry} otherwise
     */
    public Entry getNextEntry() throws IOException {
      return getAnotherPosition(Cursor.MOVE_FORWARD).getEntry();
    }

    /**
     * @return valid entry if there was a next entry,
     *         {@code #getFirstEntry} otherwise
     */
    public Entry getPreviousEntry() throws IOException {
      return getAnotherPosition(Cursor.MOVE_REVERSE).getEntry();
    }

    /**
     * Restores a current position for the cursor (current position becomes
     * previous position).
     */
    protected void restorePosition(Entry curEntry)
      throws IOException
    {
      restorePosition(curEntry, _curPos.getEntry());
    }
    
    /**
     * Restores a current and previous position for the cursor.
     */
    protected void restorePosition(Entry curEntry, Entry prevEntry)
      throws IOException
    {
      if(!_curPos.equalsEntry(curEntry) ||
         !_prevPos.equalsEntry(prevEntry))
      {
        if(!isUpToDate()) {
          updateBounds();
          _lastModCount = getIndexModCount();
        }
        _prevPos = updatePosition(prevEntry);
        _curPos = updatePosition(curEntry);
      } else {
        checkForModification();
      }
    }
    
    /**
     * Gets another entry in the given direction, returning the new entry.
     */
    private Position getAnotherPosition(boolean moveForward)
      throws IOException
    {
      DirHandler handler = getDirHandler(moveForward);
      if(_curPos.equals(handler.getEndPosition())) {
        if(!isUpToDate()) {
          restorePosition(_prevPos.getEntry());
          // drop through and retry moving to another entry
        } else {
          // at end, no more
          return _curPos;
        }
      }

      checkForModification();

      _prevPos = _curPos;
      _curPos = handler.getAnotherPosition(_curPos);
      return _curPos;
    }

    /**
     * Checks the index for modifications and updates state accordingly.
     */
    private void checkForModification()
      throws IOException
    {
      if(!isUpToDate()) {
        updateBounds();
        _prevPos = updatePosition(_prevPos.getEntry());
        _curPos = updatePosition(_curPos.getEntry());
        _lastModCount = getIndexModCount();
      }
    }

    /**
     * Updates the given position, taking boundaries into account.
     */
    private Position updatePosition(Entry entry)
      throws IOException
    {
      if(!entry.isValid()) {
        // no use searching if "updating" the first/last pos
        if(_firstPos.equalsEntry(entry)) {
          return _firstPos;
        } else if(_lastPos.equalsEntry(entry)) {
          return _lastPos;
        } else {
          throw new IllegalArgumentException("Invalid entry given " + entry);
        }
      }
      
      Position pos = findEntryPosition(entry);
      if(pos.compareTo(_lastPos) >= 0) {
        return _lastPos;
      } else if(pos.compareTo(_firstPos) <= 0) {
        return _firstPos;
      }
      return pos;
    }
    
    /**
     * Updates any the boundary info (_firstPos/_lastPos).
     */
    private void updateBounds()
      throws IOException
    {
      _firstPos = findEntryPosition(_firstPos.getEntry());
      _lastPos = findEntryPosition(_lastPos.getEntry());
    }
        
    @Override
    public String toString() {
      return getClass().getSimpleName() + " CurPosition " + _curPos +
        ", PrevPosition " + _prevPos;
    }
    
    /**
     * Handles moving the cursor in a given direction.  Separates cursor
     * logic from value storage.
     */
    private abstract class DirHandler {
      public abstract Position getAnotherPosition(Position curPos)
        throws IOException;
      public abstract Position getBeginningPosition();
      public abstract Position getEndPosition();
    }
        
    /**
     * Handles moving the cursor forward.
     */
    private final class ForwardDirHandler extends DirHandler {
      @Override
      public Position getAnotherPosition(Position curPos)
        throws IOException
      {
        Position newPos = getNextPosition(curPos);
        if((newPos == null) || (newPos.compareTo(_lastPos) >= 0)) {
          newPos = _lastPos;
        }
        return newPos;
      }
      @Override
      public Position getBeginningPosition() {
        return _firstPos;
      }
      @Override
      public Position getEndPosition() {
        return _lastPos;
      }
    }
        
    /**
     * Handles moving the cursor backward.
     */
    private final class ReverseDirHandler extends DirHandler {
      @Override
      public Position getAnotherPosition(Position curPos)
        throws IOException
      {
        Position newPos = getPreviousPosition(curPos);
        if((newPos == null) || (newPos.compareTo(_firstPos) <= 0)) {
          newPos = _firstPos;
        }
        return newPos;
      }
      @Override
      public Position getBeginningPosition() {
        return _lastPos;
      }
      @Override
      public Position getEndPosition() {
        return _firstPos;
      }
    }
  }

  /**
   * Simple value object for maintaining some cursor state.
   */
  private static final class Position implements Comparable<Position> {
    /** the last known page of the given entry */
    private final DataPage _dataPage;
    /** the last known index of the given entry */
    private final int _idx;
    /** the entry at the given index */
    private final Entry _entry;
    /** {@code true} if this entry does not currently exist in the entry list,
        {@code false} otherwise (this is equivalent to adding -0.5 to the
        _idx) */
    private final boolean _between;

    private Position(DataPage dataPage, int idx)
    {
      this(dataPage, idx, dataPage.getEntries().get(idx), false);
    }
    
    private Position(DataPage dataPage, int idx, Entry entry, boolean between)
    {
      _dataPage = dataPage;
      _idx = idx;
      _entry = entry;
      _between = between;
    }

    public DataPage getDataPage() {
      return _dataPage;
    }
    
    public int getIndex() {
      return _idx;
    }

    public int getNextIndex() {
      // note, _idx does not need to be advanced if it was pointing at a
      // between position
      return(_between ? _idx : (_idx + 1));
    }

    public int getPrevIndex() {
      // note, we ignore the between flag here because the index will be
      // pointing at the correct next index in either the between or
      // non-between case
      return(_idx - 1);
    }
    
    public Entry getEntry() {
      return _entry;
    }

    public boolean isBetween() {
      return _between;
    }

    public boolean equalsEntry(Entry entry) {
      return _entry.equals(entry);
    }
    
    public int compareTo(Position other)
    {
      if(this == other) {
        return 0;
      }

      if(_dataPage.equals(other._dataPage)) {
        // "simple" index comparison (handle between-ness)
        int idxCmp = ((_idx < other._idx) ? -1 :
                      ((_idx > other._idx) ? 1 :
                       ((_between == other._between) ? 0 :
                        (_between ? -1 : 1))));
        if(idxCmp != 0) {
          return idxCmp;
        }
      }
      
      // compare the entries.
      return _entry.compareTo(other._entry);
    }
    
    @Override
    public int hashCode() {
      return _entry.hashCode();
    }
    
    @Override
    public boolean equals(Object o) {
      return((this == o) ||
             ((o != null) && (getClass() == o.getClass()) &&
              (compareTo((Position)o) == 0)));
    }

    @Override
    public String toString() {
      return "Page = " + _dataPage.getPageNumber() + ", Idx = " + _idx +
        ", Entry = " + _entry + ", Between = " + _between;
    }
  }

  /**
   * Object used to maintain state about an Index page.
   */
  protected static abstract class DataPage {

    public abstract int getPageNumber();
    
    public abstract boolean isLeaf();
    public abstract void setLeaf(boolean isLeaf);

    public abstract int getPrevPageNumber();
    public abstract void setPrevPageNumber(int pageNumber);
    public abstract int getNextPageNumber();
    public abstract void setNextPageNumber(int pageNumber);
    public abstract int getChildTailPageNumber();
    public abstract void setChildTailPageNumber(int pageNumber);
    
    public abstract int getTotalEntrySize();
    public abstract void setTotalEntrySize(int totalSize);
    public abstract byte[] getEntryPrefix();
    public abstract void setEntryPrefix(byte[] entryPrefix);

    public abstract List<Entry> getEntries();
    public abstract void setEntries(List<Entry> entries);

    public abstract void addEntry(int idx, Entry entry)
      throws IOException;
    public abstract void removeEntry(int idx)
      throws IOException;

    public final boolean isEmpty() {
      return getEntries().isEmpty();
    }
    
    public final int getCompressedEntrySize() {
      // when written to the index page, the entryPrefix bytes will only be
      // written for the first entry, so we subtract the entry prefix size
      // from all the other entries to determine the compressed size
      return getTotalEntrySize() -
        (getEntryPrefix().length * (getEntries().size() - 1));
    }

    public final int findEntry(Entry entry) {
      return Collections.binarySearch(getEntries(), entry);
    }

    @Override
    public final int hashCode() {
      return getPageNumber();
    }

    @Override
    public final boolean equals(Object o) {
      return((this == o) ||
             ((o != null) && (getClass() == o.getClass()) &&
              (getPageNumber() == ((DataPage)o).getPageNumber())));
    }

    @Override
    public final String toString() {
      List<Entry> entries = getEntries();
      return (isLeaf() ? "Leaf" : "Node") + "DataPage[" + getPageNumber() +
        "] " + getPrevPageNumber() + ", " + getNextPageNumber() + ", (" +
        getChildTailPageNumber() + "), " +
        ((isLeaf() && !entries.isEmpty()) ?
         ("[" + entries.get(0) + ", " +
          entries.get(entries.size() - 1) + "]") :
         entries);
    }
  }


}