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A pure Java library for reading from and writing to MS Access databases.
There is a newer version: 4.0.8
/*
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 [email protected]
or at the following address:

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

package com.healthmarketscience.jackcess.impl;

import java.nio.ByteBuffer;

/**
 * Bitmask that indicates whether or not each column in a row is null.  Also
 * holds values of boolean columns.
 * @author Tim McCune
 */
public class NullMask {
  
  /** num row columns */
  private final int _columnCount;
  /** The actual bitmask */
  private final byte[] _mask;
  
  /**
   * @param columnCount Number of columns in the row that this mask will be
   *    used for
   */
  public NullMask(int columnCount) {
    _columnCount = columnCount;
    // we leave everything initially marked as null so that we don't need to
    // do anything for deleted columns (we only need to mark as non-null
    // valid columns for which we actually have values).
    _mask = new byte[(_columnCount + 7) / 8];
  }
  
  /**
   * Read a mask in from a buffer
   */
  public void read(ByteBuffer buffer) {
    buffer.get(_mask);
  }

  /**
   * Write a mask to a buffer
   */
  public void write(ByteBuffer buffer) {
    buffer.put(_mask);
  }

  /**
   * @param column column to test for {@code null}
   * @return Whether or not the value for that column is null.  For boolean
   *    columns, returns the actual value of the column (where
   *    non-{@code null} == {@code true})
   */
  public boolean isNull(ColumnImpl column) {
    int columnNumber = column.getColumnNumber();
    // if new columns were added to the table, old null masks may not include
    // them (meaning the field is null)
    if(columnNumber >= _columnCount) {
      // it's null
      return true;
    }
    return (_mask[byteIndex(columnNumber)] & bitMask(columnNumber)) == 0;
  }

  /**
   * Indicate that the column with the given number is not {@code null} (or a
   * boolean value is {@code true}).
   * @param column column to be marked non-{@code null}
   */
  public void markNotNull(ColumnImpl column) {
    int columnNumber = column.getColumnNumber();
    int maskIndex = byteIndex(columnNumber);
    _mask[maskIndex] = (byte) (_mask[maskIndex] | bitMask(columnNumber));
  }
  
  /**
   * @return Size in bytes of this mask
   */
  public int byteSize() {
    return _mask.length;
  }

  private static int byteIndex(int columnNumber) {
    return columnNumber / 8;
  }

  private static byte bitMask(int columnNumber) {
    return (byte) (1 << (columnNumber % 8));
  }
}