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A full SQL database system with JDBC driver that can be embedded in a Java application or operate as a stand-alone server with clients connecting via TCP/IP.
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/**
 * com.mckoi.database.TableDescriptions  28 Jul 2000
 *
 * Mckoi SQL Database ( http://www.mckoi.com/database )
 * Copyright (C) 2000, 2001, 2002  Diehl and Associates, Inc.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License
 * Version 2 as published by the Free Software Foundation.
 *
 * This program 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 General Public License Version 2 for more details.
 *
 * You should have received a copy of the GNU General Public License
 * Version 2 along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
 *
 * Change Log:
 * 
 * 
 */

package com.mckoi.database;

import java.io.*;
import java.util.*;

/**
 * An object that is a key part of Database.  This object maintains a list
 * of descriptions of all tables in the database.  The list contains
 * information about the columns in the table and any other misc table options.
 *
 * @author Tobias Downer
 */

public final class TableDescriptions {

  /**
   * The filename of the file that describes every table in the database.
   */
  private static final String TABLE_DESC_FILE = "MckoiDB.desc";

  /**
   * The File that contains the table descriptions list.
   */
  private File table_desc_file;

  /**
   * The File we use to temporary store the table descriptions as we save
   * them.
   */
  private File temp_desc_file;

  /**
   * The backup file for table descriptions.
   */
  private File backup_file;

  /**
   * A hash table that maps from table name to the DataTableDef object that
   * describes the table.
   */
  private HashMap table_descriptions;

  /**
   * Constructs this object with the database in the given directory.
   */
  public TableDescriptions(File database_path) {
    table_desc_file = new File(database_path, TABLE_DESC_FILE);
    temp_desc_file = new File(database_path, TABLE_DESC_FILE + ".temp");
    backup_file = new File(database_path, TABLE_DESC_FILE + ".bak");
    clear();
  }

  /**
   * Returns true if the table descriptions file exists.
   */
  public boolean exists() {
    return table_desc_file.exists() && !table_desc_file.isDirectory();
  }

  /**
   * Load the entire list of table descriptions for this database.
   */
  public void load() throws IOException {

    // Does the table description file exist?
    if (table_desc_file.exists()) {
      // The file exists so load up the table descriptions and put each table
      // in the table_descriptions map.
      DataInputStream din = new DataInputStream(
             new BufferedInputStream(new FileInputStream(table_desc_file)));

      int ver = din.readInt();
      int table_count = din.readInt();
      for (int i = 0; i < table_count; ++i) {
        DataTableDef table_desc = DataTableDef.read(din);
        String name = table_desc.getName();
        table_descriptions.put(name, table_desc);
      }

      din.close();
    }

  }

  /**
   * Updates the table description file in the database.  The table description
   * file describes every table in the database.  It is loaded when the
   * database is initialized and refreshed whenever a table alteration occurs
   * or the database is shut down.
   */
  public void save() throws IOException {

    DataOutputStream dout = new DataOutputStream(
            new BufferedOutputStream(new FileOutputStream(temp_desc_file)));

    dout.writeInt(1);
    String[] table_list = getTableList();
    dout.writeInt(table_list.length);
    for (int i = 0; i < table_list.length; ++i) {
      // Write the DataTableDef for this table
      ((DataTableDef) table_descriptions.get(table_list[i])).write(dout);
    }

    dout.flush();
    dout.close();

    // Delete the current backup file and rename the temp file to the official
    // file.
    // Cycle through the backups...

    backup_file.delete();
    table_desc_file.renameTo(backup_file);
    temp_desc_file.renameTo(table_desc_file);

  }

  /**
   * Adds a new DataTableDef object to the list of tables in the database.
   */
  void add(DataTableDef table) throws IOException {
    table_descriptions.put(table.getName(), table);
  }

  /**
   * Removes a DataTableDef object from the list with the given name.
   */
  void remove(String name) throws IOException {
    table_descriptions.remove(name);
  }

  /**
   * Returns a list of table name's sorted in alphebetical order.
   */
  public String[] getTableList() {
    Set keys = table_descriptions.keySet();
    String[] all_keys = (String[]) keys.toArray(new String[keys.size()]);
    Arrays.sort(all_keys);
    return all_keys;
  }

  /**
   * Clears this object completely.
   */
  void clear() {
    table_descriptions = new HashMap(150, 0.50f);
  }

  /**
   * Returns the DataTableDef object for the table with the given name.  The
   * description must have been loaded before this method is called.  Returns
   * null if the table was not found.
   */
  public DataTableDef getDef(String table_name) {
    return (DataTableDef) table_descriptions.get(table_name);
  }


}