/*
 * This file is part of ELKI:
 * Environment for Developing KDD-Applications Supported by Index-Structures
 *
 * Copyright (C) 2022
 * ELKI Development Team
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the GNU Affero General Public License as published by
 * the Free Software Foundation, either version 3 of the License, or
 * (at your option) any later version.
 *
 * 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 Affero General Public License for more details.
 *
 * You should have received a copy of the GNU Affero General Public License
 * along with this program. If not, see <http://www.gnu.org/licenses/>.
 */
package elki.distance;

import elki.database.ids.DBID;
import elki.database.ids.DBIDRef;

/**
 * Distance functions valid in a database context only (i.e. for DBIDs)
 * <p>
 * For any "distance" that cannot be computed for arbitrary objects, only those
 * that exist in the database and referenced by their ID.
 * <p>
 * Example: external precomputed distances
 *
 * @author Erich Schubert
 * @since 0.4.0
 *
 * @opt nodefillcolor LemonChiffon
 * @assoc - "defined on" - DBID
 */
public interface DBIDDistance extends Distance<DBID> {
  /**
   * Returns the distance between the two objects specified by their object ids.
   *
   * @param id1 first object id
   * @param id2 second object id
   * @return the distance between the two objects specified by their object ids
   */
  double distance(DBIDRef id1, DBIDRef id2);
}
