content: /*******************************************************************************
   * Copyright (c) 2010 Haifeng Li
   *   
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *  
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   *******************************************************************************/
  
  package smile.math.distance;
  
  import java.io.Serializable;
  
  /**
   * An interface to calculate a distance measure between two objects. A distance
   * function maps pairs of points into the nonnegative reals and has to satisfy
   * <ul>
   * <li> non-negativity: d(x, y) &ge; 0
   * <li> isolation: d(x, y) = 0 if and only if x = y
   * <li> symmetry: d(x, y) = d(x, y)
   * </ul>.
   * Note that a distance function is not required to satisfy triangular inequality
   * |x - y| + |y - z| &ge; |x - z|, which is necessary for a metric.
   *
   * @author Haifeng Li
   */
  public interface Distance<T> extends Serializable {
      /**
       * Returns the distance measure between two objects.
       */
      double d(T x, T y);
  }
  