/*
 * 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.data;

import java.util.HashMap;

import elki.data.type.SimpleTypeInformation;

/**
 * A ClassLabel to identify a certain class of objects that is to discern from
 * other classes by a classifier.
 * 
 * @author Arthur Zimek
 * @since 0.1
 */
public abstract class ClassLabel implements Comparable<ClassLabel> {
  /**
   * ClassLabels need an empty constructor for dynamic access. Subsequently, the
   * init method must be called.
   */
  protected ClassLabel() {
    // Initialized from factory
  }

  /**
   * Any ClassLabel should ensure a natural ordering that is consistent with
   * equals. Thus, if <code>this.compareTo(o)==0</code>, then
   * <code>this.equals(o)</code> should be <code>true</code>.
   * 
   * @param obj an object to test for equality w.r.t. this ClassLabel
   * @return true, if <code>this==obj || this.compareTo(o)==0</code>, false
   *         otherwise
   */
  @Override
  public boolean equals(Object obj) {
    if (!(obj instanceof ClassLabel)) {
      return false;
    }
    return this == obj || this.compareTo((ClassLabel) obj) == 0;
  }

  /**
   * Any ClassLabel requires a method to represent the label as a String. If
   * <code>ClassLabel a.equals((ClassLabel) b)</code>, then also
   * <code>a.toString().equals(b.toString())</code> should hold.
   */
  @Override
  public abstract String toString();

  /**
   * Returns the hashCode of the String-representation of this ClassLabel.
   */
  @Override
  public int hashCode() {
    return toString().hashCode();
  }

  /**
   * Class label factory.
   * 
   * @author Erich Schubert
   * 
   * @has - creates - ClassLabel
   * @stereotype factory
   */
  public abstract static class Factory<L extends ClassLabel> {
    /**
     * Set for reusing the same objects.
     */
    protected HashMap<String, L> existing = new HashMap<>();

    /**
     * Convert a string into a class label.
     * 
     * @param lbl String to convert
     * @return Class label instance.
     */
    public abstract L makeFromString(String lbl);

    /**
     * Get type information for the labels.
     * 
     * @return Type information
     */
    public abstract SimpleTypeInformation<? super L> getTypeInformation();
  }
}
