/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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 org.apache.lucene.search.similarities;

import org.apache.lucene.search.Explanation;

/**
 * The <em>lambda (&lambda;<sub>w</sub>)</em> parameter in information-based models.
 *
 * @see IBSimilarity
 * @lucene.experimental
 */
public abstract class Lambda {

  /** Sole constructor. (For invocation by subclass constructors, typically implicit.) */
  public Lambda() {}

  /** Computes the lambda parameter. */
  public abstract float lambda(BasicStats stats);
  /** Explains the lambda parameter. */
  public abstract Explanation explain(BasicStats stats);

  /**
   * Subclasses must override this method to return the code of the lambda formula. Since the
   * original paper is not very clear on this matter, and also uses the DFR naming scheme
   * incorrectly, the codes here were chosen arbitrarily.
   */
  @Override
  public abstract String toString();
}
