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/*
 * 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 opennlp.tools.ml.maxent.quasinewton;

/**
 * Interface for a function.
 */
public interface Function {

  /**
   * @return Retrieves the dimension value.
   */
  int getDimension();

  /**
   * Computes the function value for {@code x}.
   *
   * @param x The input vector.
   * @return Returns the computed value for {@code x}.
   *
   * @apiNote The parameter {@code x} should be validated and if inconsistencies are encountered
   * an appropriate exception should be raised, e.g. {@link IllegalArgumentException}.
   */
  double valueAt(double[] x);

  /**
   * Computes the gradient for {@code x}.
   *
   * @param x The input vector.
   * @return Returns the computed gradient for {@code x}.
   *
   * @apiNote The parameter {@code x} should be validated and if inconsistencies are encountered
   * an appropriate exception should be raised, e.g. {@link IllegalArgumentException}.
   */
  double[] gradientAt(double[] x);
}