/*
 * 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.beam.sdk.extensions.euphoria.core.client.util;

import java.util.Comparator;
import org.apache.beam.sdk.extensions.euphoria.core.annotation.audience.Audience;
import org.apache.beam.sdk.extensions.euphoria.core.client.functional.CombinableReduceFunction;
import org.apache.beam.sdk.extensions.euphoria.core.client.functional.UnaryFunction;

/** Calculate maximum. */
@Audience(Audience.Type.CLIENT)
public class Max {

  private Max() {}

  /**
   * Return {@code CombinableReduceFunction} to calculate maximum of input.
   *
   * @param <InputT> the type of elements handled
   * @param <X> the type of key by which to compare the elements
   * @param extract the key extraction function
   * @return a combiner function which delivers the "maximum" element seen; never {@code null}
   */
  public static <InputT, X extends Comparable<X>> CombinableReduceFunction<InputT> of(
      UnaryFunction<InputT, X> extract) {

    return values ->
        values
            .max(Comparator.comparing(extract::apply))
            .orElseThrow(() -> new IllegalStateException("Got empty stream on input!"));
  }
}
