content: /*
   * 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.kafka.streams.kstream.internals.foreignkeyjoin;
  
  import java.util.function.BiFunction;
  import java.util.function.Function;
  
  /**
   * An interface for extracting foreign keys from input records during foreign key joins in Kafka Streams.
   * This extractor is used to determine the key of the foreign table to join with based on the primary
   * table's record key and value.
   * <p>
   * The interface provides two factory methods:
   * <ul>
   *   <li>{@link #fromFunction(Function)} - when the foreign key depends only on the value</li>
   *   <li>{@link #fromBiFunction(BiFunction)} - when the foreign key depends on both key and value</li>
   * </ul>
   *
   * @param <KLeft>  Type of primary table's key
   * @param <VLeft>  Type of primary table's value
   * @param <KRight> Type of the foreign key to extract
   */
  @FunctionalInterface
  public interface ForeignKeyExtractor<KLeft, VLeft, KRight> {
      KRight extract(KLeft key, VLeft value);
  
      static <KLeft, VLeft, KRight> ForeignKeyExtractor<? super KLeft, ? super VLeft, ? extends KRight> fromFunction(Function<? super VLeft, ? extends KRight> function) {
          return (key, value) -> function.apply(value);
      }
  
      static <KLeft, VLeft, KRight> ForeignKeyExtractor<? super KLeft, ? super VLeft, ? extends KRight> fromBiFunction(BiFunction<? super KLeft, ? super VLeft, ? extends KRight> biFunction) {
          return biFunction::apply;
      }
  }
  