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.calcite.sql.util;
  
  import org.checkerframework.checker.nullness.qual.Nullable;
  
  import java.util.List;
  import java.util.Map;
  
  import static java.util.Objects.requireNonNull;
  
  /** Similar to {@link org.apache.calcite.util.Pair} but identity is based
   * on identity of values.
   *
   * <p>Also, uses {@code hashCode} algorithm of {@link List},
   * not {@link Map.Entry#hashCode()}.
   *
   * @param <L> Left type
   * @param <R> Right type
   */
  public class IdPair<L, R> {
    final L left;
    final R right;
  
    /** Creates an IdPair. */
    public static <L, R> IdPair<L, R> of(L left, R right) {
      return new IdPair<>(left, right);
    }
  
    protected IdPair(L left, R right) {
      this.left = requireNonNull(left, "left");
      this.right = requireNonNull(right, "right");
    }
  
    @Override public String toString() {
      return left + "=" + right;
    }
  
    @Override public boolean equals(@Nullable Object obj) {
      return obj == this
          || obj instanceof IdPair
          && left == ((IdPair) obj).left
          && right == ((IdPair) obj).right;
    }
  
    @Override public int hashCode() {
      return (31
          + System.identityHashCode(left)) * 31
          + System.identityHashCode(right);
    }
  }
  