/*
 * 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.hudi.common.util.collection;

import java.io.Serializable;
import java.util.Map;

/**
 * (NOTE: Adapted from Apache commons-lang3)
 * <p>
 * A pair consisting of two elements.
 * </p>
 *
 * <p>
 * This class is an abstract implementation defining the basic API. It refers to the elements as 'left' and 'right'. It
 * also implements the {@code Map.Entry} interface where the key is 'left' and the value is 'right'.
 * </p>
 *
 * <p>
 * Subclass implementations may be mutable or immutable. However, there is no restriction on the type of the stored
 * objects that may be stored. If mutable objects are stored in the pair, then the pair itself effectively becomes
 * mutable.
 * </p>
 *
 * @param <L> the left element type
 * @param <R> the right element type
 */
public abstract class Pair<L, R> implements Map.Entry<L, R>, Comparable<Pair<L, R>>, Serializable {

  /**
   * Serialization version.
   */
  private static final long serialVersionUID = 4954918890077093841L;

  /**
   * <p>
   * Obtains an immutable pair of from two objects inferring the generic types.
   * </p>
   *
   * <p>
   * This factory allows the pair to be created using inference to obtain the generic types.
   * </p>
   *
   * @param <L> the left element type
   * @param <R> the right element type
   * @param left the left element, may be null
   * @param right the right element, may be null
   * @return a pair formed from the two parameters, not null
   */
  public static <L, R> Pair<L, R> of(final L left, final R right) {
    return new ImmutablePair<>(left, right);
  }

  // -----------------------------------------------------------------------

  /**
   * <p>
   * Gets the left element from this pair.
   * </p>
   *
   * <p>
   * When treated as a key-value pair, this is the key.
   * </p>
   *
   * @return the left element, may be null
   */
  public abstract L getLeft();

  /**
   * <p>
   * Gets the right element from this pair.
   * </p>
   *
   * <p>
   * When treated as a key-value pair, this is the value.
   * </p>
   *
   * @return the right element, may be null
   */
  public abstract R getRight();

  /**
   * <p>
   * Gets the key from this pair.
   * </p>
   *
   * <p>
   * This method implements the {@code Map.Entry} interface returning the left element as the key.
   * </p>
   *
   * @return the left element as the key, may be null
   */
  @Override
  public final L getKey() {
    return getLeft();
  }

  /**
   * <p>
   * Gets the value from this pair.
   * </p>
   *
   * <p>
   * This method implements the {@code Map.Entry} interface returning the right element as the value.
   * </p>
   *
   * @return the right element as the value, may be null
   */
  @Override
  public R getValue() {
    return getRight();
  }

  // -----------------------------------------------------------------------

  /**
   * <p>
   * Compares the pair based on the left element followed by the right element. The types must be {@code Comparable}.
   * </p>
   *
   * @param other the other pair, not null
   * @return negative if this is less, zero if equal, positive if greater
   */
  @Override
  public int compareTo(final Pair<L, R> other) {

    checkComparable(this);
    checkComparable(other);

    Comparable thisLeft = (Comparable) getLeft();
    Comparable thisRight = (Comparable) getRight();
    Comparable otherLeft = (Comparable) other.getLeft();
    Comparable otherRight = (Comparable) other.getRight();

    if (thisLeft.compareTo(otherLeft) == 0) {
      return thisRight.compareTo(otherRight);
    } else {
      return thisLeft.compareTo(otherLeft);
    }
  }

  /**
   * <p>
   * Compares this pair to another based on the two elements.
   * </p>
   *
   * @param obj the object to compare to, null returns false
   * @return true if the elements of the pair are equal
   */
  @Override
  public boolean equals(final Object obj) {
    if (obj == this) {
      return true;
    }
    if (obj instanceof Map.Entry<?, ?>) {
      final Map.Entry<?, ?> other = (Map.Entry<?, ?>) obj;
      return getKey().equals(other.getKey()) && getValue().equals(other.getValue());
    }
    return false;
  }

  /**
   * <p>
   * Returns a suitable hash code. The hash code follows the definition in {@code Map.Entry}.
   * </p>
   *
   * @return the hash code
   */
  @Override
  public int hashCode() {
    // see Map.Entry API specification
    return (getKey() == null ? 0 : getKey().hashCode()) ^ (getValue() == null ? 0 : getValue().hashCode());
  }

  /**
   * <p>
   * Returns a String representation of this pair using the format {@code ($left,$right)}.
   * </p>
   *
   * @return a string describing this object, not null
   */
  @Override
  public String toString() {
    return new StringBuilder().append('(').append(getLeft()).append(',').append(getRight()).append(')').toString();
  }

  /**
   * <p>
   * Formats the receiver using the given format.
   * </p>
   *
   * <p>
   * This uses {@link java.util.Formattable} to perform the formatting. Two variables may be used to embed the left and
   * right elements. Use {@code %1$s} for the left element (key) and {@code %2$s} for the right element (value). The
   * default format used by {@code toString()} is {@code (%1$s,%2$s)}.
   * </p>
   *
   * @param format the format string, optionally containing {@code %1$s} and {@code %2$s}, not null
   * @return the formatted string, not null
   */
  public String toString(final String format) {
    return String.format(format, getLeft(), getRight());
  }

  private void checkComparable(Pair<L, R> pair) {
    if (!(pair.getLeft() instanceof Comparable) || !(pair.getRight() instanceof Comparable)) {
      throw new IllegalArgumentException("Elements of Pair must implement Comparable :" + pair);
    }
  }
}
