// Copyright 2014 The Bazel Authors. All rights reserved.
//
// Licensed 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 com.google.devtools.build.lib.util;

import com.google.common.base.Function;

import java.util.Comparator;
import java.util.Objects;

import javax.annotation.Nullable;

/**
 * An immutable, semantic-free ordered pair of nullable values. Avoid using it in public APIs.
 */
public final class Pair<A, B> {

  /**
   * Creates a new pair containing the given elements in order.
   */
  public static <A, B> Pair<A, B> of(@Nullable A first, @Nullable B second) {
    return new Pair<>(first, second);
  }

  /**
   * The first element of the pair.
   */
  @Nullable
  public final A first;

  /**
   * The second element of the pair.
   */
  @Nullable
  public final B second;

  /**
   * Constructor.  It is usually easier to call {@link #of}.
   */
  public Pair(@Nullable A first, @Nullable B second) {
    this.first = first;
    this.second = second;
  }

  @Nullable
  public A getFirst() {
    return first;
  }

  @Nullable
  public B getSecond() {
    return second;
  }

  @Override
  public String toString() {
    return "(" + first + ", " + second + ")";
  }

  @Override
  public boolean equals(Object o) {
    if (this == o) {
      return true;
    }
    if (!(o instanceof Pair)) {
      return false;
    }
    Pair<?, ?> p = (Pair<?, ?>) o;
    return Objects.equals(first, p.first) && Objects.equals(second, p.second);
  }

  @Override
  public int hashCode() {
    return Objects.hash(first, second);
  }

  /**
   * A function that maps to the first element in a pair.
   */
  public static <A, B> Function<Pair<A, B>, A> firstFunction() {
    return new Function<Pair<A, B>, A>() {
      @Override
      public A apply(Pair<A, B> pair) {
        return pair.first;
      }
    };
  }

  /**
   * A function that maps to the second element in a pair.
   */
  public static <A, B> Function<Pair<A, B>, B> secondFunction() {
    return new Function<Pair<A, B>, B>() {
      @Override
      public B apply(Pair<A, B> pair) {
        return pair.second;
      }
    };
  }

  /**
   * A comparator that compares pairs by comparing the first element.
   */
  public static <T extends Comparable<T>, B> Comparator<Pair<T, B>> compareByFirst() {
    return new Comparator<Pair<T, B>>() {
      @Override
      public int compare(Pair<T, B> o1, Pair<T, B> o2) {
        return o1.first.compareTo(o2.first);
      }
    };
  }
}