code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * 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 io.reactivex.rxjava3.internal.util;
  
  import java.util.*;
  
  import io.reactivex.rxjava3.functions.BiFunction;
  
  /**
   * A BiFunction that merges two Lists into a new list.
   * @param <T> the value type
   */
  public final class MergerBiFunction<T> implements BiFunction<List<T>, List<T>, List<T>> {
  
      final Comparator<? super T> comparator;
  
      public MergerBiFunction(Comparator<? super T> comparator) {
          this.comparator = comparator;
      }
  
      @Override
      public List<T> apply(List<T> a, List<T> b) {
          int n = a.size() + b.size();
          if (n == 0) {
              return new ArrayList<>();
          }
          List<T> both = new ArrayList<>(n);
  
          Iterator<T> at = a.iterator();
          Iterator<T> bt = b.iterator();
  
          T s1 = at.hasNext() ? at.next() : null;
          T s2 = bt.hasNext() ? bt.next() : null;
  
          while (s1 != null && s2 != null) {
              if (comparator.compare(s1, s2) < 0) { // s1 comes before s2
                  both.add(s1);
                  s1 = at.hasNext() ? at.next() : null;
              } else {
                  both.add(s2);
                  s2 = bt.hasNext() ? bt.next() : null;
              }
          }
  
          if (s1 != null) {
              both.add(s1);
              while (at.hasNext()) {
                  both.add(at.next());
              }
          } else {
              both.add(s2);
              while (bt.hasNext()) {
                  both.add(bt.next());
              }
          }
  
          return both;
      }
  }
  