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.concurrent.atomic.AtomicLong;
  
  import io.reactivex.rxjava3.annotations.NonNull;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  /**
   * Utility class to help with backpressure-related operations such as request aggregation.
   */
  public final class BackpressureHelper {
      /** Utility class. */
      private BackpressureHelper() {
          throw new IllegalStateException("No instances!");
      }
  
      /**
       * Adds two long values and caps the sum at {@link Long#MAX_VALUE}.
       * @param a the first value
       * @param b the second value
       * @return the sum capped at {@link Long#MAX_VALUE}
       */
      public static long addCap(long a, long b) {
          long u = a + b;
          if (u < 0L) {
              return Long.MAX_VALUE;
          }
          return u;
      }
  
      /**
       * Multiplies two long values and caps the product at {@link Long#MAX_VALUE}.
       * @param a the first value
       * @param b the second value
       * @return the product capped at {@link Long#MAX_VALUE}
       */
      public static long multiplyCap(long a, long b) {
          long u = a * b;
          if (((a | b) >>> 31) != 0) {
              if (u / a != b) {
                  return Long.MAX_VALUE;
              }
          }
          return u;
      }
  
      /**
       * Atomically adds the positive value n to the requested value in the {@link AtomicLong} and
       * caps the result at {@link Long#MAX_VALUE} and returns the previous value.
       * @param requested the {@code AtomicLong} holding the current requested value
       * @param n the value to add, must be positive (not verified)
       * @return the original value before the add
       */
      public static long add(@NonNull AtomicLong requested, long n) {
          for (;;) {
              long r = requested.get();
              if (r == Long.MAX_VALUE) {
                  return Long.MAX_VALUE;
              }
              long u = addCap(r, n);
              if (requested.compareAndSet(r, u)) {
                  return r;
              }
          }
      }
  
      /**
       * Atomically adds the positive value n to the requested value in the {@link AtomicLong} and
       * caps the result at {@link Long#MAX_VALUE} and returns the previous value and
       * considers {@link Long#MIN_VALUE} as a cancel indication (no addition then).
       * @param requested the {@code AtomicLong} holding the current requested value
       * @param n the value to add, must be positive (not verified)
       * @return the original value before the add
       */
      public static long addCancel(@NonNull AtomicLong requested, long n) {
          for (;;) {
              long r = requested.get();
              if (r == Long.MIN_VALUE) {
                  return Long.MIN_VALUE;
              }
              if (r == Long.MAX_VALUE) {
                  return Long.MAX_VALUE;
              }
              long u = addCap(r, n);
              if (requested.compareAndSet(r, u)) {
                  return r;
              }
          }
      }
  
      /**
       * Atomically subtract the given number (positive, not validated) from the target field unless it contains {@link Long#MAX_VALUE}.
       * @param requested the target field holding the current requested amount
       * @param n the produced element count, positive (not validated)
       * @return the new amount
       */
      public static long produced(@NonNull AtomicLong requested, long n) {
          for (;;) {
              long current = requested.get();
              if (current == Long.MAX_VALUE) {
                  return Long.MAX_VALUE;
              }
              long update = current - n;
              if (update < 0L) {
                  RxJavaPlugins.onError(new IllegalStateException("More produced than requested: " + update));
                  update = 0L;
              }
              if (requested.compareAndSet(current, update)) {
                  return update;
              }
          }
      }
  
      /**
       * Atomically subtract the given number (positive, not validated) from the target field if
       * it doesn't contain {@link Long#MIN_VALUE} (indicating some cancelled state) or {@link Long#MAX_VALUE} (unbounded mode).
       * @param requested the target field holding the current requested amount
       * @param n the produced element count, positive (not validated)
       * @return the new amount
       */
      public static long producedCancel(@NonNull AtomicLong requested, long n) {
          for (;;) {
              long current = requested.get();
              if (current == Long.MIN_VALUE) {
                  return Long.MIN_VALUE;
              }
              if (current == Long.MAX_VALUE) {
                  return Long.MAX_VALUE;
              }
              long update = current - n;
              if (update < 0L) {
                  RxJavaPlugins.onError(new IllegalStateException("More produced than requested: " + update));
                  update = 0L;
              }
              if (requested.compareAndSet(current, update)) {
                  return update;
              }
          }
      }
  }
  