content: /*
   * Copyright (C) 2016 The Android Open Source Project
   *
   * 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.android.internal.util;
  
  import android.os.SystemClock;
  
  import static com.android.internal.util.Preconditions.checkArgumentNonnegative;
  import static com.android.internal.util.Preconditions.checkArgumentPositive;
  
  /**
   * A class useful for rate-limiting or throttling that stores and distributes tokens.
   *
   * A TokenBucket starts with a fixed capacity of tokens, an initial amount of tokens, and
   * a fixed filling period (in milliseconds).
   *
   * For every filling period, the bucket gains one token, up to its maximum capacity from
   * which point tokens simply overflow and are lost. Tokens can be obtained one by one or n by n.
   *
   * The available amount of tokens is computed lazily when the bucket state is inspected.
   * Therefore it is purely synchronous and does not involve any asynchronous activity.
   * It is not synchronized in any way and not a thread-safe object.
   *
   * {@hide}
   */
  public class TokenBucket {
  
      private final int mFillDelta; // Time in ms it takes to generate one token.
      private final int mCapacity;  // Maximum number of tokens that can be stored.
      private long mLastFill;       // Last time in ms the bucket generated tokens.
      private int mAvailable;       // Current number of available tokens.
  
      /**
       * Create a new TokenBucket.
       * @param deltaMs the time in milliseconds it takes to generate a new token.
       * Must be strictly positive.
       * @param capacity the maximum token capacity. Must be strictly positive.
       * @param tokens the starting amount of token. Must be positive or zero.
       */
      public TokenBucket(int deltaMs, int capacity, int tokens) {
          mFillDelta = checkArgumentPositive(deltaMs, "deltaMs must be strictly positive");
          mCapacity = checkArgumentPositive(capacity, "capacity must be strictly positive");
          mAvailable = Math.min(checkArgumentNonnegative(tokens), mCapacity);
          mLastFill = scaledTime();
      }
  
      /**
       * Create a new TokenBucket that starts completely filled.
       * @param deltaMs the time in milliseconds it takes to generate a new token.
       * Must be strictly positive.
       * @param capacity the maximum token capacity. Must be strictly positive.
       */
      public TokenBucket(int deltaMs, int capacity) {
          this(deltaMs, capacity, capacity);
      }
  
      /** Reset this TokenBucket and set its number of available tokens. */
      public void reset(int tokens) {
          checkArgumentNonnegative(tokens);
          mAvailable = Math.min(tokens, mCapacity);
          mLastFill = scaledTime();
      }
  
      /** Returns this TokenBucket maximum token capacity. */
      public int capacity() {
          return mCapacity;
      }
  
      /** Returns this TokenBucket currently number of available tokens. */
      public int available() {
          fill();
          return mAvailable;
      }
  
      /** Returns true if this TokenBucket as one or more tokens available. */
      public boolean has() {
          fill();
          return mAvailable > 0;
      }
  
      /** Consumes a token from this TokenBucket and returns true if a token is available. */
      public boolean get() {
          return (get(1) == 1);
      }
  
      /**
       * Try to consume many tokens from this TokenBucket.
       * @param n the number of tokens to consume.
       * @return the number of tokens that were actually consumed.
       */
      public int get(int n) {
          fill();
          if (n <= 0) {
              return 0;
          }
          if (n > mAvailable) {
              int got = mAvailable;
              mAvailable = 0;
              return got;
          }
          mAvailable -= n;
          return n;
      }
  
      private void fill() {
          final long now = scaledTime();
          final int diff = (int) (now - mLastFill);
          mAvailable = Math.min(mCapacity, mAvailable + diff);
          mLastFill = now;
      }
  
      private long scaledTime() {
          return SystemClock.elapsedRealtime() / mFillDelta;
      }
  }
  