content: /*
   * Copyright (C) 2017 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 static com.android.internal.util.Preconditions.checkArgumentPositive;
  
  import java.lang.reflect.Array;
  import java.util.Arrays;
  
  /**
   * A simple ring buffer structure with bounded capacity backed by an array.
   * Events can always be added at the logical end of the buffer. If the buffer is
   * full, oldest events are dropped when new events are added.
   * {@hide}
   */
  public class RingBuffer<T> {
  
      // Array for storing events.
      private final T[] mBuffer;
      // Cursor keeping track of the logical end of the array. This cursor never
      // wraps and instead keeps track of the total number of append() operations.
      private long mCursor = 0;
  
      public RingBuffer(Class<T> c, int capacity) {
          checkArgumentPositive(capacity, "A RingBuffer cannot have 0 capacity");
          // Java cannot create generic arrays without a runtime hint.
          mBuffer = (T[]) Array.newInstance(c, capacity);
      }
  
      public int size() {
          return (int) Math.min(mBuffer.length, (long) mCursor);
      }
  
      public boolean isEmpty() {
          return size() == 0;
      }
  
      public void clear() {
          for (int i = 0; i < size(); ++i) {
              mBuffer[i] = null;
          }
          mCursor = 0;
      }
  
      public void append(T t) {
          mBuffer[indexOf(mCursor++)] = t;
      }
  
      /**
       * Returns object of type <T> at the next writable slot, creating one if it is not already
       * available. In case of any errors while creating the object, <code>null</code> will
       * be returned.
       */
      public T getNextSlot() {
          final int nextSlotIdx = indexOf(mCursor++);
          if (mBuffer[nextSlotIdx] == null) {
              mBuffer[nextSlotIdx] = createNewItem();
          }
          return mBuffer[nextSlotIdx];
      }
  
      /**
       * @return a new object of type <T> or null if a new object could not be created.
       */
      protected T createNewItem() {
          try {
              return (T) mBuffer.getClass().getComponentType().newInstance();
          } catch (IllegalAccessException | InstantiationException e) {
              return null;
          }
      }
  
      public T[] toArray() {
          // Only generic way to create a T[] from another T[]
          T[] out = Arrays.copyOf(mBuffer, size(), (Class<T[]>) mBuffer.getClass());
          // Reverse iteration from youngest event to oldest event.
          long inCursor = mCursor - 1;
          int outIdx = out.length - 1;
          while (outIdx >= 0) {
              out[outIdx--] = (T) mBuffer[indexOf(inCursor--)];
          }
          return out;
      }
  
      private int indexOf(long cursor) {
          return (int) Math.abs(cursor % mBuffer.length);
      }
  }
  