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 android.util.apk;
  
  import java.io.IOException;
  import java.nio.ByteBuffer;
  import java.security.DigestException;
  
  /**
   * {@link DataSource} which provides data from a {@link ByteBuffer}.
   */
  class ByteBufferDataSource implements DataSource {
      /**
       * Underlying buffer. The data is stored between position 0 and the buffer's capacity.
       * The buffer's position is 0 and limit is equal to capacity.
       */
      private final ByteBuffer mBuf;
  
      ByteBufferDataSource(ByteBuffer buf) {
          // Defensive copy, to avoid changes to mBuf being visible in buf, and to ensure position is
          // 0 and limit == capacity.
          mBuf = buf.slice();
      }
  
      @Override
      public long size() {
          return mBuf.capacity();
      }
  
      @Override
      public void feedIntoDataDigester(DataDigester md, long offset, int size)
              throws IOException, DigestException {
          // There's no way to tell MessageDigest to read data from ByteBuffer from a position
          // other than the buffer's current position. We thus need to change the buffer's
          // position to match the requested offset.
          //
          // In the future, it may be necessary to compute digests of multiple regions in
          // parallel. Given that digest computation is a slow operation, we enable multiple
          // such requests to be fulfilled by this instance. This is achieved by serially
          // creating a new ByteBuffer corresponding to the requested data range and then,
          // potentially concurrently, feeding these buffers into MessageDigest instances.
          ByteBuffer region;
          synchronized (mBuf) {
              mBuf.position(0);
              mBuf.limit((int) offset + size);
              mBuf.position((int) offset);
              region = mBuf.slice();
          }
  
          md.consume(region);
      }
  }
  