Method drain

guava/src/com/google/common/io/ReaderInputStream.java:255–259

Copy as much of the byte buffer into the output array as possible, returning the (positive) number of characters copied.

(byte[] b, int off, int len)

   * number of characters copied.
   */
  private int drain(byte[] b, int off, int len) {
    int remaining = min(len, byteBuffer.remaining());
    byteBuffer.get(b, off, remaining);
    return remaining;
  }
}