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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code is distributed in the hope that it will be useful, but WITHOUT
   * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
   * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  
  package java.security;
  
  import java.nio.ByteBuffer;
  
  import sun.security.jca.JCAUtil;
  
  /**
   * This class defines the <i>Service Provider Interface</i> (<b>SPI</b>)
   * for the {@code MessageDigest} class, which provides the functionality
   * of a message digest algorithm, such as MD5 or SHA. Message digests are
   * secure one-way hash functions that take arbitrary-sized data and output a
   * fixed-length hash value.
   *
   * <p> All the abstract methods in this class must be implemented by a
   * cryptographic service provider who wishes to supply the implementation
   * of a particular message digest algorithm.
   *
   * <p> Implementations are free to implement the Cloneable interface.
   *
   * @author Benjamin Renaud
   *
   *
   * @see MessageDigest
   */
  
  public abstract class MessageDigestSpi {
  
      // for re-use in engineUpdate(ByteBuffer input)
      private byte[] tempArray;
  
      /**
       * Returns the digest length in bytes.
       *
       * <p>This concrete method has been added to this previously-defined
       * abstract class. (For backwards compatibility, it cannot be abstract.)
       *
       * <p>The default behavior is to return 0.
       *
       * <p>This method may be overridden by a provider to return the digest
       * length.
       *
       * @return the digest length in bytes.
       *
       * @since 1.2
       */
      protected int engineGetDigestLength() {
          return 0;
      }
  
      /**
       * Updates the digest using the specified byte.
       *
       * @param input the byte to use for the update.
       */
      protected abstract void engineUpdate(byte input);
  
      /**
       * Updates the digest using the specified array of bytes,
       * starting at the specified offset.
       *
       * @param input the array of bytes to use for the update.
       *
       * @param offset the offset to start from in the array of bytes.
       *
       * @param len the number of bytes to use, starting at
       * {@code offset}.
       */
      protected abstract void engineUpdate(byte[] input, int offset, int len);
  
      /**
       * Update the digest using the specified ByteBuffer. The digest is
       * updated using the {@code input.remaining()} bytes starting
       * at {@code input.position()}.
       * Upon return, the buffer's position will be equal to its limit;
       * its limit will not have changed.
       *
       * @param input the ByteBuffer
       * @since 1.5
       */
      protected void engineUpdate(ByteBuffer input) {
          if (input.hasRemaining() == false) {
              return;
          }
          if (input.hasArray()) {
              byte[] b = input.array();
              int ofs = input.arrayOffset();
              int pos = input.position();
              int lim = input.limit();
              engineUpdate(b, ofs + pos, lim - pos);
              input.position(lim);
          } else {
              int len = input.remaining();
              int n = JCAUtil.getTempArraySize(len);
              if ((tempArray == null) || (n > tempArray.length)) {
                  tempArray = new byte[n];
              }
              while (len > 0) {
                  int chunk = Math.min(len, tempArray.length);
                  input.get(tempArray, 0, chunk);
                  engineUpdate(tempArray, 0, chunk);
                  len -= chunk;
              }
          }
      }
  
      /**
       * Completes the hash computation by performing final
       * operations such as padding. Once {@code engineDigest} has
       * been called, the engine should be reset (see
       * {@link #engineReset() engineReset}).
       * Resetting is the responsibility of the
       * engine implementor.
       *
       * @return the array of bytes for the resulting hash value.
       */
      protected abstract byte[] engineDigest();
  
      /**
       * Completes the hash computation by performing final
       * operations such as padding. Once {@code engineDigest} has
       * been called, the engine should be reset (see
       * {@link #engineReset() engineReset}).
       * Resetting is the responsibility of the
       * engine implementor.
       *
       * This method should be abstract, but we leave it concrete for
       * binary compatibility.  Knowledgeable providers should override this
       * method.
       *
       * @param buf the output buffer in which to store the digest
       *
       * @param offset offset to start from in the output buffer
       *
       * @param len number of bytes within buf allotted for the digest.
       * Both this default implementation and the SUN provider do not
       * return partial digests.  The presence of this parameter is solely
       * for consistency in our API's.  If the value of this parameter is less
       * than the actual digest length, the method will throw a DigestException.
       * This parameter is ignored if its value is greater than or equal to
       * the actual digest length.
       *
       * @return the length of the digest stored in the output buffer.
       *
       * @exception DigestException if an error occurs.
       *
       * @since 1.2
       */
      protected int engineDigest(byte[] buf, int offset, int len)
                                                  throws DigestException {
  
          byte[] digest = engineDigest();
          if (len < digest.length)
                  throw new DigestException("partial digests not returned");
          if (buf.length - offset < digest.length)
                  throw new DigestException("insufficient space in the output "
                                            + "buffer to store the digest");
          System.arraycopy(digest, 0, buf, offset, digest.length);
          return digest.length;
      }
  
      /**
       * Resets the digest for further use.
       */
      protected abstract void engineReset();
  
      /**
       * Returns a clone if the implementation is cloneable.
       *
       * @return a clone if the implementation is cloneable.
       *
       * @exception CloneNotSupportedException if this is called on an
       * implementation that does not support {@code Cloneable}.
       */
      public Object clone() throws CloneNotSupportedException {
          if (this instanceof Cloneable) {
              return super.clone();
          } else {
              throw new CloneNotSupportedException();
          }
      }
  }
  