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/*
 * Copyright (C) 2011 The Guava Authors
 *
 * 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.google.common.hash;

import static com.google.common.base.Preconditions.checkArgument;

import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.nio.ByteBuffer;
import java.nio.ByteOrder;

/**
 * A convenience base class for implementors of {@code Hasher}; handles accumulating data until an
 * entire "chunk" (of implementation-dependent length) is ready to be hashed.
 *
 * @author Kevin Bourrillion
 * @author Dimitris Andreou
 */
// TODO(kevinb): this class still needs some design-and-document-for-inheritance love
@CanIgnoreReturnValue
abstract class AbstractStreamingHasher extends AbstractHasher {
  /** Buffer via which we pass data to the hash algorithm (the implementor) */
  private final ByteBuffer buffer;

  /** Number of bytes to be filled before process() invocation(s). */
  private final int bufferSize;

  /** Number of bytes processed per process() invocation. */
  private final int chunkSize;

  /**
   * Constructor for use by subclasses. This hasher instance will process chunks of the specified
   * size.
   *
   * @param chunkSize the number of bytes available per {@link #process(ByteBuffer)} invocation;
   *     must be at least 4
   */
  protected AbstractStreamingHasher(int chunkSize) {
    this(chunkSize, chunkSize);
  }

  /**
   * Constructor for use by subclasses. This hasher instance will process chunks of the specified
   * size, using an internal buffer of {@code bufferSize} size, which must be a multiple of {@code
   * chunkSize}.
   *
   * @param chunkSize the number of bytes available per {@link #process(ByteBuffer)} invocation;
   *     must be at least 4
   * @param bufferSize the size of the internal buffer. Must be a multiple of chunkSize
   */
  protected AbstractStreamingHasher(int chunkSize, int bufferSize) {
    // TODO(kevinb): check more preconditions (as bufferSize >= chunkSize) if this is ever public
    checkArgument(bufferSize % chunkSize == 0);

    // TODO(user): benchmark performance difference with longer buffer
    // always space for a single primitive
    this.buffer = ByteBuffer.allocate(bufferSize + 7).order(ByteOrder.LITTLE_ENDIAN);
    this.bufferSize = bufferSize;
    this.chunkSize = chunkSize;
  }

  /** Processes the available bytes of the buffer (at most {@code chunk} bytes). */
  protected abstract void process(ByteBuffer bb);

  /**
   * This is invoked for the last bytes of the input, which are not enough to fill a whole chunk.
   * The passed {@code ByteBuffer} is guaranteed to be non-empty.
   *
   * 

This implementation simply pads with zeros and delegates to {@link #process(ByteBuffer)}. */ protected void processRemaining(ByteBuffer bb) { Java8Compatibility.position(bb, bb.limit()); // move at the end Java8Compatibility.limit(bb, chunkSize + 7); // get ready to pad with longs while (bb.position() < chunkSize) { bb.putLong(0); } Java8Compatibility.limit(bb, chunkSize); Java8Compatibility.flip(bb); process(bb); } @Override public final Hasher putBytes(byte[] bytes, int off, int len) { return putBytesInternal(ByteBuffer.wrap(bytes, off, len).order(ByteOrder.LITTLE_ENDIAN)); } @Override public final Hasher putBytes(ByteBuffer readBuffer) { ByteOrder order = readBuffer.order(); try { readBuffer.order(ByteOrder.LITTLE_ENDIAN); return putBytesInternal(readBuffer); } finally { readBuffer.order(order); } } private Hasher putBytesInternal(ByteBuffer readBuffer) { // If we have room for all of it, this is easy if (readBuffer.remaining() <= buffer.remaining()) { buffer.put(readBuffer); munchIfFull(); return this; } // First add just enough to fill buffer size, and munch that int bytesToCopy = bufferSize - buffer.position(); for (int i = 0; i < bytesToCopy; i++) { buffer.put(readBuffer.get()); } munch(); // buffer becomes empty here, since chunkSize divides bufferSize // Now process directly from the rest of the input buffer while (readBuffer.remaining() >= chunkSize) { process(readBuffer); } // Finally stick the remainder back in our usual buffer buffer.put(readBuffer); return this; } /* * Note: hashString(CharSequence, Charset) is intentionally not overridden. * * While intuitively, using CharsetEncoder to encode the CharSequence directly to the buffer (or * even to an intermediate buffer) should be considerably more efficient than potentially * copying the CharSequence to a String and then calling getBytes(Charset) on that String, in * reality there are optimizations that make the getBytes(Charset) approach considerably faster, * at least for commonly used charsets like UTF-8. */ @Override public final Hasher putByte(byte b) { buffer.put(b); munchIfFull(); return this; } @Override public final Hasher putShort(short s) { buffer.putShort(s); munchIfFull(); return this; } @Override public final Hasher putChar(char c) { buffer.putChar(c); munchIfFull(); return this; } @Override public final Hasher putInt(int i) { buffer.putInt(i); munchIfFull(); return this; } @Override public final Hasher putLong(long l) { buffer.putLong(l); munchIfFull(); return this; } @Override public final HashCode hash() { munch(); Java8Compatibility.flip(buffer); if (buffer.remaining() > 0) { processRemaining(buffer); Java8Compatibility.position(buffer, buffer.limit()); } return makeHash(); } /** * Computes a hash code based on the data that have been provided to this hasher. This is called * after all chunks are handled with {@link #process} and any leftover bytes that did not make a * complete chunk are handled with {@link #processRemaining}. */ protected abstract HashCode makeHash(); // Process pent-up data in chunks private void munchIfFull() { if (buffer.remaining() < 8) { // buffer is full; not enough room for a primitive. We have at least one full chunk. munch(); } } private void munch() { Java8Compatibility.flip(buffer); while (buffer.remaining() >= chunkSize) { // we could limit the buffer to ensure process() does not read more than // chunkSize number of bytes, but we trust the implementations process(buffer); } buffer.compact(); // preserve any remaining data that do not make a full chunk } }





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