/*
 * 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 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

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.
   *
   * <p>This implementation simply pads with zeros and delegates to {@link #process(ByteBuffer)}.
   */
  protected void processRemaining(ByteBuffer bb) {
    bb.position(bb.limit()); // move at the end
    bb.limit(chunkSize + 7); // get ready to pad with longs
    while (bb.position() < chunkSize) {
      bb.putLong(0);
    }
    bb.limit(chunkSize);
    bb.flip();
    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();
    buffer.flip();
    if (buffer.remaining() > 0) {
      processRemaining(buffer);
      buffer.position(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() {
    buffer.flip();
    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
  }
}
