/*
 * 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.common.base.Preconditions;

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.charset.Charset;

/**
 * Skeleton implementation of {@link HashFunction}. Provides default implementations which
 * invokes the appropriate method on {@link #newHasher()}, then return the result of
 * {@link Hasher#hash}.
 *
 * <p>Invocations of {@link #newHasher(int)} also delegate to {@linkplain #newHasher()}, ignoring
 * the expected input size parameter.
 *
 * @author Kevin Bourrillion
 */
abstract class AbstractStreamingHashFunction implements HashFunction {
  @Override public <T> HashCode hashObject(T instance, Funnel<? super T> funnel) {
    return newHasher().putObject(instance, funnel).hash();
  }

  @Override public HashCode hashUnencodedChars(CharSequence input) {
    return newHasher().putUnencodedChars(input).hash();
  }

  @Override public HashCode hashString(CharSequence input, Charset charset) {
    return newHasher().putString(input, charset).hash();
  }

  @Override public HashCode hashInt(int input) {
    return newHasher().putInt(input).hash();
  }

  @Override public HashCode hashLong(long input) {
    return newHasher().putLong(input).hash();
  }

  @Override public HashCode hashBytes(byte[] input) {
    return newHasher().putBytes(input).hash();
  }

  @Override public HashCode hashBytes(byte[] input, int off, int len) {
    return newHasher().putBytes(input, off, len).hash();
  }

  @Override public Hasher newHasher(int expectedInputSize) {
    Preconditions.checkArgument(expectedInputSize >= 0);
    return newHasher();
  }

  /**
   * 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
  protected static 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
      this.buffer = ByteBuffer
          .allocate(bufferSize + 7) // always space for a single primitive
          .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) {
      return putBytes(bytes, 0, bytes.length);
    }

    @Override
    public final Hasher putBytes(byte[] bytes, int off, int len) {
      return putBytes(ByteBuffer.wrap(bytes, off, len).order(ByteOrder.LITTLE_ENDIAN));
    }

    private Hasher putBytes(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;
    }

    @Override
    public final Hasher putUnencodedChars(CharSequence charSequence) {
      for (int i = 0; i < charSequence.length(); i++) {
        putChar(charSequence.charAt(i));
      }
      return this;
    }

    @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 <T> Hasher putObject(T instance, Funnel<? super T> funnel) {
      funnel.funnel(instance, this);
      return this;
    }

    @Override
    public final HashCode hash() {
      munch();
      buffer.flip();
      if (buffer.remaining() > 0) {
        processRemaining(buffer);
      }
      return makeHash();
    }

    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
    }
  }
}
