/*
    * 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
     }
   }
 }