Searcharoo 2007 (Medium Trust and Office 2007 indexing)

// Crc32.cs
//
// Copyright (c) 2006, 2007 Microsoft Corporation.  All rights reserved.
//
//
// Implements the CRC algorithm, which is used in zip files.  The zip format calls for
// the zipfile to contain a CRC for the unencrypted byte stream of each file.
//
// It is based on example source code published at
//    http://www.vbaccelerator.com/home/net/code/libraries/CRC32/Crc32_zip_CRC32_CRC32_cs.asp
//
// This implementation adds a tweak of that code for use within zip creation.  While
// computing the CRC we also compress the byte stream, in the same read loop. This
// avoids the need to read through the uncompressed stream twice - once to computer CRC
// and another time to compress.
//
//
// Thu, 30 Mar 2006  13:58
// 

using System;

namespace ionic.utils.zip
{
   /// <summary>
   /// Calculates a 32bit Cyclic Redundancy Checksum (CRC) using the
   /// same polynomial used by Zip.
   /// </summary>
   public class CRC32
   {      
      private UInt32[] crc32Table;
      private const int BUFFER_SIZE = 8192;
     
     private Int32 _TotalBytesRead= 0; 
     public Int32 TotalBytesRead {
       get {
	 return _TotalBytesRead;
       }
     }

      /// <summary>
      /// Returns the CRC32 for the specified stream.
      /// </summary>
      /// <param name="input">The stream over which to calculate the CRC32</param>
      /// <returns>the CRC32 calculation</returns>
      [CLSCompliant(false)]
       public UInt32 GetCrc32(System.IO.Stream input)
      {
    	return GetCrc32AndCopy(input, null) ;
      }

      /// <summary>
      /// Returns the CRC32 for the specified stream, and writes the input into the output stream.
      /// </summary>
      /// <param name="input">The stream over which to calculate the CRC32</param>
      /// <param name="output">The stream into which to deflate the input</param>
      /// <returns>the CRC32 calculation</returns>
       [CLSCompliant(false)]
       public UInt32 GetCrc32AndCopy(System.IO.Stream input, System.IO.Stream output)
      {
         unchecked
         {
            UInt32 crc32Result;
            crc32Result = 0xFFFFFFFF;
            byte[] buffer = new byte[BUFFER_SIZE];
            int readSize = BUFFER_SIZE;

	    _TotalBytesRead= 0;
            int count = input.Read(buffer, 0, readSize);
	    if (output != null) output.Write(buffer,0,count); 
	    _TotalBytesRead += count;
            while (count > 0)
            {
               for (int i = 0; i < count; i++)
               {
                  crc32Result = ((crc32Result) >> 8) ^ crc32Table[(buffer[i]) ^ ((crc32Result) & 0x000000FF)];
               }
               count = input.Read(buffer, 0, readSize);
	       if (output != null) output.Write(buffer,0,count); 
	       _TotalBytesRead += count;

            }
            
            return ~crc32Result;
         }
      }


      /// <summary>
      /// Construct an instance of the CRC32 class, pre-initialising the table
      /// for speed of lookup.
      /// </summary>
      public CRC32()
      {
         unchecked
         {
            // This is the official polynomial used by CRC32 in PKZip.
            // Often the polynomial is shown reversed as 0x04C11DB7.
            UInt32 dwPolynomial = 0xEDB88320;
            UInt32 i, j;

            crc32Table = new UInt32[256];

            UInt32 dwCrc;
            for(i = 0; i < 256; i++)
            {
               dwCrc = i;
               for(j = 8; j > 0; j--)
               {
                  if ((dwCrc & 1)==1)
                  {
                     dwCrc = (dwCrc >> 1) ^ dwPolynomial;
                  }
                  else
                  {
                     dwCrc >>= 1;
                  }
               }
               crc32Table[i] = dwCrc;
            }
         }
      }
   }

}