Calculates a 32 bit Cyclic Redundancy Checksum (CRC) using the same polynomial used by Zip.

   
 
  
// Crc32.cs
//
// 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>
    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>
    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) &amp; 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 &amp; 1) == 1)
            {
              dwCrc = (dwCrc >> 1) ^ dwPolynomial;
            }
            else
            {
              dwCrc >>= 1;
            }
          }
          crc32Table[i] = dwCrc;
        }
      }
    }
  }
}

   
     


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