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Calculate a CRC32 like the one used in zip files

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  1. using System;
  2. using System.IO;
  3.  
  4. namespace Vbaccelerator.Components.Algorithms
  5. {
  6.    /// <summary>
  7.    /// Calculates a 32bit Cyclic Redundancy Checksum (CRC) using the
  8.    /// same polynomial used by Zip.
  9.    /// </summary>
  10.    public class CRC32
  11.    {      
  12.       private UInt32[] crc32Table;
  13.       private const int BUFFER_SIZE = 1024;
  14.  
  15.       /// <summary>
  16.       /// Returns the CRC32 for the specified stream.
  17.       /// </summary>
  18.       /// <param name="stream">The stream to calculate the CRC32 for</param>
  19.       /// <returns>An unsigned integer containing the CRC32
  20.        calculation</returns>
  21.       public UInt32 GetCrc32(System.IO.Stream stream)
  22.       {
  23.          unchecked
  24.          {
  25.             UInt32 crc32Result;
  26.             crc32Result = 0xFFFFFFFF;
  27.             byte[] buffer = new byte[BUFFER_SIZE];
  28.             int readSize = BUFFER_SIZE;
  29.  
  30.             int count = stream.Read(buffer, 0, readSize);
  31.             while (count > 0)
  32.             {
  33.                for (int i = 0; i < count; i++)
  34.                {
  35.                   crc32Result = ((crc32Result) >> 8) ^ crc32Table[(buffer[i]) ^
  36.                    ((crc32Result) & 0x000000FF)];
  37.                }
  38.                count = stream.Read(buffer, 0, readSize);
  39.             }
  40.  
  41.             return ~crc32Result;
  42.          }
  43.       }
  44.  
  45.       /// <summary>
  46.       /// Construct an instance of the CRC32 class, pre-initialising the table
  47.       /// for speed of lookup.
  48.       /// </summary>
  49.       public CRC32()
  50.       {
  51.          unchecked
  52.          {
  53.             // This is the official polynomial used by CRC32 in PKZip.
  54.             // Often the polynomial is shown reversed as 0x04C11DB7.
  55.             UInt32 dwPolynomial = 0xEDB88320;
  56.             UInt32 i, j;
  57.  
  58.             crc32Table = new UInt32[256];
  59.  
  60.             UInt32 dwCrc;
  61.             for(i = 0; i < 256; i++)
  62.             {
  63.                dwCrc = i;
  64.                for(j = 8; j > 0; j--)
  65.                {
  66.                   if ((dwCrc & 1)==1)
  67.                   {
  68.                      dwCrc = (dwCrc >> 1) ^ dwPolynomial;
  69.                   }
  70.                   else
  71.                   {
  72.                      dwCrc >>= 1;
  73.                   }
  74.                }
  75.                crc32Table[i] = dwCrc;
  76.             }
  77.          }
  78.       }
  79.    }
  80. }

URL: http://www.vbaccelerator.com/home/net/code/libraries/CRC32/Crc32_zip_CRC32_CRC32_cs.asp

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