Mega Code Archive
Calculates a 32 bit Cyclic Redundancy Checksum (CRC) using the same polynomial used by Zip
<![CDATA[
// 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) & 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;
}
}
}
}
}
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