74 lines
2.4 KiB
C#
74 lines
2.4 KiB
C#
namespace TightWiki.Security
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{
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/// <summary>
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/// Performs 32-bit reversed cyclic redundancy checks.
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/// </summary>
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public class Crc32
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{
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#region Constants
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/// <summary>
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/// Generator polynomial (modulo 2) for the reversed CRC32 algorithm.
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/// </summary>
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private const UInt32 s_generator = 0xEDB88320;
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#endregion
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#region Constructors
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/// <summary>
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/// Creates a new instance of the Crc32 class.
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/// </summary>
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public Crc32()
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{
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// Constructs the checksum lookup table. Used to optimize the checksum.
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m_checksumTable = Enumerable.Range(0, 256).Select(i =>
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{
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var tableEntry = (uint)i;
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for (var j = 0; j < 8; ++j)
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{
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tableEntry = ((tableEntry & 1) != 0)
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? (s_generator ^ (tableEntry >> 1))
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: (tableEntry >> 1);
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}
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return tableEntry;
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}).ToArray();
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}
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#endregion
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#region Methods
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/// <summary>
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/// Calculates the checksum of the byte stream.
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/// </summary>
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/// <param name="byteStream">The byte stream to calculate the checksum for.</param>
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/// <returns>A 32-bit reversed checksum.</returns>
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public UInt32 Get<T>(IEnumerable<T> byteStream)
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{
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try
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{
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// Initialize checksumRegister to 0xFFFFFFFF and calculate the checksum.
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return ~byteStream.Aggregate(0xFFFFFFFF, (checksumRegister, currentByte) =>
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(m_checksumTable[(checksumRegister & 0xFF) ^ Convert.ToByte(currentByte)] ^ (checksumRegister >> 8)));
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}
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catch (FormatException e)
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{
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throw new Exception("Could not read the stream out as bytes.", e);
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}
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catch (InvalidCastException e)
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{
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throw new Exception("Could not read the stream out as bytes.", e);
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}
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catch (OverflowException e)
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{
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throw new Exception("Could not read the stream out as bytes.", e);
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}
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}
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#endregion
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#region Fields
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/// <summary>
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/// Contains a cache of calculated checksum chunks.
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/// </summary>
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private readonly UInt32[] m_checksumTable;
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#endregion
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}
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}
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