JavaScript and CSS Caching using CRC32

// Tamir Khason http://khason.net/
//
// Released under MS-PL : 6-Apr-09

using System;
using System.Collections;
using System.IO;
using System.Security.Cryptography;
using System.Text;

   /// <summary>Implements a 32-bits cyclic redundancy check (CRC) hash algorithm.</summary>
   /// <remarks>This class is not intended to be used for security purposes. For security applications use MD5, SHA1, SHA256, SHA384, 
   /// or SHA512 in the System.Security.Cryptography namespace.</remarks>
   public class CRC32 : HashAlgorithm {

      #region CONSTRUCTORS
      /// <summary>Creates a CRC32 object using the <see cref="DefaultPolynomial"/>.</summary>
      public CRC32() : this(DefaultPolynomial) {
      }

      /// <summary>Creates a CRC32 object using the specified polynomial.</summary>
      /// <remarks>The polynomical should be supplied in its bit-reflected form. <see cref="DefaultPolynomial"/>.</remarks>
      [CLSCompliant(false)]
      public CRC32(uint polynomial) {
         HashSizeValue = 32;
         _crc32Table = (uint[])_crc32TablesCache[polynomial];
         if (_crc32Table == null) {
            _crc32Table = CRC32._buildCRC32Table(polynomial);
            _crc32TablesCache.Add(polynomial, _crc32Table);
         }
         Initialize();
      }

      // static constructor
      static CRC32() {
         _crc32TablesCache = Hashtable.Synchronized(new Hashtable());
         _defaultCRC = new CRC32();
      }
      #endregion

      #region PROPERTIES
      /// <summary>Gets the default polynomial (used in WinZip, Ethernet, etc.)</summary>
      /// <remarks>The default polynomial is a bit-reflected version of the standard polynomial 0x04C11DB7 used by WinZip, Ethernet, etc.</remarks>
      [CLSCompliant(false)]
      public static readonly uint DefaultPolynomial = 0xEDB88320; // Bitwise reflection of 0x04C11DB7;
      #endregion

      #region METHODS
      /// <summary>Initializes an implementation of HashAlgorithm.</summary>
      public override void Initialize() {
         _crc = _allOnes;
      }

      /// <summary>Routes data written to the object into the hash algorithm for computing the hash.</summary>
      protected override void HashCore(byte[] buffer, int offset, int count) {
         for (int i = offset; i < count; i++) {
            ulong ptr = (_crc & 0xFF) ^ buffer[i];
            _crc >>= 8;
            _crc ^= _crc32Table[ptr];
         }
      }

      /// <summary>Finalizes the hash computation after the last data is processed by the cryptographic stream object.</summary>
      protected override byte[] HashFinal() {
         byte[] finalHash = new byte[4];
         ulong finalCRC = _crc ^ _allOnes;

         finalHash[0] = (byte)((finalCRC >> 0) & 0xFF);
         finalHash[1] = (byte)((finalCRC >> 8) & 0xFF);
         finalHash[2] = (byte)((finalCRC >> 16) & 0xFF);
         finalHash[3] = (byte)((finalCRC >> 24) & 0xFF);

         return finalHash;
      }

      /// <summary>Computes the CRC32 value for the given ASCII string using the <see cref="DefaultPolynomial"/>.</summary>
      public static int Compute(string asciiString) {
         _defaultCRC.Initialize();
         return ToInt32(_defaultCRC.ComputeHash(asciiString));
      }

      /// <summary>Computes the CRC32 value for the given input stream using the <see cref="DefaultPolynomial"/>.</summary>
      public static int Compute(Stream inputStream) {
         _defaultCRC.Initialize();
         return ToInt32(_defaultCRC.ComputeHash(inputStream));
      }

      /// <summary>Computes the CRC32 value for the input data using the <see cref="DefaultPolynomial"/>.</summary>
      public static int Compute(byte[] buffer) {
         _defaultCRC.Initialize();
         return ToInt32(_defaultCRC.ComputeHash(buffer));
      }

      /// <summary>Computes the hash value for the input data using the <see cref="DefaultPolynomial"/>.</summary>
      public static int Compute(byte[] buffer, int offset, int count) {
         _defaultCRC.Initialize();
         return ToInt32(_defaultCRC.ComputeHash(buffer, offset, count));
      }

      /// <summary>Computes the hash value for the given ASCII string.</summary>
      /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
      public byte[] ComputeHash(string asciiString) {
         byte[] rawBytes = ASCIIEncoding.ASCII.GetBytes(asciiString);
         return ComputeHash(rawBytes);
      }

      /// <summary>Computes the hash value for the given input stream.</summary>
      /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
      new public byte[] ComputeHash(Stream inputStream) {
         byte[] buffer = new byte[4096];
         int bytesRead;
         while ((bytesRead = inputStream.Read(buffer, 0, 4096)) > 0) {
            HashCore(buffer, 0, bytesRead);
         }
         return HashFinal();
      }

      /// <summary>Computes the hash value for the input data.</summary>
      /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
      new public byte[] ComputeHash(byte[] buffer) {
         return ComputeHash(buffer, 0, buffer.Length);
      }

      /// <summary>Computes the hash value for the input data.</summary>
      /// <remarks>The computation preserves the internal state between the calls, so it can be used for computation of a stream data.</remarks>
      new public byte[] ComputeHash(byte[] buffer, int offset, int count) {
         HashCore(buffer, offset, count);
         return HashFinal();
      }
      #endregion

      #region PRIVATE SECTION
      private static uint _allOnes = 0xffffffff;
      private static CRC32 _defaultCRC;
      private static Hashtable _crc32TablesCache;
      private uint[] _crc32Table;
      private uint _crc;

      // Builds a crc32 table given a polynomial
      private static uint[] _buildCRC32Table(uint polynomial) {
         uint crc;
         uint[] table = new uint[256];

         // 256 values representing ASCII character codes. 
         for (int i = 0; i < 256; i++) {
            crc = (uint)i;
            for (int j = 8; j > 0; j--) {
               if ((crc & 1) == 1)
                  crc = (crc >> 1) ^ polynomial;
               else
                  crc >>= 1;
            }
            table[i] = crc;
         }

         return table;
      }

      private static int ToInt32(byte[] buffer) {
         return BitConverter.ToInt32(buffer, 0);
      }
      #endregion

   }