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ZelWiki.Security/CRC32.cs

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