/// <summary>
/// Performs 32-bit reversed cyclic redundancy checks.
/// https://rosettacode.org/wiki/CRC-32
/// GNU Free Documentation License 1.2 
/// </summary>

using System;
using System.Collections.Generic;
using System.Linq;

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 InvalidOperationException("Could not read the stream out as bytes.", e);
    }
    catch (InvalidCastException e)
    {
      throw new InvalidOperationException("Could not read the stream out as bytes.", e);
    }
    catch (OverflowException e)
    {
      throw new InvalidOperationException("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
}