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using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
namespace com.clusterrr.TuyaNet
{
/// <summary>
/// Performs 32-bit reversed cyclic redundancy checks.
/// </summary>
internal 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 InvalidDataException("Could not read the stream out as bytes.", e);
}
catch (InvalidCastException e)
{
throw new InvalidDataException("Could not read the stream out as bytes.", e);
}
catch (OverflowException e)
{
throw new InvalidDataException("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
}
}
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