// Copyright Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
namespace Amazon.TranscribeStreamingService
{
///
/// Performs 32-bit reversed cyclic redundancy checks.
///
public class Crc32
{
private static Crc32? _singleton;
public static Crc32 Singleton {
get {
if(_singleton == null) _singleton = new Crc32();
return _singleton;
}
}
#region Constants
///
/// Generator polynomial (modulo 2) for the reversed CRC32 algorithm.
///
private const UInt32 s_generator = 0xEDB88320;
#endregion
#region Constructors
///
/// Creates a new instance of the Crc32 class.
///
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
///
/// Calculates the checksum of the byte stream.
///
/// The byte stream to calculate the checksum for.
/// A 32-bit reversed checksum.
public UInt32 Get(IEnumerable 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
///
/// Contains a cache of calculated checksum chunks.
///
private readonly UInt32[] m_checksumTable;
#endregion
}
}