/// <summary>
/// Initializes a new instance of the <see cref="Crc"/> class.
/// </summary>
/// <param name="parameters"></param>
private Crc([NotNull] CrcParameters parameters)
{
Check.NotNull(parameters, nameof(parameters));
_parameters = parameters;
LookupTable = GetOrCreateLookupTable(parameters);
}
private static ulong[] GetOrCreateLookupTable(CrcParameters p)
{
int refIn = p.ReflectIn ? 1 : 0;
int refOut = p.ReflectOut ? 1 : 0;
var cacheKey = $"{p.Width}:{p.Polynomial}{p.InitialValue}{p.XorOutValue}{refIn}{refOut}";
return(LookupTableCache.GetOrAdd(
cacheKey,
t =>
{
var lookupTable = new ulong[256];
var topBit = (ulong)1 << (p.Width - 1);
// Calculate lookup table
for (var i = 0; i < lookupTable.Length; i++)
{
var r = (ulong)i;
if (p.ReflectIn)
{
r = ReflectBits(r, p.Width);
}
else if (p.Width > 8)
{
r <<= p.Width - 8;
}
//var r = (ulong)inByte << (width - 8);
for (var j = 0; j < 8; j++)
{
r = (r & topBit) != 0 ? (r << 1) ^ p.Polynomial : r << 1;
}
if (p.ReflectIn)
{
r = ReflectBits(r, p.Width);
}
lookupTable[i] = r & (ulong.MaxValue >> (64 - p.Width));
}
// Done
return lookupTable;
}
));
}
