/// <summary>Retrieves a standard set of CRC parameters.</summary>
/// <param name="standard">The name of the standard parameter set to retrieve.</param>
/// <returns>The CRC Parameters for the given standard.</returns>
public static CRCParameters GetParameters(CRCStandard standard)
{
CRCParameters param = null;
switch (standard)
{
case CRCStandard.CRC8:
param = new CRCParameters(8, 0x07, 0, 0, false);
break;
case CRCStandard.CRC16_IBM:
param = new CRCParameters(16, 0x8005, 0, 0, false);
break;
case CRCStandard.CRC16_CCITT:
param = new CRCParameters(16, 0x1021, 0xFFFF, 0, false);
break;
case CRCStandard.CRC16_ARC:
param = new CRCParameters(16, 0x8005, 0, 0, true);
break;
case CRCStandard.CRC16_XMODEM:
param = new CRCParameters(16, 0x8408, 0, 0, true);
break;
case CRCStandard.CRC16_ZMODEM:
param = new CRCParameters(16, 0x1021, 0, 0, false);
break;
case CRCStandard.CRC24:
param = new CRCParameters(24, 0x1864CFB, 0xB704CE, 0, false);
break;
case CRCStandard.CRC32:
param = new CRCParameters(32, 0x04C11DB7, 0xFFFFFFFF, 0xFFFFFFFF, true);
break;
case CRCStandard.CRC32_JAMCRC:
param = new CRCParameters(32, 0x04C11DB7, 0xFFFFFFFF, 0, true);
break;
case CRCStandard.CRC32_BZIP2:
param = new CRCParameters(32, 0x04C11DB7, 0xFFFFFFFF, 0xFFFFFFFF, false);
break;
case CRCStandard.CRC64_ISO:
param = new CRCParameters(64, 0x000000000000001B, 0, 0, true);
break;
case CRCStandard.CRC64_ECMA:
param = new CRCParameters(64, 0x42F0E1EBA9EA3693, -1, -1, false);
break;
}
return(param);
}
/// <summary>Build the CRC lookup table for a given polynomial.</summary>
static private void BuildLookup(CRCParameters param)
{
if (lookupTables.Contains(param))
{
// No sense in creating the table twice.
return;
}
var table = new long[256];
var topBit = (long)1 << (param.Order - 1);
long widthMask = (((1 << (param.Order - 1)) - 1) << 1) | 1;
// Build the table.
for (var i = 0; i < table.Length; i++)
{
table[i] = i;
if (param.ReflectInput)
{
table[i] = Reflect(i, 8);
}
table[i] = table[i] << (param.Order - 8);
for (var j = 0; j < 8; j++)
{
if ((table[i] & topBit) != 0)
{
table[i] = (table[i] << 1) ^ param.Polynomial;
}
else
{
table[i] <<= 1;
}
}
if (param.ReflectInput)
{
table[i] = Reflect(table[i], param.Order);
}
table[i] &= widthMask;
}
// Add the new lookup table.
lookupTables.Add(param, table);
}
/// <summary>Initializes a new instance of the CRC class.</summary>
/// <param name="param">The parameters to utilize in the CRC calculation.</param>
public CRC(CRCParameters param)
{
lock (this) {
if (param == null)
{
throw new ArgumentNullException("param", "The CRCParameters cannot be null.");
}
parameters = param;
HashSizeValue = param.Order;
BuildLookup(param);
lookup = (long[])lookupTables[param];
if (param.Order == 64)
{
registerMask = 0x00FFFFFFFFFFFFFF;
}
else
{
registerMask = (long)(Math.Pow(2, (param.Order - 8)) - 1);
}
Initialize();
}
}
// Pre-build the more popular lookup tables.
static CRC()
{
lookupTables = new Hashtable();
BuildLookup(CRCParameters.GetParameters(CRCStandard.CRC32));
}