/*******************************************************************************
* Copyright 2014 Trevor Robinson
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
******************************************************************************/
package com.scurrilous.circe.crc;
/**
* Implements a "normal" MSB-first long-width CRC function using a lookup table.
* Does not support bit-widths less than 8.
*/
final class NormalLongCrc extends AbstractLongCrc {
private final long widthMask;
private final long[] table = new long[256];
NormalLongCrc(String algorithm, int bitWidth, long poly, long init, long xorOut) {
super(algorithm, bitWidth, init, xorOut);
if (bitWidth < 8)
throw new IllegalArgumentException("invalid CRC width");
widthMask = bitWidth < 64 ? ((1L << bitWidth) - 1) : ~0L;
final long top = 1L << (bitWidth - 1);
for (int i = 0; i < 256; ++i) {
long crc = (long) i << (bitWidth - 8);
for (int j = 0; j < 8; ++j)
crc = (crc & top) != 0 ? (crc << 1) ^ poly : crc << 1;
table[i] = crc & widthMask;
}
}
@Override
protected long resumeRaw(long crc, byte[] input, int index, int length) {
for (int i = 0; i < length; ++i)
crc = table[(int) ((crc >>> (bitWidth - 8)) ^ input[index + i]) & 0xff] ^ (crc << 8);
return crc & widthMask;
}
}