/*******************************************************************************
* 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 int-width CRC function using a lookup table.
* Does not support bit-widths less than 8.
*/
final class NormalIntCrc extends AbstractIntCrc {
private final int widthMask;
private final int[] table = new int[256];
NormalIntCrc(String algorithm, int bitWidth, int poly, int init, int xorOut) {
super(algorithm, bitWidth, init, xorOut);
if (bitWidth < 8)
throw new IllegalArgumentException("invalid CRC width");
widthMask = bitWidth < 32 ? ((1 << bitWidth) - 1) : ~0;
final int top = 1 << (bitWidth - 1);
for (int i = 0; i < 256; ++i) {
int crc = 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 int resumeRaw(int crc, byte[] input, int index, int length) {
for (int i = 0; i < length; ++i)
crc = table[((crc >>> (bitWidth - 8)) ^ input[index + i]) & 0xff] ^ (crc << 8);
return crc & widthMask;
}
}