com.scurrilous.circe.crc.NormalByteCrc Maven / Gradle / Ivy
/*******************************************************************************
* 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 byte-width CRC function using a lookup table.
*/
final class NormalByteCrc extends AbstractIntCrc {
private final byte[] table = new byte[256];
NormalByteCrc(String algorithm, int bitWidth, int poly, int init, int xorOut) {
super(algorithm, bitWidth, init, xorOut);
if (bitWidth > 8)
throw new IllegalArgumentException("invalid CRC width");
final int widthMask = (1 << bitWidth) - 1;
final int shpoly = poly << (8 - bitWidth);
for (int i = 0; i < 256; ++i) {
int crc = i;
for (int j = 0; j < 8; ++j)
crc = (crc & 0x80) != 0 ? (crc << 1) ^ shpoly : crc << 1;
table[i] = (byte) ((crc >> (8 - bitWidth)) & widthMask);
}
}
@Override
protected int resumeRaw(int crc, byte[] input, int index, int length) {
for (int i = 0; i < length; ++i)
crc = table[(crc << (8 - bitWidth)) ^ (input[index + i] & 0xff)] & 0xff;
return crc;
}
}