PngCrc.java
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You 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 org.apache.commons.imaging.formats.png;
// should just use ints, not longs
final class PngCrc {
/* Table of CRCs of all 8-bit messages. */
private final long[] crcTable = new long[256];
/* Flag: has the table been computed? Initially false. */
private boolean crcTableComputed;
public long continuePartialCrc(final long oldCrc, final byte[] buf, final int len) {
return updateCrc(oldCrc, buf);
}
/*
* Update a running CRC with the bytes buf[0..len-1]--the CRC should be initialized to all 1's, and the transmitted value is the 1's complement of the final
* running CRC (see the crc() routine below)).
*/
/* Gets the CRC of the bytes buf[0..len-1]. */
public int crc(final byte[] buf, final int len) {
return (int) (updateCrc(0xffffffffL, buf) ^ 0xffffffffL);
}
public long finishPartialCrc(final long oldCrc) {
return oldCrc ^ 0xffffffffL;
}
/* Make the table for a fast CRC. */
private void makeCrcTable() {
long c;
int n;
int k;
for (n = 0; n < 256; n++) {
c = n;
for (k = 0; k < 8; k++) {
if ((c & 1) != 0) {
c = 0xedb88320L ^ c >> 1;
} else {
c = c >> 1;
}
}
crcTable[n] = c;
}
crcTableComputed = true;
}
public long startPartialCrc(final byte[] buf, final int len) {
return updateCrc(0xffffffffL, buf);
}
private long updateCrc(final long crc, final byte[] buf) {
long c = crc;
int n;
if (!crcTableComputed) {
makeCrcTable();
}
for (n = 0; n < buf.length; n++) {
// Debug.debug("crc[" + n + "]", c + " (" + Long.toHexString(c) +
// ")");
c = crcTable[(int) ((c ^ buf[n]) & 0xff)] ^ c >> 8;
}
return c;
}
}