Calculates the CRC32 - 32 bit Cyclical Redundancy Check. This check is
+ * used in numerous systems to verify the integrity of information. It's also
+ * used as a hashing function. Unlike a regular checksum, it's sensitive to
+ * the order of the characters. It produces a 32 bit Java int.
+ *
+ *
This Java programme was translated from a C version I had written. This + * software is in the public domain. + * + *
When calculating the CRC32 over a number of strings or byte arrays the + * previously calculated CRC is passed to the next call. In this way the CRC + * is built up over a number of items, including a mix of strings and byte + * arrays. + * + *
+ * int crcCalc = CRC32.crc32("Hello World");
+ * crcCalc = CRC32.crc32("How are you?", crcCalc);
+ * crcCalc = CRC32.crc32("I'm feeling really good, how about you?", crcCalc);
+ *
+ *
+ * The line int crcCalc = CRC32.crc32("Hello World"); is
+ * equivalent to int crcCalc = CRC32.crc32("Hello World", -1);.
+ * When starting a new CRC calculation the "previous crc" is set to 0xFFFFFFFF
+ * (or -1).
+ *
+ * @author Michael Lecuyer (mjl@theorem.com)
+ * @author Michael Bayne (slight modifications)
+ */
+public class CRC32
+{
+ /**
+ * Convenience mithod for generating a CRC from a single
+ * String.
+ *
+ * @param buffer string to generate the CRC32.
+ * @return 32 bit CRC.
+ */
+ public static int crc32 (String buffer)
+ {
+ return crc32(buffer, 0xFFFFFFFF);
+ }
+
+ /**
+ * Convenience method for generating a CRC from a byte array.
+ *
+ * @param buffer byte array to generate the CRC32.
+ * @return 32 bit CRC.
+ */
+ public static int crc32 (byte buffer[])
+ {
+ return crc32(buffer, 0xFFFFFFFF);
+ }
+
+ /**
+ * Convenience method for generating a CRC from a series of
+ * String's.
+ *
+ * @param buffer string to generate the CRC32.
+ * @param crc previously generated CRC32.
+ * @return 32 bit CRC.
+ */
+ public static int crc32 (String buffer, int crc)
+ {
+ return crc32(buffer.getBytes(), crc);
+ }
+
+ /**
+ * Convenience method for generating a CRC from a series of
+ * byte arrays.
+ *
+ * @param buffer byte array to generate the CRC32
+ * @param crc previously generated CRC32.
+ * @return 32 bit CRC.
+ */
+ public static int crc32 (byte buffer[], int crc)
+ {
+ return crc32(buffer, 0, buffer.length, crc);
+ }
+
+ /**
+ * General CRC generation function.
+ *
+ * @param buffer byte array to generate the CRC32.
+ * @param start byte start position.
+ * @param count number of byte's to include in CRC calculation.
+ * @param lastcrc previously generated CRC32.
+ * @return 32 bit CRC
+ */
+ public static int crc32 (byte buffer[], int start, int count, int lastcrc)
+ {
+ int crc = lastcrc;
+ for (int ii = start; count-- != 0; ii++) {
+ int temp1 = crc >>> 8;
+ int temp2 = _crcTable[(crc ^ buffer[ii]) & 0xFF];
+ crc = temp1 ^ temp2;
+ }
+ return crc;
+ }
+
+ protected static int[] _crcTable = new int[256];
+ protected static final int CRC32_POLYNOMIAL = 0xEDB88320;
+
+ static {
+ int crc;
+ for (int ii = 0; ii <= 255; ii++) {
+ crc = ii;
+ for (int jj = 8; jj > 0; jj--) {
+ if ((crc & 1) == 1) {
+ crc = (crc >>> 1) ^ CRC32_POLYNOMIAL;
+ } else {
+ crc >>>= 1;
+ }
+ }
+ _crcTable[ii] = crc;
+ }
+ }
+}