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astex
Class CRC32

java.lang.Object
  extended by astex.CRC32

public class CRC32
extends java.lang.Object

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. 

 CRC32 crc = new CRC32();
 int crcCalc = crc.crc32("Hello World");
 crcCalc = crc.crc32("How are you?", crcCalc);
 crcCalc = crc.crc32("I'm feeling really good, how about you?", crcCalc);
The line int crcCalc = crc.crc32("Hello World"); is equivalent to int crcCalc = crc.crc32("Hello World", -1);. When starting a new CRC calculation the "previous crc" is set to 0xFFFFFFFF (or -1).

The table only needs to be built once. You may use it to generate many CRC's.

Method Summary
static int crc32(byte[] buffer)
          Convenience method for generating a CRC from a byte array.
static int crc32(byte[] buffer, int crc)
          Convenience method for generating a CRC from a series of byte arrays.
static int crc32(byte[] buffer, int start, int count, int lastcrc)
          General CRC generation function.
static int crc32(int[] buffer, int start, int count, int lastcrc)
          General CRC generation function.
static int crc32(java.lang.String buffer)
          Convenience mithod for generating a CRC from a single String.
static int crc32(java.lang.String buffer, int crc)
          Convenience method for generating a CRC from a series of String's.
static void main(java.lang.String[] args)
          Tests CRC32.
 
Methods inherited from class java.lang.Object
equals, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Method Detail

main

public static void main(java.lang.String[] args)
Tests CRC32.
Eg: java CRC32 "Howdy, I'm A Cowboy"

Parameters:
args - the string used to calculate the CRC32

crc32

public static int crc32(java.lang.String buffer)
Convenience mithod for generating a CRC from a single String.

Parameters:
buffer - string to generate the CRC32
Returns:
32 bit CRC

crc32

public static int crc32(byte[] buffer)
Convenience method for generating a CRC from a byte array.

Parameters:
buffer - byte array to generate the CRC32
Returns:
32 bit CRC

crc32

public static int crc32(java.lang.String buffer,
                        int crc)
Convenience method for generating a CRC from a series of String's.

Parameters:
buffer - string to generate the CRC32
crc - previously generated CRC32.
Returns:
32 bit CRC

crc32

public static int crc32(byte[] buffer,
                        int crc)
Convenience method for generating a CRC from a series of byte arrays.

Parameters:
buffer - byte array to generate the CRC32
crc - previously generated CRC32.
Returns:
32 bit CRC

crc32

public static int crc32(byte[] buffer,
                        int start,
                        int count,
                        int lastcrc)
General CRC generation function.

Parameters:
buffer - byte array to generate the CRC32
start - byte start position
count - number of byte's to include in CRC calculation
crc - previously generated CRC32.
Returns:
32 bit CRC

crc32

public static int crc32(int[] buffer,
                        int start,
                        int count,
                        int lastcrc)
General CRC generation function.

Parameters:
buffer - byte array to generate the CRC32
start - byte start position
count - number of byte's to include in CRC calculation
crc - previously generated CRC32.
Returns:
32 bit CRC
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