Java CRC32: not the same as CRC from C#
I have to compare files with java with a CRC32 code provided by a C# script. When I calculate the CRC32 with java.util.zip.CRC32 the result is completely different ...
My guess is that the polynom = 0x2033 of the C# script is not the same as used in zip.CRC32. Is it possible to set the polynom ? Or any ideas of a java-class for calculating a CRC32 where you can define your own polynom?
UPD开发者_运维问答ATE: problem is not the polymnom. This is the same between C# and Java
This is my code, maybe something is wrong in the way I read the file?
package com.mine.digits.internal.contentupdater;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.zip.CRC32;
public class CRC
{
public static String doConvert32(File file)
{
byte[] bytes = readBytesFromFile(file); // readFromFile(file).getBytes();
CRC32 x = new CRC32();
x.update(bytes);
return (Long.toHexString(x.getValue())).toUpperCase();
}
/** Read the contents of the given file. */
private static byte[] readBytesFromFile(File file)
{
try
{
InputStream is = new FileInputStream(file);
long length = file.length();
if (length > Integer.MAX_VALUE) {
// File is too large
}
byte[] bytes = new byte[(int)length];
int offset = 0;
int numRead = 0;
while (offset < bytes.length && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0)
{
offset += numRead;
}
// Ensure all the bytes have been read in
if (offset < bytes.length) {
System.out.println("Could not completely read file " + file.getName());
}
// Close the input stream and return bytes
is.close();
return bytes;
}
catch (IOException e)
{
System.out.println("IOException " + file.getName());
return null;
}
}
}
Thanks a lot, Frank
The standard (IEEE) CRC32 polynomial is 0x04C11DB7 which corresponds to:
x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 +
x^10 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
This is what java.util.zip.CRC32 uses. Don't know about the C# script you mention...
You can find this code snippet useful:
- http://www.cs.princeton.edu/introcs/51data/CRC32.java.html
CRC-32 is a specific CRC variant according to IEEE 802.3 and is using the polynom 0x04C11DB7. If your C# library is using the polynom 0x2033, it is /not/ an implementation of CRC-32.
If you need Java code to calculate arbitrary CRC variants, googling "java crc" will give you several examples.
Solved by copying the code from C# and convert it to a Java-class...
So now both use the same code, only had to do some minor changes for unsigned <> signed bytes differences.
1 + x + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 + x^11 + x^12 + x^16 + x^22 + x^23 + x^26 (0x04C11DB7) Java uses the above polynomial for CRC 32 calculation and is different from IEEE 802.3 standard which has a 32 bit power of x in addition.
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