VB to Java, WTF?

Hey /prog/

I'm trying to port a CRC16 function from VB to Java, and I cannot for the life of me see what I'm doing wrong.  Any ideas?

Here's the VB, and passing in [ 0x00 0x01 0x40 0x30 ] returns [ 0x18 0x2E ]:

    Function CRC16(ByVal data() As Byte) As Byte()
        Dim CRC16Lo As Byte, CRC16Hi As Byte
        Dim CL As Byte, CH As Byte
        Dim SaveHi As Byte, SaveLo As Byte
        Dim i As Integer
        Dim Flag As Integer
        CRC16Lo = &HFF
        CRC16Hi = &HFF
        CL = &H8
        CH = &H84
        For i = 0 To UBound(data)
            CRC16Lo = CRC16Lo Xor data(i)
            For Flag = 0 To 7
                SaveHi = CRC16Hi
                SaveLo = CRC16Lo
                CRC16Hi = CRC16Hi \ 2
                CRC16Lo = CRC16Lo \ 2
                If ((SaveHi And &H1) = &H1) Then
                    CRC16Lo = CRC16Lo Or &H80
                End If
                If ((SaveLo And &H1) = &H1) Then
                    CRC16Hi = CRC16Hi Xor CH
                    CRC16Lo = CRC16Lo Xor CL
                End If
            Next Flag
        Next i
        Dim ReturnData(1) As Byte
        ReturnData(0) = CRC16Hi
        ReturnData(1) = CRC16Lo
        CRC16 = ReturnData
    End Function

Here's the Java, and passing in the same bytes as above returns [ 0x63 0xF5 ]

    public static byte[] get(byte[] data) {
       
        byte lo = (byte)0xFF;
        byte hi = (byte)0xFF;
       
        byte cl = (byte)0x08;
        byte ch = (byte)0x84;
       
        byte sl = 0;
        byte sh = 0;
       
        for(byte b : data) {
           
            lo ^= b;
           
            for(int i=0; i<8; i++) {
               
                sh = hi;
                sl = lo;
               
                hi >>= 2;
                lo >>= 2;
               
                if((sh & 1) == 1) {
                    lo |= 0x80;
                }
               
                if((sl & 1) == 1) {
                    hi ^= ch;
                    lo ^= cl;
                }
               
            }
        }
       
        return new byte[] {
            hi,
            lo
        };
       
    }

I'm not a VB developer so it's probably my understanding of arithmetic there that's screwing me up, but if anybody can point out how I'm an idiot, I'll be forever grateful.

out

Don't you want >>>=?

>>3
Yeah, I realized that mistake but it's not my problem -- I'm getting the same result back (63:f5)

Thanks anyway

>>4
Oh, found the problem. You're doing <<= 2 instead of <<= 1. VB's backslash operator isn't a bitshift, it's integer division.

Fuck, I meant >>=.

I figured it out, if anybody wants to know, it's because Java is fucktarded and forces signedness.  Once I wasted a bunch of memory by using integers so they didn't overflow it worked fine.  Here's the code which appears to work:

    public static byte[] get(byte[] data) {
       
        int lo = 0xFF;
        int hi = 0xFF;
       
        int cl = 0x08;
        int ch = 0x84;
       
        int sl = 0;
        int sh = 0;
       
        for(byte b : data) {
           
            lo ^= b;
           
            for(int i=0; i<8; i++) {
               
                sh = hi;
                sl = lo;
               
                hi /= 2;
                lo /= 2;
               
                if((sh & 1) == 1) {
                    lo |= 0x80;
                }
               
                if((sl & 1) == 1) {
                    hi ^= ch;
                    lo ^= cl;
                }
               
            }
        }
       
        return new byte[] {
            (byte)(hi & 0xFF),
            (byte)(lo & 0xFF)
        };
       
    }

>>7
You changed from bitshifting to division.

>>8
Yeah, I did.  I was barking up the wrong tree on that.  As >>5 said, \ is integer division, not bit shifting.

OP here

This: 
lo ^= b;
Should be:
lo ^= b & 0xFF;

And then it appears to work for any input

porting CRC16
Just Google "CRC16 java" and you have a working implementation.

>>11
http://en.wikipedia.org/wiki/Cyclic_redundancy_check

>CRC-16-IBM    x16 + x15 + x2 + 1
>CRC-16-CCITT    x16 + x12 + x5 + 1
>CRC-16-T10-DIF    x16 + x15 + x11 + x9 + x8 + x7 + x5 + x4 + x2 + x + 1
>CRC-16-DNP    x16 + x13 + x12 + x11 + x10 + x8 + x6 + x5 + x2 + 1
>CRC-16-DECT    x16 + x10 + x8 + x7 + x3 + 1
>CRC-16-Fletcher    Not a CRC; see Fletcher's checksum

This is none of those

CRC is lame. use builtin SHA-1/MD-5 hashes which are part of Java.
http://www.javablogging.com/sha1-and-md5-checksums-in-java/

>>12
http://en.wikipedia.org/wiki/Computation_of_CRC
It should be trivial to figure out the polynomial from the code, and then modify existing code to use it.

Java
GC
Found your problem!

Also,

GC is shit.