Problem with 8 bit CheckSum

[bitstra]

Hello,

I have a problem woth calculating the accurate oh an 8 bit CheckSum for an
A-GPS data fie by skytraq...

in Linux/posix the right checksum is calculated for the array of bytes like this:

//for bytearray ephdata[]

csum is the checksum over all bytes of file
csumb is the checksum over the first 10000 bytes of the file

// checksum
unsigned char csuma, csumb = 0;
for (i = 0; i < 0x10000; i++)
csumb += ephdata;
csuma = csumb;
for (; i < ephbytes; i++)
csuma += ephdata;

I've try'd to make that way in b4a - but with all variants I could not get the accurate checksum value.

Here is one of my b4a-examples:

Dim bc, bcB As Byte
Dim c, cB As Int
'c=0:cB=0
For i=0 To buffer.Length-1
'c=c+(Bit.ParseInt(buffer(i),16)+255)
bc=Bit.xor(bc,buffer(i)+255)
If i<=9999 Then bcB=Bit.Xor(bcB,buffer(i)+255)
'cB=cB+(Bit.ParseInt(buffer(i),16)+255)
Next

c=bc Mod 256
cB=bcB Mod 256

The hardware manufacturer of the device, for that I want to use the checksum says:

'The checksums are the 8-bit exclusive OR of the data, to be transmitted'...

Any Idea???


regards

bitstra

 

[Erel]

What is the type of buffer?

This code seems a bit strange:

Bit.ParseInt(buffer(i),16)

 

[bitstra]

Hi Erel,

thank you for your reply. As you can see, the bit strange lines are remarked - not used...

The buffer type is byte() from a binary file read:

Dim in As File
If File.Exists(File.DirRootExternal,"Eph.dat")=True Then 
    If File.size(File.DirRootExternal,"Eph.dat")>0 Then 
     Dim buffer(File.size(File.DirRootExternal,"Eph.dat")) As Byte
     in = File.OpenInput(File.DirRootExternal,"Eph.dat")
     count = in.ReadBytes(buffer, 0, buffer.length)
     byteCnt=count
     
     Dim bc, bcB As Byte
     Dim c, cB As Int
     'c=0:cB=0
     For i=0 To buffer.Length-1
       'c=c+(Bit.ParseInt(buffer(i),16)+255)
      bc=Bit.xor(bc,buffer(i)+255)
      If i<=9999 Then bcB=Bit.Xor(bcB,buffer(i)+255) 
      'cB=cB+(Bit.ParseInt(buffer(i),16)+255)
     Next
     
     c=bc Mod 256
     cB=bcB Mod 256
     'c=  c-(Round(c /256)*256)
     'cB= cB-(Round(cB/256)*256)
     
     
     
     'cB=cB Mod 256
     
     Log("csum: " & c & "/ csumB: " & cB)

          in.Close
         End If
End If

???

regards


bitstra

 

[Erel]

Now that you are using [ code ] tags, I can see the comments...

buffer(i) + 255 is wrong.

You should use Bit.And(buffer(i), 0xFF) to convert it to an unsigned byte value.