B4J Question Misbehaving Byte array

barx

barx

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I guess i'm missing something here. If I have the following code:

B4X: 
Dim serial() As Byte = Array As Byte(0x53, 0x32, 0x33, 0x31, 0x33, 0x31, 0x57, 0x34, 0x33, 0x39)
Dim LoginString() As Byte = Array As Byte(0x55, 0x79, 0x80, 0x00, 0x00, 0x00, 0x00, 0x01, 0x0b, serial(0), serial(1), serial(2), serial(3), serial(4), serial(5), serial(6), serial(7), serial(8), serial(9), 0x01)
Dim Chksum() As Byte = CalcChecksum(LoginString)

And then Break at the CalcChecksum method and poke into the passed data
2017-04-02 (1).png
As you can see, the first 2 bytes are fine but the 3rd is wrong. It is 0xFFFFFF80 instead of 0x80.

I've been tinkering about and basically anything > 0x79 shows wrong. Anthing < 0x79 is fine.

Any ideas?
 
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Daestrum

Daestrum

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Looks like the debugger just sign extends it to 32bit to display it.
All bytes in java are signed and can have a value of -128 to 127, not like 'c' where an unsigned byte can have a value of 0 to 255.
 
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barx

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Daestrum said:
Looks like the debugger just sign extends it to 32bit to display it.
All bytes in java are signed and can have a value of -128 to 127, not like 'c' where an unsigned byte can have a value of 0 to 255.
Click to expand...


So how do I make a byte of say 0xaa. I have tried just using 170 (the decimal of 0xaa) but still the same. I don't believe it is just the ide showing it wrongly as the resultant checksum that is returned is calculated wrongly.
 
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Daestrum

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Putting them in the byte buffer use 0xaa
If you want to see the value as 170 then you will need to put the byte value into a short using something like
B4X: 
Dim s As Short
s = Bit.And(yourValue,0xff)
log(s)
 
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Daestrum

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Can't see your checksum calculation but it's probably similar to this
Note one way (s) will calculate correctly, (s1) will be incorrect.
B4X: 
Dim s As Short = 0
Dim s1 As Short = 0
For Each d As Byte In c ' c is the byte buffer
s = s + Bit.And(d,0xff) ' makes value 0 to 255
s1 = s1 + d ' wrong - not allowing for -ve values
Next
' this is correct as it only adds +ve numbers
Log("s "& s)
Log(Bit.UnsignedShiftRight(Bit.And(s,0xff00),8)&" "&Bit.And(s,0xff))
' this is wrong value as it calculated on the value not the 0 - 255 value
Log("s1 "& s1) ' wrong value as not allowing for -ve values
Log(Bit.UnsignedShiftRight(Bit.And(s1,0xff00),8)&" "&Bit.And(s1,0xff))
 
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barx

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Thanks for that @Daestrum

This is current sub. I had found that adding 0x100 to negative values, seemed to sort the issue, though I have not fully tested it nor thought about the theory too much to know if it theoretically should work.

B4X: 
Sub CalcChecksum(Data() As Byte) As Byte()
   
    Dim Total As Int
    Dim Byte1, Byte2 As Byte = 0x00
   
    For i = 0 To Data.Length - 1
        If Data(i) < 0 Then
            Total = Total + Data(i) + 0x100
        Else
            Total = Total + Data(i)
        End If
       
    Next
   
    Byte2 = Bit.And(Total, 0xff)
    Byte1 = Bit.ShiftRight(Total, 8)
   
    Return Array As Byte(Byte1, Byte2)
   
End Sub

I will try out your solution as it looks a lot neater
 
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