Android Question LCR calculate

[Lello1964]

I have to calculate LRC of a character string using this method:

LRC byte is calculated with an operation of Exclusive OR (XOR) on Data

Example :
uint8 LRC(const uint8* str, const uint16 length)
{
uint16 i=length;
uint8 r=0x00;
while (i--)
r^=*str++;
return r;
}


can you help me ?

 

[Star-Dust]

Longitudinal redundancy check - Wikipedia

it.m.wikipedia.org

 

[emexes]

I have to calculate LRC of a character string using this method:
can you help me ?

I can try. ?

'''uint8 LRC(const uint8* str, const uint16 length)
Sub LRC(A() As Byte, L As Int) As Byte

    '''uint8 r=0x00;
    Dim R As Byte = 0

    '''uint16 i=length;
    '''while (i--) {
    For I = 0 to L - 1
        '''r^=*str++;
        R = Bit.Xor(R, A(I))
    Next

    '''return r;
    Return R

End Sub

and your first major confusion will be that B4A Bytes are Java Bytes are signed (-128 to 127) rather than unsigned (0 to 255)

and the second tidy-up is that you can use A.Length to get the side of A() ie no need for the second parameter

and what's kinda funny is that the B4A compiler is going to convert our code straight back to a curly-braces language

 

[Star-Dust]

perhaps it is better to start from the Java code that is present on Wikipedia rather than from the other code precisely to avoid this difference in types

 

[Lello1964]

#if JAVA
    Public byte calculateLRC(byte[] data) {
        byte checksum = 0;
        For (int i = 0; i <= data.length - 1; i++) {
            checksum = (byte) ((checksum + data[i]) & 0xFF);
        }
        checksum = (byte) (((checksum ^ 0xFF) + 1) & 0xFF);
        Return checksum;
    }
#End If

I tried this java code but it gives me compile error:

src\b4a\example11\main.java:437: error: <identifier> expected
Public byte calculateLRC(byte[] data) {
^
1 erro

 

[Star-Dust]

Public to lowercase?

 

[emexes]

Probably better to get it working in regular B4A first, rather than adding complexity and confusion eg:

Public byte calculateLRC(byte[] data) {
    byte checksum = 0;
    For (int i = 0; i <= data.length - 1; i++) {
        checksum = (byte) ((checksum + data[i]) & 0xFF);
    }
    checksum = (byte) (((checksum ^ 0xFF) + 1) & 0xFF);
    Return checksum;
}

producing a different check-value than:

Example :
uint8 LRC(const uint8* str, const uint16 length)
{
uint16 i=length;
uint8 r=0x00;
while (i--)
r^=*str++;
return r;
}

 

[emexes]

and your first major confusion will be that B4A Bytes are Java Bytes are signed (-128 to 127) rather than unsigned (0 to 255)

This is not an actual problem, just something that confuses many people when they first encounter it.

The actual bit-pattern is correct.

If the negative shades of its numeric value irk you sufficiently, then: Log(Bit.And(SignedByte, 0xFF))

 

[Serge Bertet]

This post seems to be close to want you want to acheive:

Modbus CRC16 calculation

Hello, I have to calculate the CRC16 of a MODBUS string. Unfortunately I have no idea how can I do that. I have found a code for Visual Basic, how I can convert it for Basic4Android? Public Function CRC(buf() As Byte, lbuf As Integer) As Integer...

www.b4x.com

 

[Star-Dust]

Probably better to get it working in regular B4A first, rather than adding complexity and confusion eg:



producing a different check-value than:

What I meant was to translate into B4X starting from Java and thus there will be no type compatibility problems.

The algorithm is described on Wikipedia, in the version that was posted (I don't know if it's swift or MatLab or something else) it seems that it lacks to add the value 1 at the end of the calculation.

 

[DonManfred]

can you help me ?

What is the value of the bytearray (source) and what is the expected output?

The Library is based on the JAVA-Code from Wikipedia (see link in #2)

    Dim lrc As LRC
    Dim values() As Byte = Array As Byte(15,63,255,128)
    Dim lrccrc As Byte = lrc.calculateLRC(values)
    Log($"$8{lrccrc}"$)