XORing bytes (for LRC calculation)

[jasonwood]

I am trying to figure out how to XOR two bytes together in FileMaker.

Example:

I start with the binary values 49 and 50 which would be represented in binary as 00110001 and 00110010.

I want to XOR them together (matching bits become 0 and different ones become 1) so that I end up with 3 (binary representation 00000011).

This is needed in order to calculate an LRC (Longitudinal Redundancy Check), which I need to append to my serial commands (using troi serial plugin) to a credit card terminal.

Where do I start?

[comment]

Try:

BitwiseXOR ( n ; m ) =

Let ( [

bitXOR = Mod ( n ; 2 ) xor  Mod ( m ; 2 ) ;

nextN = Div ( n ; 2 ) ;

nextM = Div ( m ; 2 ) 

] ;

Case ( nextN or nextM ; 2 * BitwiseXOR ( nextN ; nextM ) ) + bitXOR

)

[jasonwood]

well I don't really understand it but it works and it's WAY better than my brute force solution:

Let ([$byte1=byte1;$byte2=byte2];"") &



If ( ( (      $byte1      ) ≥ 128 ) xor ( (      $byte2      ) ≥ 128 ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 128 ; Let ($byte1=$byte1-128;"") ; "" ) &

If ( $byte2 ≥ 128 ; Let ($byte2=$byte2-128;"") ; "" ) &



If ( ( ( $byte1 ) ≥ 64   ) xor ( ( $byte2 ) ≥   64 ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 64 ; Let ($byte1=$byte1-64;"") ; "" ) &

If ( $byte2 ≥ 64 ; Let ($byte2=$byte2-64;"") ; "" ) &



If ( ( ( $byte1 ) ≥ 32   ) xor ( ( $byte2 ) ≥   32 ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 32 ; Let ($byte1=$byte1-32;"") ; "" ) &

If ( $byte2 ≥ 32 ; Let ($byte2=$byte2-32;"") ; "" ) &



If ( ( ( $byte1 ) ≥ 16   ) xor ( ( $byte2 ) ≥   16 ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 16 ; Let ($byte1=$byte1-16;"") ; "" ) &

If ( $byte2 ≥ 16 ; Let ($byte2=$byte2-16;"") ; "" ) &



If ( ( ( $byte1 ) ≥  8    ) xor ( ( $byte2 ) ≥    8  ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 8 ; Let ($byte1=$byte1-8;"") ; "" ) &

If ( $byte2 ≥ 8 ; Let ($byte2=$byte2-8;"") ; "" ) &



If ( ( ( $byte1 ) ≥  4    ) xor ( ( $byte2 ) ≥    4  ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 4 ; Let ($byte1=$byte1-4;"") ; "" ) &

If ( $byte2 ≥ 4 ; Let ($byte2=$byte2-4;"") ; "" ) &



If ( ( ( $byte1 ) ≥  2    ) xor ( ( $byte2 ) ≥    2  ) ; "1" ; "0" ) & 



If ( $byte1 ≥ 2 ; Let ($byte1=$byte1-2;"") ; "" ) &

If ( $byte2 ≥ 2 ; Let ($byte2=$byte2-2;"") ; "" ) &



If ( ( ( $byte1 ) ≥  1    ) xor ( ( $byte2 ) ≥    1  ) ; "1" ; "0" )

Thanks!

[jasonwood]

I don't suppose that could be modified to do the whole sequence of comparisons (I was going to use a scripted loop for this.

Basically I'm XORing a lot more than 2 values, which would come from one field (with a separator or carriage return, whatever works best).

First XOR the first two, then XOR the result with the next value, and so forth until the last value.

[comment]

I think it would be better to use another custom function for the "sequencing". This function would call the first function to do the XORing.