Calculate CRC-6 GSM using lookup table-CodePudding

I am trying to calculate CRC-6 GSM but the result is wrong ($16). For '123456789' it should return $13 according to https://reveng.sourceforge.io/crc-catalogue/all.htm#crc.cat.crc-6-gsm

var Table: array[0..255] of Byte = (
$3F, $10, $0E, $21, $32, $1D, $03, $2C, 
$25, $0A, $14, $3B, $28, $07, $19, $36, 
$0B, $24, $3A, $15, $06, $29, $37, $18, 
$11, $3E, $20, $0F, $1C, $33, $2D, $02, 
$38, $17, $09, $26, $35, $1A, $04, $2B, 
$22, $0D, $13, $3C, $2F, $00, $1E, $31, 
$0C, $23, $3D, $12, $01, $2E, $30, $1F, 
$16, $39, $27, $08, $1B, $34, $2A, $05, 
$31, $1E, $00, $2F, $3C, $13, $0D, $22, 
$2B, $04, $1A, $35, $26, $09, $17, $38, 
$05, $2A, $34, $1B, $08, $27, $39, $16, 
$1F, $30, $2E, $01, $12, $3D, $23, $0C, 
$36, $19, $07, $28, $3B, $14, $0A, $25, 
$2C, $03, $1D, $32, $21, $0E, $10, $3F, 
$02, $2D, $33, $1C, $0F, $20, $3E, $11, 
$18, $37, $29, $06, $15, $3A, $24, $0B, 
$23, $0C, $12, $3D, $2E, $01, $1F, $30, 
$39, $16, $08, $27, $34, $1B, $05, $2A, 
$17, $38, $26, $09, $1A, $35, $2B, $04, 
$0D, $22, $3C, $13, $00, $2F, $31, $1E, 
$24, $0B, $15, $3A, $29, $06, $18, $37, 
$3E, $11, $0F, $20, $33, $1C, $02, $2D, 
$10, $3F, $21, $0E, $1D, $32, $2C, $03, 
$0A, $25, $3B, $14, $07, $28, $36, $19, 
$2D, $02, $1C, $33, $20, $0F, $11, $3E, 
$37, $18, $06, $29, $3A, $15, $0B, $24, 
$19, $36, $28, $07, $14, $3B, $25, $0A, 
$03, $2C, $32, $1D, $0E, $21, $3F, $10, 
$2A, $05, $1B, $34, $27, $08, $16, $39, 
$30, $1F, $01, $2E, $3D, $12, $0C, $23, 
$1E, $31, $2F, $00, $13, $3C, $22, $0D, 
$04, $2B, $35, $1A, $09, $26, $38, $17
);

function GSM_Update(Msg: PByte; Length: Integer): Byte;
var i: Integer;
    H: Byte;
begin
  H :=  $00;

  for i:=0 to Length-1 do begin

    H := Table[H xor Msg^];  //I believe this line might be wrong but don't know how to change it
    Inc(Msg);
  end;

  Result := H xor $3F;
end;

CodePudding user response：

I'm not certain about the Pascal operators and syntax, but for the table in your code, that line needs to be:

    H := (not Table[(H shl 2) xor Msg^]) and $3F;

This could be simplified and sped up by using a more appropriate table. I would replace each byte in the table by its one's complement shifted left by two bits. I.e. $00, $bc, $c4, $78, ... Then that line would be what you currently have:

    H := Table[H xor Msg^];

and the last line would need to be changed to:

  Result := (H shr 2) xor $3F;

CodePudding user response：

In Pascal for loop limits are inclusive. When you write

for i := 1 to 1 do writeLn(i)

it will print one line showing the value 1. Pascal is influenced by mathematics. In math the limits specified to the ∑ summation operator are also inclusive. By the way, in Delphi you are allowed to make multiple assignments to its result variable, and since you’re using value parameters you can save i, too. Thus your function works without any local variables:

function GSMupdate(msg: pByte; length: integer): byte;
begin
    result := 0;
    for length := 1 to length - 1 do // ← adjustment
    begin
        result := table[result xor msg^];
        inc(msg);
    end;
    result := result xor $3F;
end;