Author: Mike Shkolnik
How can I calculate a checksum by modulus 10 which is used in barcodes?
Answer:
Solve 1:
I want to publish a code for checksum calculation by modulus 10 which is used in
the barcodes. I must say that this "mod10" is specifical so readf an article if
you're interested.
This algorithm is very popular for UPC barcodes (Universal Product Code), hash code
or serial number generation for applications etc...
The basic algorithm:
add the values of the digits in the odd positions (1, 3, 5...)
multiply this result by 3
add the values of the digits in the even positions (2, 4, 6...)
sum the results of steps 2 and 3
the check digit is the smallest number which, when added to the result in step 4,
produces a multiple of 10.
Small example. Assume the source data is 08137919805
0+1+7+1+8+5=22
22*3=66
8+3+9+9+0=29
66+29=95
95+??=100 where ?? is a 5 (our checksum)
My implementation in the Pascal:
1 function Mod10(const Value: string): Integer;
2 var
3 i, intOdd, intEven: Integer;
4 begin
5 {add all odd seq numbers}
6 intOdd := 0;
7 i := 1;
8 while (i <= Length(Value)) do
9 begin
10 Inc(intOdd, StrToIntDef(Value[i], 0));
11 Inc(i, 2);
12 end;
13
14 {add all even seq numbers}
15 intEven := 0;
16 i := 2;
17 while (i <= Length(Value)) do
18 begin
19 Inc(intEven, StrToIntDef(Value[i], 0));
20 Inc(i, 2);
21 end;
22
23 Result := 3 * intOdd + intEven;
24 {modulus by 10 to get}
25 Result := Result mod 10;
26 if Result <> 0 then
27 Result := 10 - Result
28 end;
You can expand or optimize this algorithm for own needs.
For example, I modified it and now I use it for any characters (not only digits) in
source value.
The original algorithm I used for UPC-barcode validation in the SMReport Designer
and the my extended algorithm I use in the serial number generation as part of the
protection schema (in the shareware projects).
Solve 2:
29 function BarCodeValid(ACode: string): boolean;
30 var
31 I: integer;
32 SumOdd, SumEven: integer;
33 ADigit, AChecksumDigit: integer;
34 begin
35 SumOdd := 0;
36 SumEven := 0;
37 for I := 1 to (Length(ACode) - 1) do
38 begin
39 ADigit := StrToIntDef(ACode[I], 0);
40 if (I mod 2 = 0) then
41 begin
42 SumEven := SumEven + ADigit;
43 end
44 else
45 begin
46 SumOdd := SumOdd + ADigit;
47 end; {if}
48 end; {for}
49 AChecksumDigit := StrToIntDef(ACode[Length(ACode)], 0);
50 Result := ((SumOdd * 3 + SumEven + AChecksumDigit) mod 10 = 0);
51 end; {--BarCodeValid--}
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