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CheckDigit

Function CheckDigit( const aData : string) : integer;

Example​

procedure ScriptEvent(var Value: Variant);
begin
  Value := CheckDigit('941234554321'); // 9
end;

Usage​

CheckDigit calculates a GS1-style decimal check result using alternating 1 and 3 weights.

Parameters

NameTypeDescription
aDatastring, constSource characters excluding the check digit. Each character is converted independently to an integer; use the decimal digits 0 through 9.

Returns

For a weighted sum with remainder 1 through 9, returns 9 through 1 respectively. In Velox, a zero remainder returns 10 rather than the standard check digit 0. An empty string also returns 10.

Behaviour

The complete input participates in the calculation. The function does not remove an existing check digit, trim whitespace or restrict the input to a recognised GS1 key length.

Errors

SysUtils.StrToInt raises EConvertError for a character it cannot convert. The function does not catch or translate that exception. An arithmetic overflow can also surface when overflow checking is enabled.

Usage notes

CheckDigit and CheckSum are not aliases. They use equivalent alternating-weight orientation for normal digit strings, but their zero-remainder and empty-input results differ.

Additional Technical Info​

CheckDigit scans a decimal data string from left to right, applies alternating factors of 1 and 3, and returns ten minus the weighted sum modulo ten. For ordinary non-zero remainders this produces the expected GS1 modulo-10 check digit.

Implementation

Velox registers the function directly. The implementation obtains the Delphi string Length, selects the initial factor index from Length mod 2, and scans positions 1 through Length. Each one-character indexed string value is passed to SysUtils.StrToInt, multiplied by factor 1 or 3, and added to an Integer accumulator. The factor alternates after every character. The final statement is exactly 10 - (sum mod 10); it does not apply a second modulo operation.

The parity choice gives the rightmost source digit a factor of 3: an even-length input starts with factor 1, while an odd-length input starts with factor 3.

Edge cases and quirks

  • A zero remainder produces 10. GS1's standard calculation subtracts from the nearest equal or higher multiple of ten, so the check digit for a zero remainder is 0. Callers requiring that standard result must handle this implementation quirk or use CheckSum.
  • An empty input skips the loop and returns 10 rather than signalling missing data.
  • A space, sign, letter or other character that cannot be converted as a standalone integer raises a conversion exception. There is no preliminary digit validation.
  • The function accepts arbitrary length. It does not distinguish GTIN-8, GTIN-12, GTIN-13, GTIN-14 or other GS1 structures.
  • The accumulator is a Delphi Integer. Extremely long inputs can overflow; the outcome then depends on the product build's overflow-checking setting because the function supplies no guard.

Side effects

The operation reads its input and local variables only. It does not mutate state, log or perform I/O.

Performance and concurrency

Time is linear in the number of characters and auxiliary storage is constant. One conversion is performed per character. The function uses no shared state and is re-entrant.

Related entries

  • CheckSum returns 0 for a zero remainder and is the safer existing helper when that GS1 result is required.

External references

Created 2026-07-15