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CheckSum

Function CheckSum( const aData : string) : integer;

Example​

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

Usage​

CheckSum calculates a decimal GS1-style check digit using right-to-left 3 and 1 weighting.

Parameters

NameTypeDescription
aDatastring, constSource characters excluding the check digit. Each character is converted independently; supply decimal digits without spaces or punctuation.

Returns

An integer from 0 through 9. An empty input returns 0. A result of 0 means the weighted sum is already a multiple of ten.

Behaviour

The function calculates only from the supplied sequence. It does not identify or remove an existing check digit, validate a GS1 key length, or determine which application identifier owns the data.

Errors

An EConvertError from StrToInt propagates unchanged. If Velox overflow checking is enabled, an overflowing total can raise an arithmetic exception; otherwise normal build-specific integer overflow behaviour applies. No failure is logged or swallowed here.

Usage notes

The GS1 check digit is different from the modulo-103 symbol check character used by Code 128. ReportBuilder handles its Code 128 symbol calculation separately.

Additional Technical Info​

CheckSum calculates the standard decimal modulo-10 check digit for a digit string that does not yet contain its check digit. It weights the rightmost source digit by 3, the next by 1, and continues alternating towards the left.

Implementation

Velox registers the function directly in its common scripting import. The implementation makes two right-to-left passes over the Delphi string. The first starts at Length(aData), steps back by two and totals those digits; the second starts at Length(aData) - 1, steps back by two and totals the intervening digits. Each character is extracted with the one-based System.Copy(aData, index, 1) overload and converted by SysUtils.StrToInt.

The first total is multiplied by 3, the second is added, and the result is reduced modulo ten. A non-zero remainder is subtracted from ten; a zero remainder is left as zero.

Edge cases and quirks

  • The empty string produces 0 because both loops are skipped and the initial totals remain zero. This is an arithmetic result, not proof that empty data is a valid GS1 structure.
  • The input is not trimmed. Whitespace, brackets, separators and non-digit characters reach StrToInt one character at a time and normally raise EConvertError.
  • Copy uses one-based string positions in this path. Because each loop runs only while its position is greater than zero, it does not request an out-of-range digit.
  • Arbitrary lengths are accepted. The weight orientation remains anchored to the rightmost character.
  • The totals and result use Delphi Integer. Extremely long inputs can overflow; the function does not set a maximum or provide checked wide arithmetic.

Side effects

The function uses local arithmetic and temporary one-character strings only. It does not mutate its input or any shared state.

Performance and concurrency

The two passes together process every character once, so time is linear in input length and auxiliary arithmetic storage is constant. Each digit extraction creates a temporary string. The function has no shared state and is re-entrant.

Related entries

  • CheckDigit uses a similar weighting scheme but returns 10 instead of 0 for a zero remainder.
  • EAN13toEAN128 calls CheckSum after constructing its 13-digit legacy data value.

External references

Created 2026-07-15