lint_csum.c File Reference

Purpose

The csum linter ensures that the data has a valid numeric check digit.

Remarks

The process for validating the standard GS1 numeric check digit is described in the GS1 General Specifications section "Standard check digit calculations for GS1 data structures".

Functional Description

gs1_lint_csum()

GS1_SYNTAX_DICTIONARY_API gs1_lint_err_t gs1_lint_csum	(	const char *const	data,
		size_t *const	err_pos,
		size_t *const	err_len
	)

Use to ensure that the AI component has a valid numeric check digit.

Parameters

[in]	data	Pointer to the null-terminated data to be linted. Must not be NULL.
[out]	err_pos	To facilitate error highlighting, the start position of the bad data is written to this pointer, if not NULL.
[out]	err_len	The length of the bad data is written to this pointer, if not NULL.

Returns

GS1_LINTER_OK if okay.

GS1_LINTER_INCORRECT_CHECK_DIGIT if the check digit is incorrect.

GS1_LINTER_TOO_SHORT_FOR_CHECK_DIGIT if the data is too short.

GS1_LINTER_NON_DIGIT_CHARACTER if the data contains a non-digit character.

{
 
        int weight;
        int parity = 0;
        const char *p;
        size_t len, pos;
 
        assert(data);
 
        len = strlen(data);
 
        /*
         * Data must include at least the check digit.
         *
         */
        if (*data == '\0')
                GS1_LINTER_RETURN_ERROR(
                        GS1_LINTER_TOO_SHORT_FOR_CHECK_DIGIT,
                        0,
                        0
                );
 
        /*
         * Data must consist of all digits.
         *
         */
        if ((pos = strspn(data, "0123456789")) != len)
                GS1_LINTER_RETURN_ERROR(
                        GS1_LINTER_NON_DIGIT_CHARACTER,
                        pos,
                        1
                );
 
        /*
         * Calculate the sum of the numeric values of the data, excluding the
         * check digit, weighted by alternating ...3:1:3 values, from right to
         * left.
         *
         * The check digit is valid if its value, when added to the data
         * checksum, makes the overall sum a multiple of 10.
         *
         */
        weight = len % 2 == 0 ? 3 : 1;
        p = data;
        while (*(p+1)) {
                parity += weight * (*p++ - '0');
                weight = 4 - weight;
        }
        parity = (10 - parity % 10) % 10;
 
        if (parity + '0' != *p)
                GS1_LINTER_RETURN_ERROR(
                        GS1_LINTER_INCORRECT_CHECK_DIGIT,
                        len - 1,
                        1
                );
 
        GS1_LINTER_RETURN_OK;
 
}