Fix: re-expose GTIN#check_digit as public method

[Narnach]

Restore BarcodeValiation::GTIN#check_digit as public method (broken in v2.3.0).

PR #3 changed the internals of BarcodeValidation::GTIN to support different GTIN formats and to allow conversion between them. While this is a very nice feature, it unfortunately made the previously public method GTIN#check_digit (which was added via BarcodeValidation::Mixin) private.

In v2.2.0 we relied on this in our tests to fix the check digit of randomly generated barcodes. v2.3.0 makes both check_digit and expected_check_digit private, so it does not publicly expose any of the tools we'd need for our purposes.

The simplest fix seemed to make check_digit public, which restores the API that was available in v2.2.0. I've added a spec that calls the method and its validation as a means to ensure it remains public.

The readme still references check_digit as a public method, which reinforces that it was probably made private by accident.

(Note that I read the commit template, but it seems copy/pasted from another project that has interface tests and authentication. I did try to keep the first paragraph of my PR suitable for a changelog, because I'm not sure if that is relevant or not.)

[wvengen]

Thanks for the PR! I'm bumping into this as well. @marketplacer if you find the time to merge this, very welcome :)

[madhums]

This would be very helpful for me too! @nrw505 @beet could you please have a look?

[beet]

Making the check digit private was a conscious decision to treat it as an internal implementation detail of the GTIN objects' #valid? public predicate method:

def valid?
  valid_length == length && check_digit.valid?
end

Could you walk us through how you're using the check digit directly?

[Narnach]

I can see how hiding implementation details is a good reason to shrink your public API. In this case, though, check digits are part of the barcode domain and hiding it means you're hiding a useful domain object. In our case we relied on it, so we did not have to re-implement it ourselves (which seems wasteful).

In addition to reading and validating existing barcodes, we have two reasons for generating them:

• When we generate barcodes for internal use
• When we generate random unique barcodes during testing, and we would prefer them to be valid

One example of how we use the public check digit would be this:

# Takes a barcode string and if needed corrects the check digit to make it valid.
#
# @param [String] bc A barcode that may or may not have a valid check digit.
# @return [String] A barcode that has a valid check digit.
def repair_check_digit(bc)
  bc = bc.dup
  bc[-1] = BarcodeValidation.scan(bc).check_digit.expected.to_s
  bc
end

[beet]

@Narnach Nice one, thanks for the explanation, makes perfect sense.

[madhums]

Ah great! Can we merge and do a release? @beet :)

[beet]

Just released Release v2.5.0 · marketplacer/barcodevalidation

[Narnach]

Nice, thank you @beet!