Implement price/weight normalization behavior

[Narnach]

Status: rough draft

We've got country-specific price/weight normalization logic implemented in Questionmark's Hoard system for the Netherlands and (IIRC) Belgium. A PR can start by extracting the logic and sharing it.

Design idea:

• Barcodes must be able to self-identify the country they are intended for. This breaks down into official rules and our own QM-specific rules. Speculative: implement GTIN::Base#country with official rules and possibly see if we can get this merged into upstream barcodevalidation when we have a working implementation.
  • Our custom classes may not be able to deterministically return a country.
• Implement GTIN::Base#normalize(country: self.country) which we can use to implement normalization rules. Probably use a helper class per country. Return true/false based on whether normalization could happen or not. We can implement a #normalize! version that raises an error when we don't know the country or are unable to normalize.
  • The optional country argument helps to provide external information about the country a barcode "belongs" to.
• Implement a #normalized? method that can check if it's normalized or not. I'm unsure what's desired behavior for barcodes that we don't know the normalization behavior for. Do we also need to implement #can_normalize? to make this deterministic?

[wvengen]

Great to move this logic here, yes.
Note that one cannot deduce the country from the barcode, but it is an input.
Perhaps we can instantiate a barcode with a country parameter. This is needed elsewhere too, e.g. if we implement operator == it would also need to do normalisation, I think it belongs in an instance attribute.

Links focused on NL situation:
http://developers.thequestionmark.org/2016/12/14/partial-barcode-sql
http://developers.thequestionmark.org/2017/02/13/storing-barcodes

[Narnach]

Note that one cannot deduce the country from the barcode, but it is an input.

My understanding was flawed, thanks for correcting me 😄

In that case it makes sense to have #country accessor as well, so we can base normalization behavior based on this being present. It also feeds reliably into #can_normalize? because we can then use the presence + value of country in addition to other factors.

[wvengen]

I'd expected country as input when initializing the class. I don't think that would fit with Barcodevalidation, so indeed an accessor would make sense - or a function to set it, e.g.

BarcodeValidation.scan('1234567890').country(:nl)

We could consider if there could be a default country or not. Right now, an instance of our apps has a static country from an environment variable, there it would make our code easier. But we may add multi-country support to single app instances, then the default would need to be set on each request / for each BarcodeValidation instance (also fine).

[Narnach]

I'd lean towards the "manually pass a country to an instance" approach to keep the behavior as generic as possible and don't get in the way of future multi-tenancy situations, or legitimate cases where you'd want to compare normalized barcodes from multiple countries.

[Narnach]

Another aspect that's useful to call out here is that normalization requires we split a barcode into its component parts: prefix, affix, check_digit, and that affix can thus be split into product id, price and weight.

• http://developers.thequestionmark.org/2016/12/14/partial-barcode-sql