Consistent single unit level price rounding

[samuelvogel]

We created https://github.com/shopware/shopware/pull/847 before, which was not accepted. We wan't to split it up in multiple smaller PRs for smaller change sets and better understandability.

Generally we need to check which cases can actually happen in Shopware, depending on:

• With which precision article prices are saved (can be done including or without taxes per customer group) in the database
• Net & gross baskets & orders
• Percentage discounts and so forth

Then we should create a PR for every one of the problems with a (realistic) example and the fix.

We already have a couple of open PRs regarding rounding as well, which we should not overlap:

• https://github.com/shopware/shopware/pull/556
• https://github.com/shopware/shopware/pull/915

References:

• Different rounding methods: https://drupalcommerce.org/sites/default/files/docs/TaxesPDF.pdf
• Same problem in Apache OFBiz: https://issues.apache.org/jira/browse/OFBIZ-6436
• General explanation on how to round on invoices: https://makandracards.com/makandra/1505-invoices-how-to-properly-round-and-calculate-totals

[fixpunkt]

I'd suggest taking https://makandracards.com/makandra/1505-invoices-how-to-properly-round-and-calculate-totals with a grain of salt, since the author states right at the end of the second paragraph:

All other totals in your invoice will be some sum or difference of these two rounded numbers [line item total and VAT paid], so no further rounding is necessary.

Which is only correct if you don't take the fact that we're working on a machine with finite precision into account, cf. #26.

[samuelvogel]

That article is not really about our specific problem anyways, as it doesn't go deeper than "item level", which means a position of the invoice which might have a quantity higher than one compared to "unit level" (what we want). But it explains some of the general high-level rounding choices.