Sales tax, GST and other tax calculations
=========================================

This is an erlang project which has one method to calculate tax for
single price.

Tax system is a global nightmare. There are services that should
provide you the solution but if you want to start from something very
small, say one man's web service, you want to do things as cheap as
possible.

There are countries that do not have any tax system by goverment. Like
USA is one but it has states and each state has own state
tax. Additional to these there are also counties or cities that can
set tax for goods. Zip codes differ from tax areas which in practice
means we should not use zip code to calculate tax but there are no
sane information by government to have these numbers. So, we do a
shortcut: from wikipedia
(https://en.wikipedia.org/wiki/Sales_taxes_in_the_United_States) we
can find maximum tax rate for each state and we use that. It still
much less than general tax range in Europe. Thus, there is a mapping
file which maps Zip code to FIPS and we can map FIPS code to state,
handy.

Goods and Services Tax (GST) is used in Canada
(https://en.wikipedia.org/wiki/Sales_taxes_in_Canada), Hong Kong,
India, Malaysia, New Zealand, Singapore and Australia. In Canada the
tax differs by province and that can be determined by first letter of
Zip (or postal code).

In Europe all countries have taxes and that should be easy
(jsonvat.com, https://github.com/adamcooke/json-vat). Also similar
mapping is used with other countries
(https://en.wikipedia.org/wiki/Value-added_tax).


Howto use
---------

> Price = 100,
> CountryCode = "US",
> Zip = "76543",
> Rates = tax:rates(CountryCode, Zip),
#{ name=><<"Alabama">>, state_tax=>4, tax=> 2.5}
> {Vat, Gst, StateTax, AreaTax, BeforeTax } = tax:calc_included(100, Rates),
#{ total=>100, withouttax=>93.5, name=><<"Alabama">>, state_tax=>4, tax=> 2.5}