Discounts + VAT

[challengee]

Fixes #59 and fixes #44

[challengee]

@mohawke this should be ready, what do you think?

[mohawke]

does this fix an issue?

[mohawke]

Maybe just calculate the VAT amount and Discount based on the given data?

A = invoice amount (given)
Vr = VAT rate (given)
T = invoice total (given)

D = discount
V = VAT amount

1)  A - D + V = T
2) (A - D)*Vr = V

This leads to:

D = [ A*( 1 + Vr ) - T ] / ( 1 + Vr )

Then you can use the rounded D here ( we'll call it Dr ):

V = (A - Dr)*Vr 

Then round this V to Vr and put Vr and Dr on the invoice. This should be correct according to VAT regulations.

@challengee : what do you think?

[challengee]

Seems like a good idea, I'll give this a try

[mohawke]

Looks correct now!