canadian mortgages in the SX editor

[Ludovic Nicolle]

Hey guys,

so this is my first post, so be indulgent if possible. thanks.

As a canadian user trying to use the Loan Druid and SX editor, one 
feature that was missing was the ability to specify a nominal interest 
rate that has an implicit compounding frequency that is different than 
the actual compounding frequency of the interest paid.
(Yes I know, this sounds weird, but this is the way canadian mortgage 
rates work, probably because 200 years ago doing a multiplication was 
more costly than actually building the house, so they reduced the 
number of calculations done.)

Anyway, there is a somewhat easy way to translate the canadian posted 
rate to an effective periodic rate. One can use that rate with the 
"standard" formula and get the good results, but you have to know the 
proper formula, and enter it properly in the Sx editor, and know scheme 
keywords. Not very user friendly.

So I have created a patch that contains 4 functions. One to do that 
translation of nominal to effective rates based on the 2 compounding 
frequencies. Three other functions are the cpd equivalent of the 
gnc:pmt, gnc:ipmt and gnc:ppmt functions, with the conversion builtin, 
so it's easier to use in the SX editor.

One more feature that was bugging me while working on this is that the 
sum of interest and principal was often not equal to the payment. So 
the the 3 cpd_ functions in the patch have a builtin rounding at the 
2nd decimal. This precision is hardcoded, so i don't really like it, 
but I think it can be fixed in a later patch.

So i created this bug http://bugzilla.gnome.org/show_bug.cgi?id=168700 
with the patch attached.

jsled, this is your realm, so I guess you are the one who should give 
me the most feedback. ;-)

Ludo