scheduled loan payment setup question

Conrad Canterford conrad@mail.watersprite.com.au
24 Dec 2002 10:00:50 +1100


On Tue, 2002-12-24 at 07:52, Brian Smith wrote:
> On Mon, 23 Dec 2002 12:37:46 -0600
>   "John W. Eaton" <jwe@bevo.che.wisc.edu> wrote:
> >current principal.  This function would simply have to 
> >look up the
> >balance given an account name and then return
> >   balance * interest / 12
> Your solution is pretty close to 
> correct - it works for most mortgage loans, although car 
> loans are typically compounded daily and you would need to 
> use balance * interest / 365 * days since last payment. 

And this demonstrates the fundamental problem that any loan druid is
going to have. These two options may work for some (maybe even most)
people around the world, but they by no means cover all. In Australia,
we have loans that calculate interest daily, but credit it monthly, we
have loans structured around fortnightly payment and interest intervals,
fixed repayment loans (for which the function provided in the loan druid
works just fine) and probably a half a dozen more wierd combinations.
This is before we include rounding/truncation differences and other
oddities. The druid is not ever going to be able to cope with all this
reliably.

What I believe should be the next step then, is to allow user-provided
(scheme, probably) routines to be used to calculate the interest in the
druid - precisely what John was asking for. Unfortunately, unless there
is something I'm not aware of, there is no user interface to allow the
"plugging in" of such routines just at the moment.

So, one of you needs to raise a feature suggestion (enhancement) in
bugzilla for this.
(http://bugzilla.gnome.org/enter_bug.cgi?product=GnuCash)

Once that feature in implemented, we need to set up a "Contributions"
directory on the website where users can submit things like these scheme
routines for the general benefit of the whole Gnucash community.

I don't have any fantastic ideas as to where possible calculation
methods can be collected, and at the risk of making Josh and/or Chris
very irrate with me, I'm going to suggest that they also be raised in
bugzilla as "enhancements" - please mark them clearly as being
alternative interest calculation methods and be VERY thorough in
describing how the interest is calculated and credited in your situation
(and do please check the accuracy with your bank).
If you're going to do this, I'd suggest checking the already existing
ones to make sure yours is different to others (if it is the same, add a
"Me too" comment to the existing one). The URL to check on current
enhancement requests for SX is (as one line in browser):
http://bugzilla.gnome.org/query.cgi?product=GnuCash&component=Scheduled+Transactions&bug_severity=enhancement

And once again, I'll point out that even with all this, there will still
be a need to check the results against your statements, because of the
rounding/truncation issues and whatever other magic the banks perform in
these calculations. The numbers should be right to within a reasonable
margin, but they may not be exact.

Hope this answers the questions.

Conrad.