reproducible, & possibly configurable rounding

[Josh Berkus]

On 05/07/2014 04:01 PM, Bradley M. Kuhn wrote:
> I originally characterized Tripun's project as a "implement fixed point
> arithmetic"; which I naïvely thought was the "only way to handle the
> rounding problem properly".  I've since been educated: there is more
> than one way to get this done right, and I think the way we're leaning
> is to clearly document options, why we picked the option we did, and
> (ideally) make Ledger-CLI more configurable on this point by the user.

???

Ledger is written in Python, which has the decimal type for fixed-point
arithmatic.  You shouldn't have to implement that.

I don't know of a non-cpu intensive method for handling splits, though.
 Here's the basic issue:

Imagine that a particular kind of revenue gets automatically split
evenly between 4 earmarks.  Then you receive $14.21 in that account.
How do you split it so that you don't lose a penny?

When I had to implement it before, my routine looked like this:

- split amount naively, rounding to two decimals (for USD)
- compare that against the original amount
- figure out the number of "lost" pennies
- reassign those pennies to the splits, based on random selection with
the probability of each penny being assigned to a split as the % of that
split, with no duplicates.

I can write some primitive python code up for this process if it'll help.


-- 
Josh Berkus
PostgreSQL Experts Inc.
http://pgexperts.com