Rounding in the price db.

[David Carlson]

On 9/1/2015 5:13 PM, John Ralls wrote:
>> On Aug 28, 2015, at 8:17 PM, John Ralls <jralls at ceridwen.us> wrote:
>>
>>
>>> On Aug 28, 2015, at 6:43 AM, Geert Janssens <geert.gnucash at kobaltwit.be> wrote:
>>>
>>> On Friday 28 August 2015 08:55:53 John Ralls wrote:
>>>> I’ve pushed a feature branch, single-price, to my Github repo
>>>> (https://github.com/jralls/gnucash <https://github.com/jralls/gnucash>) which covers most of what we’ve
>>>> discussed here. I’m still wrestling with the math of how to sensibly
>>>> handle rounding itself, so what’s there still uses the hard-coded
>>>> 10^6 denom. The branch is from maint because I’d like to put these
>>>> changes in 2.6.8.
>>>>
>>>> The actual changes are explained in the commit notes. In my limited
>>>> testing it appears to work and to provide stability when doing
>>>> multiple transactions with the same exchange rate. Please test some
>>>> more; I’m sure I didn’t think of every possible variation.
>>>>
>>> Hi John,
>>>
>>> Thanks for your work on this.
>>>
>>> From my first tests and reading the code I have the following observations:
>>>
>>> - Let me start with a nit-pick: while reading through the commits I got confused by the return values of check_account and check_edit. They return TRUE if the check fails (that is when anything is not ok to continue with a transfer). From a distance that seems backwards. I usually expect a check function to return TRUE if all checks pass correctly.
>>>
>>> - Next I created a vendor bill, posted it and then paid it in a foreign currency. This also pops up the transfer dialog. I entered a price and continued. This added the price to the db (as expected). Next I remove the payment transaction (from the bank account in the foreign currency) and issue a new payment via the payment dialog, again in the same foreign currency. This time however, I enter a to amount directly (ensuring it would result in a completely different price). Check the price db and note the existing price hasn't changed.
>>>
>>> - The currencies I was playing with are € (from currency) and HKD (to currency). Before your changes my price db listed a HKD security EUR currency. On your branch the code now adds a EUR security in HKD currency. That change is fine in itself. I prefer your normalized way to store (currency) exchange rates in the db. The issue with this however is that F::Q won't return an exchange rate for the new price, while it did (and still does) for the old one.
>>>
>>>> As for the math, here’s the conundrum: I proposed earlier to base the
>>>> rounding on what would make a 1 scu change in the “to” commodity. The
>>>> problems with that idea are that it depends entirely on the amount in
>>>> the “from” commodity and that prices are often quoted in fractions of
>>>> a scu. For example, the Wall Street Journal website quotes the Yen at
>>>> 120.98 to the USD. The Yen’s scu is 1, and the change in the rate to
>>>> make a 1¥ change in the value is different if the USD amount is $10
>>>> from what it would be if the amount was $1000. Carry that to its
>>>> illogical conclusion and we need infinite precision, and that’s
>>>> ignoring the fact that we need infinite precision to exactly
>>>> represent a lot of rational fractions, but since all the real money
>>>> systems use decimal math nowadays that’s not really germane.
>>>>
>>>> So I have a new proposal: If the commodities are both currencies,
>>>> store exchange rates in the direction where the rate > 1, set the
>>>> denominator to 1000, and round-half-up. The price retrieval code
>>>> already checks in both directions. If only one of the commodities is
>>>> a currency then it’s a price and we store it in the currency with the
>>>> denominator = the currency’s scu * 10000.
>>>>
>>> See above: the check in both directions is not reliable/borked.
>>>
>>>> That leaves commodity-commodity prices. The most common example in
>>>> modern life is stock-for-stock exchanges resulting from mergers or
>>>> spin-offs. These tend to be one-offs, so no rounding required. Barter
>>>> exchange, where one exchanges one commodity for another (e.g. two
>>>> bushels of corn for a cow), is similarly fractional rather than
>>>> decimal, so again not rounding is appropriate. The third case is the
>>>> problem: Bitcoin and similar pseudo-currencies. For maint I think
>>>> we’re going to have to leave those prices unrounded as well, but
>>>> perhaps for master we should consider creating a separate commodity
>>>> category so that users can create commodities that GnuCash treats as
>>>> currencies.
>>>>
>>> That looks like a very sensible proposal to me.
>> Geert,
>>
>> Thanks for testing. I agree that the check_foo() semantics are clumsy. I did it that way to avoid negating the return value in the if conditional, but in retrospect that would be clearer, so I’ll flip it.
>>
>> Roger that the checks aren’t reliably bidirectional. I’ll dig into that. I hadn’t yet changed anything with regards to which direction prices are recorded, at least not on purpose, so I’ll have to track that down too.
>>
>> I coded up the price-rounding algorithm on the flight back today and played with it a little. I think it may need some adjustment.
> I’ve pushed more changes to single-price which I think address Geert’s comments and some tweaks to maximize significant digit preservation while keeping denominators <= 10E6 in most cases. Please test some more!
>
> Regards,
> John Ralls
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I kind of lost track of how it is supposed to work for the user.  If I
am slow to enter a buy or sell for a certain date, will GnuCash
overwrite a price that was downloaded earlier for the same security and
date or possibly manually entered earlier for that date?  Sometimes I
create a buy or sell by duplicating a transaction that has the wrong
information then do several transaction saves as I am changing to the
correct accounts, amounts and date. 

David C