> On Aug 28, 2015, at 8:17 PM, John Ralls <jra...@ceridwen.us> wrote: > > >> On Aug 28, 2015, at 6:43 AM, Geert Janssens <geert.gnuc...@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 _______________________________________________ gnucash-devel mailing list gnucash-devel@gnucash.org https://lists.gnucash.org/mailman/listinfo/gnucash-devel
