Proposal for modifying gnucash to use exact quantities
Phillip J Shelton
shelton@usq.edu.au
Tue, 25 Jul 2000 10:16:44 +1000
Not really up on what is available my self so my comment is `Are there any engines
out there that already do this type of math?
And would it be safe to say you are looking at some kind of integer math?
Phill
Bill Gribble wrote:
> 3. gnc_numeric : representing exact numbers
> -------------------------------------------
>
> The gnc_numeric data type represents exact quantities such as monetary
> values and prices. Its purpose is to provide an arithmetic engine for
> dealing with exact quantities which can be used to handle financial
> computations in gnucash.
>
> Requirements: [modified from a posting to the list]
>
> 1. Maximum magnitude. We should be able to represent very large
> monetary sums as exact integer quantities of
> smallest-currency-units. A rule of thumb is that we should be
> able to represent national budgets to the SCU
> (smallest-currency-unit) with several orders of magnitude to
> spare, even in very small-valued currencies.
>
> 2. Genericity. We must be able to handle fixed-point operations
> including addition, subtraction, multiplication, and division for
> numbers in different units. SCU values must be allowed to be
> non-decimal fractions of the currency (such as share prices in
> 64ths of a dollar, or old-style English pence/shilling/pound
> currency units).
>
> We must be flexible enough to allow a fixed-point "number of
> shares" (perhaps represented as an integer number of
> thousandths-of-a-share) to be multiplied by a fixed-point "price
> per share" (perhaps represented as an integer number of
> 64ths-of-a-dollar) to get a fixed-point "total transaction value"
> (perhaps represented as an integer number of pennies).
>
> 3. Rounding/truncation control. When performing multiplication and
> division between fixed-point quantities, rounding/truncation may
> be performed if the desired denomination of the output differs
> from the input (as in the price * num = total example above).
> There must be a mechanism to control when and how
> rounding/truncation is done, if there are several choices.
>
> 4. Intermediate results. If an overall computation does not
> overflow the bounds of the representation, the computation must
> be performed correctly and without an error flag even if
> intermediate results might overflow. Where overflow is possible
> in intermediate results, an extended-range format must be used
> temporarily to store intermediate results.
>
> 5. Input/output conversion. There must be an interface API which
> will allow for conversion to and from common C data types
> with a mechanism for signaling overflow/underflow and
> loss-of-information in the conversion.
>
> 6. Compatibility with existing codebase. The implementation of
> a new numeric format must be compatible with both the C and
> Scheme portions of gnucash.
>
> 7. Abstraction. Since we have made at least one fundamental mistake
> in specifying the original representation of monetary values in
> gnucash, we have to assume that we may make others. We are going
> to have to do significant surgery on gnucash to remove instances
> of double math using built-in operators; by all means, let's
> replace *all* such math with a c-function-based API that can have
> its implementation repaired at a later time, even if some simple
> operations (such as addition) can be implemented directly as C
> operations on "long long" (if that happens to end up as the
> underlying representation).
>
> 8. Fixed storage size. Considering that the gnucash engine may at
> some time have a database backend, it's important to have a
> numeric format that has a fixed storage size and (optimally) can
> be directly operated on by SQL operators.
>