[swift-evolution] Pre-proposal: Safer Decimal Calculations

Michael Gottesman mgottesman at apple.com
Wed Mar 23 10:44:27 CDT 2016

> On Mar 23, 2016, at 5:26 AM, Rainer Brockerhoff via swift-evolution <swift-evolution at swift.org> wrote:
> On 3/22/16 23:20, Michael Gottesman via swift-evolution wrote:
>>> On Mar 18, 2016, at 3:42 PM, Rainer Brockerhoff via swift-evolution <swift-evolution at swift.org> wrote:
>>> First draft towards a tentative pre-proposal:
>>> https://gist.github.com/rbrockerhoff/6874a5698bb479886e83
>>> ------
>>> Pre-proposal: Safer Decimal Calculations
>>> Proposal: TBD
>>> Author(s): Rainer Brockerhoff
>>> Status: TBD
>>> Review manager: TBD
>>> ...
>>> Full Decimal type proposal
>>> There are cogent arguments that prior art/habits and the already complex
>>> interactions between Double, Float, Float80 and CGFloat are best left alone.
>>> However, there remains a need for a precise implementation of a workable
>>> Decimal value type for financial calculations. IMHO repurposing the
>>> existing NSDecimalNumber from Objective-C is not the best solution.
>>> As most experienced developers know, the standard solution for financial
>>> calculations is to internally store fixed-point values — usually but not
>>> always in cents — and then print the “virtual” point (or decimal comma,
>>> for the rest of us) on output.
>>> I propose, therefore, an internal data layout like this:
>>> UInt16 - position of the “virtual” point, starting at 0
>>> UInt16 - data array size - 1
>>> [Int32] - contiguous data array, little-endian order, grown as needed.
>>> Note that both UInt16 fields being zero implies that the number is
>>> reduced to a 32-bit Integer. Number literals in Swift can be up to 2048
>>> bits in size, so the maximum data array size would be 64, although it
>>> could conceivably grow beyond that. The usual cases of the virtual point
>>> position being 0 or 2 could be aggressively optimized for normal
>>> arithmetic operators.
>>> Needless to say such a Decimal number would accept and represent
>>> literals such as 0.01 with no problems. It would also serve as a BigNum
>>> implementation for most purposes.
>>> No doubt implementing this type in the standard library would allow for
>>> highly optimized implementations for all major CPU platforms. In
>>> particular, the data array should probably be [Int64] for 64-bit platforms.
>> Rainer: I quickly skimmed this. Just to make sure I am understanding 100%: you are proposing a fixed point decimal calculation or a floating point decimal calculation. The former, no?
> Right, fixed-point. (NSDecimalNumber is decimal floating-point, of course).

Ok. (That was the source of my confusion).

> I'll be updating my pre-proposal in a few days with the received feedback.

When you feel that this is ready, we should have Steve Canon +CC look at this.

> -- 
> Rainer Brockerhoff  <rainer at brockerhoff.net>
> Belo Horizonte, Brazil
> "In the affairs of others even fools are wise
> In their own business even sages err."
> http://brockerhoff.net/blog/
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