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 >> <[email protected]> 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). I'll be updating my pre-proposal in a few days with the received feedback. -- Rainer Brockerhoff <[email protected]> Belo Horizonte, Brazil "In the affairs of others even fools are wise In their own business even sages err." http://brockerhoff.net/blog/ _______________________________________________ swift-evolution mailing list [email protected] https://lists.swift.org/mailman/listinfo/swift-evolution
