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

[Dany St-Amant]

> Le 18 mars 2016 à 18:42, Rainer Brockerhoff via swift-evolution <swift-evolution at swift.org> a écrit :
> 
> First draft towards a tentative pre-proposal:
> https://gist.github.com/rbrockerhoff/6874a5698bb479886e83
> ------
> 
> 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.

By using a table of signed Int32, I think you'll lose one bit every time the number is extended.
Should the sign of the number be hidden in the data array size, either as the sign or as the lsb?
- second field = (data array size - 1) << 1 | (negative ? 1 : 0)

Dany