Decimal floating-point

[stephentyrone]

Implement IEEE 754 Decimal128 (and possibly Decimal64 as well, though it's less important). This has a dependency on fixed-width integer types because we want to be able to leverage UInt128 arithmetic for the implementation.

[paulrs]

We would like to second the request for this feature. For financial applications where values will not exceed the limits of UInt128 and where precision in values is key, having a Decimal128 implementation (or Decimal64) would be very helpful.

[djehrlich]

I agree with @paulrs that Decimal128 would be extremely valuable for financial applications.

[jasonbobier]

Especially since Foundation's Decimal causes a fatal error in this simple case:

let d = Decimal(exactly: 0)

and Apple seems unwilling to fix it (I reported it January 17, 2021).

[xwu]

@jasonbobier The fatal error was simply that the API was unimplemented; it's been implemented in the open-source swift-corelibs-foundation for some years now and Apple's proprietary implementation will pick up the changes on some timeline of their choosing.

[jasonbobier]

@xwu sigh "Some timeline of their choosing" is accurate. I have bugs from 2003 still open with them.

[KTSnowy]

Is there any update on this?

[mgriebling]

I have an implementation of this including Decimal32, Decimal64, and Decimal128. Decimal32 is mostly tested but the other implementations require testing. This is a pure Swift implementation based on the Intel Corp Decimal Floating-Point Math Library v2.2. BTW, this is a IEEE 754 implementation.
It supports both BID (Binary Integer Decimal) and DPD (Densely Packed Decimal) encodings. IEEE 754 allows both encodings.

How would I upload this code so others can contribute? See https://github.com/mgriebling/MGDecimal for details.