util/decimal: Investigate EricLagergren/decimal package

[nvanbenschoten]

This library looks really good, and could be a justifiable alternative to our current inf/dec library.

Specially, reasons to consider switching libraries are that this new package:

• stores small integers inside a compact int64, speeding up operations and potentially avoiding allocations
• includes support for a larger number of arithmetic methods (Log, Exp, etc.)
  so we could remove our custom implementations
• uses a Context to define how arithmetic operations react (common in a lot of other languages)
• even comes with a FizzBuz function!

Either way, we should continue to monitor golang/go#12127.

[ericlagergren]

Hey! Just noticed this. Feel free to ask about anything you want. :)

[tamird]

Also, our current decimal library appears not to handle scientific notation, while PG does. EricLagergren/decimal also handles scientific notation, which is good.

[tamird]

@ericlagergren it seems that you don't have an equivalent of https://godoc.org/gopkg.in/inf.v0#Dec.UnscaledBig, which we need for our serialization of decimals.

[ericlagergren]

@tamird That could easily be added. Unless the decimal spec (http://speleotrove.com/decimal/decarith.html) has a name for the method, I'd probably lean towards a Mantissa method or something like that. But, yeah, could be easily added.

[tamird]

I haven't read the spec, but Mantissa SGTM.

[tamird]

Note: there are some optimizations in 72a5c19 which are baked into github.com/EricLagergren/decimal and should be removed when this is implemented.

[nvanbenschoten]

apd is CockroachDB's new Arbitrary-precision decimal library. The library fully replaced inf.Dec in #13551, and will be the decimal package we use going forward.

[tamalsaha]

@nvanbenschoten, do you mind elaborating why write a new library? I am trying to find a big.Decimal lib for some financial calculation. So, I am curious.

[ericlagergren]

@tamalsaha from a reddit thread: https://np.reddit.com/r/golang/comments/5zkzrq/apd_an_arbitraryprecision_decimal_package/dezaw3v/

[nvanbenschoten]

@tamalsaha the reddit thread and blog post Eric linked to does a pretty good job explaining our rationale. The decision had less to do with specific shortcomings in other libraries and more to do with independent requirements for our use case, like panic-free operation and configurable precision.