Обсуждение: Unsigned 64 bit integer to numeric

Hi,

Probably a simple question, but I don't see a simple answer so far. In
one extension I want to convert uint64 into a numeric to put it
eventually into a jsonb object. As far as I see in numeric.c there are
functions only for signed int64. Is there a way to achive this with
uint64 (without duplicating significant part of numeric implementation
in the extension)?

>>>>> "Dmitry" == Dmitry Dolgov <9erthalion6@gmail.com> writes:

 Dmitry> Hi,

 Dmitry> Probably a simple question, but I don't see a simple answer so
 Dmitry> far. In one extension I want to convert uint64 into a numeric
 Dmitry> to put it eventually into a jsonb object. As far as I see in
 Dmitry> numeric.c there are functions only for signed int64. Is there a
 Dmitry> way to achive this with uint64 (without duplicating significant
 Dmitry> part of numeric implementation in the extension)?

Sure. Flip the top bit; convert the value as if signed; then subtract
-(2^63) from the result. (Easier to subtract -(2^63) than to add 2^63,
since the former can itself be represented in a signed int64 for easy
conversion to numeric.)

-- 
Andrew (irc:RhodiumToad)

> On Wed, Dec 04, 2019 at 11:49:20AM +0000, Andrew Gierth wrote:
>
> >>>>> "Dmitry" == Dmitry Dolgov <9erthalion6@gmail.com> writes:
>
>  Dmitry> Hi,
>
>  Dmitry> Probably a simple question, but I don't see a simple answer so
>  Dmitry> far. In one extension I want to convert uint64 into a numeric
>  Dmitry> to put it eventually into a jsonb object. As far as I see in
>  Dmitry> numeric.c there are functions only for signed int64. Is there a
>  Dmitry> way to achive this with uint64 (without duplicating significant
>  Dmitry> part of numeric implementation in the extension)?
>
> Sure. Flip the top bit; convert the value as if signed; then subtract
> -(2^63) from the result. (Easier to subtract -(2^63) than to add 2^63,
> since the former can itself be represented in a signed int64 for easy
> conversion to numeric.)

Indeed, looks like this does the trick, thank you!