From: | Dean Rasheed <dean(dot)a(dot)rasheed(at)gmail(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, Lætitia Avrot <laetitia(dot)avrot(at)gmail(dot)com>, Alvaro Herrera <alvherre(at)2ndquadrant(dot)com>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: [Patch] Log10 and hyperbolic functions for SQL:2016 compliance |
Date: | 2019-03-11 11:58:17 |
Message-ID: | CAEZATCUVK18HxMp4Xz_XuGKNZiKG05oPHTvkVYGMdKFuo4XYDg@mail.gmail.com |
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On Sun, 3 Feb 2019 at 15:12, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
> Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
> > The spec doesn't require the inverse functions (asinh, acosh, atanh),
> > but surely there is no principled reason to omit them?
>
> +1 --- AFAICS, the C library has offered all six since C89.
>
+1 for including the inverse functions. However, it looks to me like
the inverse functions are C99-specific, so they might not be available
on all supported platforms. If they're not, we may need to provide our
own implementations.
I did a bit of research and had play. It looks like it might not be
too hard to provide our own implementations, but it does take a bit of
care to avoid rounding and overflow errors. Attached are some
standalone C programs where I tested various algorithms. A decent
approach seems to be to either use log1p() (which is itself
C99-specific, hence that's the first thing I played with) or to use a
single round of Newton-Raphson to correct rounding errors, in a
similar way to how we implement cbrt() on platforms that don't have
that.
Of course that may all be moot -- those functions may in fact be
available everywhere we care about, but it was interesting to play
around with them anyway.
Regards,
Dean
Attachment | Content-Type | Size |
---|---|---|
log1p.c | text/x-csrc | 7.3 KB |
asinh.c | text/x-csrc | 5.1 KB |
acosh.c | text/x-csrc | 4.2 KB |
atanh.c | text/x-csrc | 5.2 KB |
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