Re: [PATCH] Improve geometric types

On 09/27/2018 07:21 PM, Alvaro Herrera wrote:
> If you look at the differing results carefully, there's this one:
>
> *** 3249,3255 ****
> ! [(0,0),(3,0),(4,5),(1,6)] | (-5,-12) | [(0,-0),(-15,-36),(40,-73),(67,-42)]
> --- 3249,3255 ----
> ! [(0,0),(3,0),(4,5),(1,6)] | (-5,-12) | [(0,0),(-15,-36),(40,-73),(67,-42)]
>
> (Third column is first multiplied by second).
>
> I wonder why the expected file has a -0 only in the second position and
> not both first and second. These are both positive zeroes being
> multiplied by a negative number. Why is 0 * -12 = -0 yet 0 * -5 = 0?
> What is going on? Is the sign suppressed for negative zeros only in the
> first coordinate? I suppose this is just a side effect of how
> float8_mi, _pl, _mul work (in point_mul_point).
>
> Anyway maybe your test case should use more of the float8 op
> combinations in order to show the difference.
>

I may be missing what you're saying, but point_mul_point is not just a
simple multiplication of coordinates, i.e.

(x1,y1) * (x2,y2) != (x1*x2, y1*y2)

It essentially does this:

((x1 * x2 - y1 * y2), (x1 * y2 + x2 * y1))

so I wouldn't be surprised if this was a difference between _pl and _mi.

regards

--
Tomas Vondra http://www.2ndQuadrant.com
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