Re: faster representation of pow on some áreas of subsurface?

[Linus Torvalds]

On Sun, Jan 19, 2014 at 1:41 PM, Dirk Hohndel <dirk at hohndel.org> wrote:
>
> OK, I compared the results for a number of test dives and they are identical.

They had better be. The "Stop doing the (very expensive) pow()
calculation pointlessly" patch should be mathematically identical to
what we did before, just written slightly differently. So at most, it
might cause some single-ULP difference due to different order of
operations or similar. But it does the exact same deco calculations,
just does them more efficiently.

> So I’ll take that old patch. Next I’ll look at the test patches.

So the two test patches actually cause different math to be done, and
might thus show slight differences. I don't think they'll show
anything that is visible to the human eye, though.

For example, when we don't do the deco calculation every second
(instead just doing it every sample), that means that we now no longer
do the depth interpolation between samples:

    int depth = interpolate(entry[-1].depth, entry[0].depth, j - t0, t1 - t0);

goes away, and we just do the "add_segment()" with the end entry
depth, and the time between entries. Now, we *have* done depth
interpolation to get the actual dive-plot entries (see
populate_plot_entries()), but that is done at 10s intervals rather
than 1s intervals.

But quite frankly, while the math changes, I don't think it really
makes much sense to do depth interpolation (and deco calculations) at
1s segments. There's really nothing that says that our interpolated
values are any better than the sample values, so..

As far as I can tell, the only real reason for the 1s interpolation
was that we originally had a deco model that *only* worked in 1s
increments. You added the ability to do non-1s periods in commit
2c336032568 ("Consider previous dives when calculating deco"), because
then you want to take the surface interval into account (and doing
*that* one second at a time is insane), but the "within the dive" deco
was doing that 1s interpolation because of the old limited interface.

                     Linus