Have fewer if conditions for straight guide propagation.

[mtbc]

Reduces "if" conditions in Python code in an attempt to reduce register consumption or warp divergence.

In practice, I don't see any improvement, but opening this PR in case it forms a useful basis for further work.

[mtbc]

So, here, if I try,

is_better = vdots[i] < 0 and best_time > times[i]
if is_better:
    best_time = times[i]
    side = i

then it's still (relatively) fast but with,

is_better = vdots[i] < 0 and best_time > times[i]
best_time = is_better and times[i] or best_time
side = is_better and i or side

it becomes extremely slow. Have you any tips, @ckendrick?

[ckendrick]

There might be an issue with how side is being set. I ran into an issue where I was setting the side to an incorrect value which caused neutrons to get stuck in the loop for all max_bounces number of iterations. If you set max_bounces = 100 and it is significantly faster, then this is probably the issue. Also, running test_guide.py in interactive mode should show differences if the side is incorrect.

Sorry I haven't opened a PR yet for the optimized tapered guide. The equivalent loop from mine is

i = 0
for k in range(4):
    side_hit = vdots[k] < 0.0
    t2 = times[k] / vdots[k]
    best_time = side_hit and t2 < t1
    i += best_time * int(k + 1)
    t1 = ((not best_time) * t1) + best_time * t2
if i == 0:
    break

[mtbc]

Aha, thank you. Nice trick with using the numerical aspect of the truth value more directly than I.

And, yes, reducing max_bounces correspondingly speeds the run. Maybe my two alternatives aren't logically equivalent and I've yet to spot why!

[mtbc]

I ran into an issue where I was setting the side to an incorrect value ...

and so was I,

... my two alternatives aren't logically equivalent ...

so they weren't. 😃

Will push a followup commit but still doesn't improve performance significantly.