Am Freitag, 6. Mai 2005 02:24 schrieb Greg Buchholz:
Josef Svenningsson wrote:
I think the thing that really kills you is the way you shuffle around the list in the advance function. Your commented out rotations function has the same problem. Here is my attempt to solve the problem a little more efficiently:
We're heading in the right direction anyway. I can now compute 1 million iteration in about 2 minutes (with +RTS -H750M -K100M). Well almost, since it now doesn't compute the right answer, so something must be amiss in the shuffling section. Now if we can get it to us a little less than 1G of memory, we'll be in good shape.
Thanks,
Greg Buchholz
The problem is that a prime crept in in Josef's code, so to calculate the positions and velocities, the updated versions of the planets are used, it should be update f newlist (a:as) = a' : update f (newlist . (a:)) as where a' = f a (newlist as) instead of update f newlist (a:as) = a' : update f (newlist . (a':)) as where a' = f a (newlist as). Besides, offset_momentum does not use the parameter n at all and I think that all the indexing in the computation of the potential energy is rather inefficient. I rewrote it as myEnergy :: [Planet] -> Double myEnergy pps@(p:ps) = kinetic - (pot pps) where kinetic = 0.5 * (sum $ map (\q -> dot (vel q) (vel q) * mass q) pps) pot = sum . pots pots [] = [] pots (p:ps) = sum [m * (mass q) / (dist p q) | q <- ps]: pots ps where m = mass p, it didn't make it any faster, though. I did not yet space-profile Cheers, Daniel