Nathan Wagner <nw...@hydaspes.if.org> writes: > On Wed, Nov 04, 2015 at 12:51:52PM -0500, Tom Lane wrote: >> I'm not very impressed with the first patch: it might save a few >> geqo_randint() calls, but it seems to do so at the price of making the >> swap choices less random --- for instance it sure looks to me like the >> last array element is now less likely to participate in swaps than >> other elements. Unless you can prove that actually the swapping is >> still unbiased, I'm inclined to reject this part.
> If I have understood the original code correctly, we need to select two > different random integers between 0 and num_gene-1, inclusive. That > happens to be num_gene possible results. > Having chosen the first one, which I will call "swap1", we now only have > num_gene-1 possible results, which need to range from either 0 to > swap1-1 or from swap1+1 to num_gene-1, which is num_gene-1 possible > results. I treat this as a single range from 0 to num_gene-2 and > generate a number within that range, which I will call "swap2". > If swap2 is between 0 and swap1-1, it is in the first range, and no > adjustment is necessary. If it is greater than or equal to swap1, then > it is in the second range. However the generated swap2 in the second > range will be between swap1 and num_gene-2, whereas we need it to be > between swap1+1 and num_gene-1, so I add one to swap2, adjusting the > range to the needed range. Ah, after thinking some more, I see how that works. I tend to think that your other proposal of swap1 = geqo_randint(root, num_gene - 1, 0); swap2 = geqo_randint(root, num_gene - 2, 0); if (swap2 === swap1) swap2 = num_gene - 1; would be clearer, since only the forbidden case gets remapped. However, really the whole argument is moot, because I notice that geqo_mutation() is only called in the "#ifdef CX" code path, which we don't use. So there's little point in improving it. (There's a fair amount of dead code in /geqo/, which I've never had the energy to clean up, but maybe we should do that sometime. It seems unlikely that anyone will ever be interested in experimenting with the ifdef'ed-out code paths.) regards, tom lane -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers