On Jan 4, 2011, at 5:27 PM, Christopher Jefferson wrote:
On 4 Jan 2011, at 16:39, Howard Hinnant wrote:
On Jan 4, 2011, at 11:05 AM, Anthony Williams wrote:
Howard Hinnant <howard.hinnant@gmail.com> writes:
On Jan 4, 2011, at 6:22 AM, Christopher Jefferson wrote:
On 4 Jan 2011, at 03:40, Howard Hinnant wrote:
Warming this up for tr2:
http://home.roadrunner.com/~hinnant/combinations.html
Comments welcome. Real world use welcome.
For TR2, it would be nice to produce a range object, so we could write:
for(perm: reversible_permutation(first, mid, last) ) f(perm);
Not positive, but I believe (if one could figure out how to code it) would suffer the exponential cost of incrementation demonstrated for next_partial_permutation and next_combination in "Performance Considerations" shown in:
Couldn't the range adaptor object --- or the iterators generated from it --- store the intermediate information about where in the series of swaps it was? Sort of a continuation in your for_each_permutation() functions.
I don't know. Show me the code. :-)
I am currently working on the code. I believe I am getting similar performance. However, I currently have a very heavy-duty iterator (it contains a heap-allocated array, effectively storing the stack of an algorithm like yours). Very important that it doesn't get post-incremented!
Cool, and kudos! :-) I suspect that the ultimate solution to this problem will be writing combine_discontinuous and permute in a non-recursive manner, and then saving state as you are doing. If they are non-recursive that will eliminate the need for storing the stack of states and thus the heap allocation. I tried but failed after running out of time on this endeavor. Warning: it's addictive. ;-) And to get reversible permutations going you need combine_discontinuous3 (or its equivalent) in a non-recursive manner. Ultimately the logic needed here is the ability to call the functor for all permutations of 3 distinct ranges, treated as if they were one continuous range. Then you could skip the combination step altogether. Again, I ran out of time to work this direction. Anyway, I'm glad you're working on this. for_each_* can easily be built out of next_* if there is no performance penalty for doing so. But the reverse is not true, at least as far as I've discovered. And if I had my druthers, I'd like to be able to choose either the next_* or for_each_* API without performance concerns, depending only upon my application (whether or not I needed to break out of the loop early). -Howard