linear/relative or binary search algorithm?
Milian Wolff
mail at milianw.de
Mon Oct 26 16:39:22 UTC 2009
On Monday, 26. October 2009 19:13:54 Jonathan Schmidt-Dominé - Developer
wrote:
> Hi Milian!
>
> > I have now implemented my two ideas, i.e. a relative search (relative to
> > the last search) and a binary search. Of course the latter is faster for
> > random access, while the former is faster for linear access.
> >
> > To decide which one to use I'm now running callgrind on each of the three
> > (i.e. also the initial algorithm) on the huge phpfunctions.php file.
> >
> > Thank god I have access to always-on machines at my university :)
> >
> > So far I'd say: Implement binary search as it's always fast. I doubt the
> > additional speed in the linear/relative algorithm is significant together
> > with the other stuff going on during a declaration build.
>
> Binary-Search is complicate. Iterative usage is a common usecase and a
> small if with a comparison and a + or - is very cheap. But try it out. ;)
>
> > I'll publish the results tomorrow (lets hope it finishes in ~12h ;-) ).
> > Maybe we can find some more bottlenecks this way :)
> >
> > Oh and please review my implementation of the binary search algorithm,
> > you can find it here:
> >
> > http://websvn.kde.org/trunk/playground/devtools/kdevelop-pg-
> > qt/tests/benchmarks.cpp?revision=HEAD&view=markup
> >
> > Note that I never wrote such an algorithm before, hence feedback is much
> > appreciated. Esp. the part where I increase/decrease the iterator by
> > float(upperBound - lowerBound)/2. Is that OK?
> >
> > As you can see in the benchmarks, I've added some tests to verify that
> > all algorithms return the same results.
> >
> > I'd be also interested in testing whether it's significantly slower (if
> > not even faster!) to use e.g. QVector or QList together with qBinaryFind,
> > or a std::vector with a binary_search. Of course that would mean I'd have
> > to benchmark _filling_ the lookup table as well.
>
> qBinaryFind would also work with pointers. But it searches for exact
> matches,, positionAt does not do the same.
True, could be that we cannot use existing functions... hmm!
--
Milian Wolff
mail at milianw.de
http://milianw.de
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