Henning Thielemann wrote:
It seems that algorithms on graphs can be implemented particularly
efficient in low-level languages with pointers and in-place updates. E.g.
topological sort needs only linear time, provided that dereferencing
pointers requires constant time. I could simulate pointer dereferencings
and pointer updates by Map yielding linear logarithmic time for
topological sort. I wonder if it is possible to write a linear time
topological sort using lazy evaluation, since the runtime system of
Haskell implementations is a graph processor based on pointers.
First of all, topological sorting is only linear time because the 32 or
64 bit used to label nodes aren't counted. Put differently, random
access in constant time to a collection of n elements doesn't exist.
That being said, we want to use arrays of course. Preferably in a
"whole-meal" way that doesn't involve incremental state updates. A few
minutes ago, I stumbled upon the lazyarray packages which points to
the paper
Thomas Johnsson.
"Efficient Graph Algorithms Using Lazy Monolithic Arrays"
http://citeseer.ist.psu.edu/95126.html
which offers such a way! (Although I currently don't quite understand
why this works, and these ad-hoc unique numbers bother me.)
Regards,
apfelmus
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
