Most pythonic way of rotating a circular list to a canonical point

Oscar Benjamin oscar.j.benjamin at gmail.com
Sun Aug 2 01:10:07 CEST 2015


On Sun, 2 Aug 2015 00:05 Oscar Benjamin <oscar.j.benjamin at gmail.com> wrote:


On Sat, 1 Aug 2015 22:06 Lukas Barth <mail at tinloaf.de> wrote:


Nice idea! But I actually really need those "canonic rotations", since I'm
hashing them somewhere.


Do you really need the canonical rotation or just a hash that is invariant
under rotations?

I don't know of a solution to the former that is better than what you
already have but the latter is an easier problem: Find the minimum element.
Compute the hash of the rotated sequence for each occurrence of the least
common element. Add those hashes together or multiply them or some similar
operation. That's your hash that will compare equal for any rotation of a
given sequence.



I said "least common" but meant "minimum". A potential improvement would be
to use the minimum element of the least common elements instead of just the
minimum element. Presumably in most cases there would be at least some
unique elements so if you take the minimum of those then you would only
need to compute 1 sequence hash. Maybe this is unnecessary though.

--
Oscar
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