#20669: LatticePoset: add function to get all sublattices
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Reporter: jmantysalo | Owner:
Type: enhancement | Status: needs_work
Priority: major | Milestone: sage-7.3
Component: combinatorics | Resolution:
Keywords: latticeposet | Merged in:
Authors: Jori Mäntysalo | Reviewers: Travis Scrimshaw
Report Upstream: N/A | Work issues:
Branch: | Commit:
u/jmantysalo/all_sublattices | 1bb87bd775eece24a586b9ccedb7dfd6ca0b5488
Dependencies: | Stopgaps:
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Comment (by jmantysalo):
Replying to [comment:3 tscrim]:
> I think you should use a backtracking algorithm instead of a recursion.
It will be faster (and you don't have to constantly get things like
`self.cardinality()`), never run up against the Python maximum recursion
limit, and have better input (which `set()` and `0` should be defaults
anyways).
Maximun recursion limit is 999 by default, and that is too small only if
we have a `ChainPoset(n)` with `n > 999`; recursion stack will have at
most as many steps as the lattice has elements. And it will have `2^999`
sublattices anyway, so this is not valid argument.
For example after `B4 = Posets.BooleanLattice(4)` it takes 0,02 seconds to
count 732 sublattices with
`len(list(B4._hasse_diagram.sublattices_iterator(set(), 0)))`, whereas it
took 1,13 seconds with `len(B4.sublattices())`. So if one wants to
optimize lattices, then bottleneck is usually not in `hasse_diagram.py`
but in `lattices.py`.
Of course I can change this to use backtracking, but... Then why not move
this to static sparce graphs class, if we really want as fast code as
possible?
--
Ticket URL: <http://trac.sagemath.org/ticket/20669#comment:4>
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