[Haskell-cafe] Data structure to manage collection of sets with
efficient lookup, intersection?
Patrick.Surry at portraitsoftware.com
Mon May 5 08:26:35 EDT 2008
New to Haskell, with a mental block about how to represent this
I have an unknown function f which is defined on subsets of some
universal set (say integers 1...N). I know the values of f for some
subsets, and using those can infer values on other subsets.
So what I need is a way to manage a collection of subsets s_i (and the
associated values f(s_i)) so that I can efficiently (a) check whether a
subset s is already 'known' in my collection, and (b) find all subsets t
in the collection that intersect s.
In a "traditional" language, I'd likely create a dictionary with keys
s_i containing the f(s_i) as values, along with a separate dictionary
keyed on the elements of the universal set (1...N) in which each entry
is a list of all s_i containing the given element of the universal set.
Together they let me check, given a subset s, whether I know f(s), and
also get the list of all known subsets intersecting s (by the union of
the lists of sets containing each element of s).
I can't quite wrap my head around how to do this efficiently in Haskell,
maybe because of the way the above is duplicating information about the
subsets across two different data structures?
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