OK, here is a more serious proposal, comprised of simple, fast operations:
To hash a set, you take each of the items in the set, compute the hash
value, xor it with the golden number, and square it.
Add these results together.
Essentially, it's the sum of the squares of the hashes, the xor is thrown
in there primarily so common numbers like 0 have a meaningful effect on the
hash (without the xor there's no difference between #{0 1} and #{1}, for
example).
Here's Clojure code:
(declare my-hash)
(defn set-hash [coll]
(reduce + (for [item (seq coll)]
(let [hash-item (.intValue (bit-xor 0x9e3779b9 (my-hash
item)))]
(unchecked-multiply-int hash-item
hash-item)))))
(defn my-hash [i]
(if (set? i) (set-hash i) (hash i)))
Note that the .intValue call can go away if you convert this to Java and
use int xor rather than Clojure's xor.
This gives really nice spread-out results for similar sets:
=> (my-hash #{{1 2} 3})
1067103816
=> (my-hash #{{1 3} 2})
3094912306
=> (my-hash #{{1 4} {2 3}})
-3227863472
=> (my-hash #{{1 3} {2 4}})
2855562010
=> (my-hash #{[1 1] [2 2]})
3474272896
=> (my-hash #{[1 2] [2 1]})
-1060041718
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