While searching for MultiSet or Bag resources, I found this implementation by Achim Passen at: A simple multiset/bag implementation for Clojure.
However, I found I could meet my needs by adding functions to treat
vectors, or other collection types, as unordered. These can then be used
directly, or in the definition of your own specialized types.
For the hash, from Clojure Data Structures:
(defn hash-unordered [collection]
(-> (reduce unchecked-add-int 0 (map hash collection))
(mix-collection-hash (count collection))))
For equality:
(defn equals-unordered [coll-a coll-b]
"Treat collections as unordered for 1st level of comparison."
(or (identical? coll-a coll-b)
(and (empty? coll-a) (empty? coll-b))
(and (= (hash-unordered coll-a) (hash-unordered coll-b))
(let [set-a (set coll-a)
set-b (set coll-b)]
(and (= set-a set-b)
(loop [[item & items] (seq set-a)]
(let [finder #(= item %)
found-a (filter finder coll-a)
found-b (filter finder coll-b)]
(if (not= (count found-a) (count found-b))
false
(if (empty? items)
true
(recur items))))))))))
Neither Achim's deftype, nor the above, is likely as efficient as
implementation as a core collection MultiSet.
The attached file has a trivial implementation of a deftype showing the
functions in use. It defines a shopping cart where the items are kept on a
vector, which is treated as an unordered collection. A transcript of
playing with the shopping cart follows:
user=> (load-file "shopping_cart.clj")
#'example.multi-set/make-shopping-cart
user=> (use 'example.multi-set)
nil
user=> (def cart0 (make-shopping-cart 'carrots 'beans 'eggs))
#'user/cart0
user=> cart0
(make-shopping-cart carrots beans eggs)
user=> (def cart1 (make-shopping-cart 'eggs 'carrots 'beans))
#'user/cart1
user=> cart1
(make-shopping-cart eggs carrots beans)
user=> (def cart2 (make-shopping-cart 'ham 'eggs 'carrots 'beans))
#'user/cart2
user=> cart2
(make-shopping-cart ham eggs carrots beans)
user=> (= cart0 cart1)
true
user=> (= cart1 cart2)
false
Greg
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shopping_cart.clj
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