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  1. +115 −0 README
  2. +396 −0 src/main/clojure/clojure/data/priority_map.clj
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+A priority map is very similar to a sorted map,
+but whereas a sorted map produces a
+sequence of the entries sorted by key, a priority
+map produces the entries sorted by value.
+In addition to supporting all the functions a
+sorted map supports, a priority map
+can also be thought of as a queue of [item priority] pairs.
+To support usage as a versatile priority queue,
+priority maps also support conj/peek/pop operations.
+## Usage
+The standard way to construct a priority map is with priority-map:
+user=> (def p (priority-map :a 2 :b 1 :c 3 :d 5 :e 4 :f 3))
+user=> p
+{:b 1, :a 2, :c 3, :f 3, :e 4, :d 5}
+So :b has priority 1, :a has priority 2, and so on.
+Notice how the priority map prints in an order sorted by its priorities (i.e., the map's values)
+We can use assoc to assign a priority to a new item:
+user=> (assoc p :g 1)
+{:b 1, :g 1, :a 2, :c 3, :f 3, :e 4, :d 5}
+or to assign a new priority to an extant item:
+user=> (assoc p :c 4)
+{:b 1, :a 2, :f 3, :c 4, :e 4, :d 5}
+We can remove an item from the priority map:
+user=> (dissoc p :e)
+{:b 1, :a 2, :c 3, :f 3, :d 5}
+An alternative way to add to the priority map is to conj a [item priority] pair:
+user=> (conj p [:g 0])
+{:g 0, :b 1, :a 2, :c 3, :f 3, :e 4, :d 5}
+or use into:
+user=> (into p [[:g 0] [:h 1] [:i 2]])
+{:g 0, :b 1, :h 1, :a 2, :i 2, :c 3, :f 3, :e 4, :d 5}
+Priority maps are countable:
+user=> (count p)
+Like other maps, equivalence is based not on type, but on contents.
+In other words, just as a sorted-map can be equal to a hash-map,
+so can a priority-map.
+user=> (= p {:b 1, :a 2, :c 3, :f 3, :e 4, :d 5})
+You can test them for emptiness:
+user=> (empty? (priority-map))
+user=> (empty? p)
+You can test whether an item is in the priority map:
+user=> (contains? p :a)
+user=> (contains? p :g)
+It is easy to look up the priority of a given item, using any of the standard map mechanisms:
+user=> (get p :a)
+user=> (get p :g 10)
+user=> (p :a)
+user=> (:a p)
+Priority maps derive much of their utility by providing priority-based seq.
+Note that no guarantees are made about the order in which items of the same priority appear.
+user=> (seq p)
+([:b 1] [:a 2] [:c 3] [:f 3] [:e 4] [:d 5])
+Because no guarantees are made about the order of same-priority items, note that
+rseq might not be an exact reverse of the seq. It is only guaranteed to be in
+descending order.
+user=> (rseq p)
+([:d 5] [:e 4] [:c 3] [:f 3] [:a 2] [:b 1])
+This means first/rest/next/for/map/etc. all operate in priority order.
+user=> (first p)
+[:b 1]
+user=> (rest p)
+([:a 2] [:c 3] [:f 3] [:e 4] [:d 5])
+Priority maps support metadata:
+user=> (meta (with-meta p {:extra :info}))
+{:extra :info}
+But perhaps most importantly, priority maps can also function as priority queues.
+peek, like first, gives you the first [item priority] pair in the collection.
+pop removes the first [item priority] from the collection.
+(Note that unlike rest, which returns a seq, pop returns a priority map).
+user=> (peek p)
+[:b 1]
+user=> (pop p)
+{:a 2, :c 3, :f 3, :e 4, :d 5}
+It is also possible to use a custom comparator:
+user=> (priority-map-by (comparator >) :a 1 :b 2 :c 3)
+{:c 3, :b 2, :a 1}
+## License
+Copyright (C) 2011 FIXME
+Distributed under the Eclipse Public License, the same as Clojure.
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