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;; A priority map is a map from items to priorities,
;; offering queue-like peek/pop as well as the map-like ability to
;; easily reassign priorities and other conveniences.
;; by Mark Engelberg (mark.engelberg@gmail.com)
;; May 20, 2011
(ns
^{:author "Mark Engelberg",
:doc "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.
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
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)
6
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})
true
You can test them for emptiness:
user=> (empty? (priority-map))
true
user=> (empty? p)
false
You can test whether an item is in the priority map:
user=> (contains? p :a)
true
user=> (contains? p :g)
false
It is easy to look up the priority of a given item, using any of the standard map mechanisms:
user=> (get p :a)
2
user=> (get p :g 10)
10
user=> (p :a)
2
user=> (:a p)
2
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}
All of these operations are efficient. Generally speaking, most operations
are O(log n) where n is the number of distinct priorities. Some operations
(for example, straightforward lookup of an item's priority, or testing
whether a given item is in the priority map) are as efficient
as Clojure's built-in map.
The key to this efficiency is that internally, not only does the priority map store
an ordinary hash map of items to priority, but it also stores a sorted map that
maps priorities to sets of items with that priority.
A typical textbook priority queue data structure supports at the ability to add
a [item priority] pair to the queue, and to pop/peek the next [item priority] pair.
But many real-world applications of priority queues require more features, such
as the ability to test whether something is already in the queue, or to reassign
a priority. For example, a standard formulation of Dijkstra's algorithm requires the
ability to reduce the priority number associated with a given item. Once you
throw persistence into the mix with the desire to adjust priorities, the traditional
structures just don't work that well.
This particular blend of Clojure's built-in hash sets, hash maps, and sorted maps
proved to be a great way to implement an especially flexible persistent priority queue.
Connoisseurs of algorithms will note that this structure's peek operation is not O(1) as
it would be if based upon a heap data structure, but I feel this is a small concession for
the blend of persistence, priority reassignment, and priority-sorted seq, which can be
quite expensive to achieve with a heap (I did actually try this for comparison). Furthermore,
this peek's logarithmic behavior is quite good (on my computer I can do a million
peeks at a priority map with a million items in 750ms). Also, consider that peek and pop
usually follow one another, and even with a heap, pop is logarithmic. So the net combination
of peek and pop is not much different between this versatile formulation of a priority map and
a more limited heap-based one. In a nutshell, peek, although not O(1), is unlikely to be the
bottleneck in your program.
All in all, I hope you will find priority maps to be an easy-to-use and useful addition
to Clojure's assortment of built-in maps (hash-map and sorted-map).
"}
clojure.data.priority-map
(:use clojure.test)
(:import clojure.lang.MapEntry java.util.Map clojure.lang.PersistentTreeMap))
; Note that the plan is to eventually support subseq, but this will require
; some changes to core:
;; user=> (subseq p < 3)
;; ([:b 1] [:a 2])
;; user=> (subseq p >= 3)
;; ([:c 3] [:f 3] [:e 4] [:d 5])
(declare pm-empty)
; A Priority Map is comprised of a sorted map that maps priorities to hash sets of items
; with that priority (priority->set-of-items),
; as well as a hash map that maps items to priorities (item->priority)
; Priority maps may also have metadata
(deftype PersistentPriorityMap [priority->set-of-items item->priority __meta]
Object
(toString [this] (str (.seq this)))
clojure.lang.ILookup
; valAt gives (get pm key) and (get pm key not-found) behavior
(valAt [this item] (get item->priority item))
(valAt [this item not-found] (get item->priority item not-found))
clojure.lang.IPersistentMap
(count [this] (count item->priority))
(assoc [this item priority]
(let [current-priority (get item->priority item nil)]
(if current-priority
;Case 1 - item is already in priority map, so this is a reassignment
(if (= current-priority priority)
;Subcase 1 - no change in priority, do nothing
this
(let [item-set (get priority->set-of-items current-priority)]
(if (= (count item-set) 1)
;Subcase 2 - it was the only item of this priority
;so remove old priority entirely
;and conj item onto new priority's set
(PersistentPriorityMap.
(assoc (dissoc priority->set-of-items current-priority)
priority (conj (get priority->set-of-items priority #{}) item))
(assoc item->priority item priority))
;Subcase 3 - there were many items associated with the item's original priority,
;so remove it from the old set and conj it onto the new one.
(PersistentPriorityMap.
(assoc priority->set-of-items
current-priority (disj (get priority->set-of-items current-priority) item)
priority (conj (get priority->set-of-items priority #{}) item))
(assoc item->priority item priority)))))
; Case 2: Item is new to the priority map, so just add it.
(PersistentPriorityMap.
(assoc priority->set-of-items
priority (conj (get priority->set-of-items priority #{}) item))
(assoc item->priority item priority)))))
(empty [this] pm-empty)
; cons defines conj behavior
(cons [this e] (let [[item priority] e] (.assoc this item priority)))
; Like sorted maps, priority maps are equal to other maps provided
; their key-value pairs are the same.
(equiv [this o] (.equiv item->priority o))
(hashCode [this] (.hashCode item->priority))
(equals [this o] (or (identical? this o) (.equals item->priority o)))
;containsKey implements (contains? pm k) behavior
(containsKey [this item] (contains? item->priority item))
(entryAt [this k]
(let [v (.valAt this k this)]
(when-not (identical? v this)
(MapEntry. k v))))
(seq [this]
(seq (for [[priority item-set] priority->set-of-items, item item-set]
(MapEntry. item priority))))
;without implements (dissoc pm k) behavior
(without
[this item]
(let [priority (item->priority item ::not-found)]
(if (= priority ::not-found)
;; If item is not in map, return the map unchanged.
this
(let [item-set (priority->set-of-items priority)]
(if (= (count item-set) 1)
;;If it is the only item with this priority, remove that priority's set completely
(PersistentPriorityMap. (dissoc priority->set-of-items priority)
(dissoc item->priority item))
;;Otherwise, just remove the item from the priority's set.
(PersistentPriorityMap.
(assoc priority->set-of-items priority (disj item-set item)),
(dissoc item->priority item)))))))
java.io.Serializable ;Serialization comes for free with the other things implemented
clojure.lang.MapEquivalence
Map ;Makes this compatible with java's map
(size [this] (count item->priority))
(isEmpty [this] (zero? (count item->priority)))
(containsValue [this v] (contains? (priority->set-of-items this) v))
(get [this k] (.valAt this k))
(put [this k v] (throw (UnsupportedOperationException.)))
(remove [this k] (throw (UnsupportedOperationException.)))
(putAll [this m] (throw (UnsupportedOperationException.)))
(clear [this] (throw (UnsupportedOperationException.)))
(keySet [this] (set (keys this)))
(values [this] (vals this))
(entrySet [this] (set this))
clojure.lang.IPersistentStack
(peek [this]
(when-not (.isEmpty this)
(let [f (first priority->set-of-items)]
(MapEntry. (first (val f)) (key f)))))
(pop [this]
(if (.isEmpty this) (throw (IllegalStateException. "Can't pop empty priority map"))
(let [f (first priority->set-of-items),
item-set (val f)
item (first item-set),
priority (key f)]
(if (= (count item-set) 1)
;If the first item is the only item with its priority, remove that priority's set completely
(PersistentPriorityMap.
(dissoc priority->set-of-items priority)
(dissoc item->priority item))
;Otherwise, just remove the item from the priority's set.
(PersistentPriorityMap.
(assoc priority->set-of-items priority (disj item-set item)),
(dissoc item->priority item))))))
clojure.lang.IFn
;makes priority map usable as a function
(invoke [this k] (.valAt this k))
(invoke [this k not-found] (.valAt this k not-found))
clojure.lang.IObj
;adds metadata support
(meta [this] __meta)
(withMeta [this m] (PersistentPriorityMap. priority->set-of-items item->priority m))
clojure.lang.Reversible
(rseq [this]
(seq (for [[priority item-set] (rseq priority->set-of-items), item item-set]
(MapEntry. item priority)))))
;; clojure.lang.Sorted
;; ; These methods provide support for subseq
;; (comparator [this] (.comparator ^PersistentTreeMap priority->set-of-items))
;; (entryKey [this entry] (val entry))
;; (seqFrom [this k ascending]
;; (let [sets (if ascending (subseq priority->set-of-items >= k) (rsubseq priority->set-of-items <= k))]
;; (seq (for [[priority item-set] sets, item item-set]
;; (MapEntry. item priority)))))
;; (seq [this ascending]
;; (if ascending (seq this) (rseq this))))
(def ^:private pm-empty (PersistentPriorityMap. (sorted-map) {} {}))
(defn- pm-empty-by [comparator] (PersistentPriorityMap. (sorted-map-by comparator) {}))
; The main way to build priority maps
(defn priority-map
"keyval => key val
Returns a new priority map with supplied mappings"
[& keyvals]
(reduce conj pm-empty (partition 2 keyvals)))
(defn priority-map-by
"keyval => key val
Returns a new priority map with supplied mappings"
[comparator & keyvals]
(reduce conj (pm-empty-by comparator) (partition 2 keyvals)))
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