### A* in Clojure

Thanks to Mark Engelberg there’s now a nice priority map in contrib. With such a facility, it becomes really easy to write a nice implementation of the A* algorithm.

(ns net.cgrand.clj-me.a-star (:use [clojure.contrib.priority-map :only [priority-map]])) (defn A* "Finds a path between start and goal inside the graph described by edges (a map of edge to distance); estimate is an heuristic for the actual distance. Accepts a named option: :monotonic (default to true). Returns the path if found or nil." [edges estimate start goal & {mono :monotonic :or {mono true}}] (let [f (memoize #(estimate % goal)) ; unsure the memoization is worthy neighbours (reduce (fn [m [a b]] (assoc m a (conj (m a #{}) b))) {} (keys edges))] (loop [q (priority-map start (f start)) preds {} shortest {start 0} done #{}] (when-let [[x hx] (peek q)] (if (= goal x) (reverse (take-while identity (iterate preds goal))) (let [dx (- hx (f x)) bn (for [n (remove done (neighbours x)) :let [hn (+ dx (edges [x n]) (f n)) sn (shortest n Double/POSITIVE_INFINITY)] :when (< hn sn)] [n hn])] (recur (into (pop q) bn) (into preds (for [[n] bn] [n x])) (into shortest bn) (if mono (conj done x) done))))))))

Example:

(defn euclidian-distance [a b] ; multidimensional (Math/sqrt (reduce + (map #(let [c (- %1 %2)] (* c c)) a b)))) ;; generate a grid graph whose outlying edges are one-way (defn grid [x y w h] (into {} (for [i (range w) j (range h) :let [x0 (+ x i) y0 (+ y j) x1 (inc x0) y1 (inc y0)]] {[[x0 y0] [x1 y0]] 1 [[x1 y0] [x1 y1]] 1 [[x1 y1] [x0 y1]] 1 [[x0 y1] [x0 y0]] 1}))) (def g (apply dissoc (grid 0 0 4 4) (keys (grid 1 1 2 2)))) (comment user=> (A* g euclidian-distance [0 3] [4 2]) ([0 3] [1 3] [2 3] [3 3] [3 2] [4 2]) )

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Hey, I have generic AI search (including A*) in clojure (http://github.com/aria42/mochi/blob/master/src/mochi/search.clj)

and it was using java.util.PriorityQueue, clojure.contrib.priority-map would be much better.

This appears to be not A* but a simplified algorithm which assumes that your heuristic is monotonic.

@foo I may have commited an oversight in implementing A* so I’d really like to hear more from you. However note that the closed set is augmented (monotonic A*) only when the option :monotonic is set to true.

it’s still moonspeak.

[...] may also be interesting to compare this a* search function to that of cgrande’s. a* algorithm, dijkstra's algorithm, [...]

Thanks for the nice example. I couple of minor comments:

The Math/sqrt is kinda slow and seems unnecessary in Eucl-dist; and if all the costs were integers, one could avoid Math/sqrt and replace the Double constant in A* with a Long (cheaper comparisons?).

The memoize is indeed worthy (question you posed above) as lookups of the same node may be done multiple times (node x may be rejected once but selected at a later time). I wonder though whether memoize is a better choice than sticking values into a regular hash-map.(?)

Hi,

Thanks for your post, insightful as usual ☺.

Just passing by, I wondered if you had considered using cheplist [*].

Best Regards,

B.

[*]http://nakkaya.com/2011/10/10/cheaplist-in-clojure/

@Bernard, no I haven’t, merci pour le pointeur.

This is brilliant and beautiful!

My favorite part is the code to trace back through the preds to recover the path: (reverse (take-while identity (iterate preds goal))) … what could be more succinct?

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