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Last active November 18, 2020 03:08
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Implementing Tarjan in Clojure, step by step
;; having a map with only one entry is a bit of a waste, so env now directly maps a node
;; to a "lowest" stack or nil if not "on stack" anymore
(defn tarjan [nodes succs]
(letfn [(sc [env node]
(if (contains? env node)
(let [stack (::stack env)
env (assoc env node stack ::stack (conj stack node))
env (reduce (fn [env succ]
(let [env (sc env succ)]
(if-let [stack (env succ)]
(assoc env node
(min-key count stack (env node)))
env (succs node))]
(if (= stack (env node))
(loop [scc #{} nodes (::stack env) env env]
(let [curr (peek nodes)
scc (conj scc curr)
env (assoc env curr nil)]
(if (= curr node)
(-> env
(update-in [::sccs] conj scc)
(assoc ::stack stack))
(recur scc (pop nodes) env))))
(::sccs (reduce sc {::stack () ::sccs #{}} nodes))))
=> (def g {:c #{:d} :a #{:b :c} :b #{:a :c}})
=> (tarjan (keys g) g)
#{#{:c} #{:d} #{:a :b}}
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xsc commented Nov 4, 2016

@cgrand Thank you for this! Would you consider adding a license (maybe MIT?) to this gist?
I'd like to use your implementation in a library but without a license that's not possible.

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