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Optimize degeneracy ordering #108

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Optimize degeneracy ordering #108

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18 changes: 6 additions & 12 deletions src/loom/alg.cljc
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Original file line number Diff line number Diff line change
Expand Up @@ -663,25 +663,19 @@ can use these functions."
(defn degeneracy-ordering
"Returns sequence of vertices in degeneracy order."
[g]
;; The strategy here is to repeatedly remove the node with smallest
;; degree in the remaining subgraph.
(loop [ordered-nodes []
node-degs (->> (zipmap (nodes g)
(map (partial out-degree g) (nodes g)))
(into (pm/priority-map)))
k 0]
(into (pm/priority-map)))]
(if (empty? node-degs)
ordered-nodes
(let [[n deg] (first node-degs)
;; This will be the adjacent nodes still in node-degs (not in ordered-nodes) decr'd by 1
updated-degs (->> (map (juxt identity node-degs) (successors g n))
(filter second)
(map (juxt first (comp dec second)))
(into {}))]
(let [[n _] (first node-degs)]
(recur (conj ordered-nodes n)
(reduce (fn [n-ds [n d]] ;; Update this assoc'ing the updated-degs found above
(assoc n-ds n d))
(reduce (fn [m n'] (if (contains? m n') (update m n' dec) m))
(dissoc node-degs n)
updated-degs)
(max k deg))))))
(successors g n)))))))

(defn- bk-gen [g [r p x] stack]
(let [v-pivot (reduce (partial max-key (partial out-degree g)) p)]
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