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From "ASF GitHub Bot (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (GEARPUMP-349) Graph#topologicalOrderIterator is slow for large graph
Date Thu, 14 Sep 2017 12:20:00 GMT

    [ https://issues.apache.org/jira/browse/GEARPUMP-349?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16166159#comment-16166159
] 

ASF GitHub Bot commented on GEARPUMP-349:
-----------------------------------------

Github user manuzhang commented on a diff in the pull request:

    https://github.com/apache/incubator-gearpump/pull/223#discussion_r138876219
  
    --- Diff: core/src/main/scala/org/apache/gearpump/util/Graph.scala ---
    @@ -355,19 +393,7 @@ class Graph[N, E](vertexList: List[N], edgeList: List[(N, E, N)])
extends Serial
        * check whether there is a loop
        */
       def hasCycle(): Boolean = {
    -    @tailrec
    -    def detectCycle(graph: Graph[N, E]): Boolean = {
    -      if (graph.edges.isEmpty) {
    -        false
    -      } else if (graph.vertices.nonEmpty && !graph.vertices.exists(graph.inDegreeOf(_)
== 0)) {
    -        true
    -      } else {
    -        graph.removeZeroInDegree
    -        detectCycle(graph)
    -      }
    -    }
    -
    -    detectCycle(copy)
    +    tryTopologicalOrderIterator.isFailure
    --- End diff --
    
    can we cache result here ?


> Graph#topologicalOrderIterator is slow for large graph
> ------------------------------------------------------
>
>                 Key: GEARPUMP-349
>                 URL: https://issues.apache.org/jira/browse/GEARPUMP-349
>             Project: Apache Gearpump
>          Issue Type: Improvement
>          Components: core
>    Affects Versions: 0.8.4
>            Reporter: Manu Zhang
>            Assignee: Huafeng Wang
>             Fix For: 0.8.5
>
>
> The algorithm is as follows
> 1. find zero in-degree nodes from a copied graph. 
> 2. remove nodes from the copied graph and add them to the output
> 3. repeat 1
> The issue is that step 1 traverses all remaining nodes each time, which costs the algorithm
{{O(n^2)}} time
> {{Graph#hasCycle}} has a similar issue



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