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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 12:23:00 GMT

    [ https://issues.apache.org/jira/browse/GEARPUMP-349?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16166167#comment-16166167
] 

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_r138876832
  
    --- Diff: core/src/main/scala/org/apache/gearpump/util/Graph.scala ---
    @@ -243,13 +259,34 @@ class Graph[N, E](vertexList: List[N], edgeList: List[(N, E, N)])
extends Serial
        * The node returned by Iterator is stable sorted.
        */
       def topologicalOrderIterator: Iterator[N] = {
    -    val newGraph = copy
    -    var output = List.empty[N]
    -
    -    while (!newGraph.isEmpty) {
    -      output ++= newGraph.removeZeroInDegree
    +    tryTopologicalOrderIterator.get
    +  }
    +
    +  private def tryTopologicalOrderIterator: Try[Iterator[N]] = {
    +    Try {
    +      val indegreeMap = mutable.Map.empty[N, Int]
    --- End diff --
    
    why not generate `indegreeMap` with `vertices.map` ?


> 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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