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interaction of topologicalSort with withSubgraph #104

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rbodik opened this Issue Apr 1, 2018 · 1 comment

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rbodik commented Apr 1, 2018

Hello, topologicalSort seems to find a cycle in an acyclic directed graph when that graph is edge-filtered. Is this expected behavior? This cycle can't be found with findCycle.

import scalax.collection.mutable.Graph
import scalax.collection.edge.Implicits._
import scalax.collection.GraphPredef._

val g1 = Graph( 1 ~> 3,                         2 ~> 3)
val g2 = Graph((1 ~+> 3)("a"), (1 ~+> 2)("b"), (2 ~+> 3)("a"))

// The next two calls return a topo sort, as expected. 
g1.get(1).topologicalSort()
g2.get(1).topologicalSort()

// The next topologicalSort finds a cycle on node 3.  
// I would expect the filtered graph to have a topo sort exactly as in graph `g1` rooted in `g1.get(1)`.
g2.get(1).withSubgraph(edges = _.label == "a").topologicalSort() 

// The cycle on node 3 can't be found with findCycle
g2.get(1).withSubgraph(edges = _.label == "a").findCycle
g2.get(3).findCycle()

// For an insider, the next three return values may shed some light into the unexpected behavior.
g1.get(1)
g2.get(1)
g2.get(1).withSubgraph(edges = _.label == "a")
@peter-empen

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peter-empen commented Apr 3, 2018

Ras, thank you for reporting.

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