izumi.fundamentals.graphs.tools.cycles.LoopDetector.scala Maven / Gradle / Ivy

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package izumi.fundamentals.graphs.tools.cycles

import izumi.fundamentals.graphs.struct.IncidenceMatrix
import izumi.fundamentals.graphs.tools.cycles.LoopDetector.Cycles

import scala.annotation.nowarn
import scala.collection.mutable

// TODO: this class is not required for distage
trait LoopDetector {
  def findCyclesForNode[T](node: T, graph: IncidenceMatrix[T]): Option[Cycles[T]]
  def findLoopMember[T](graph: IncidenceMatrix[T]): Option[T]

  final def findCyclesForNodes[T](nodes: Set[T], graph: IncidenceMatrix[T]): Set[Cycles[T]] = {
    nodes.flatMap(findCyclesForNode(_, graph))
  }
}

@nowarn("msg=Unused import")
object LoopDetector {
  import scala.collection.compat._

  final case class Loop[T](loop: Seq[T]) extends AnyVal

  final case class Cycles[T](node: T, loops: Seq[Loop[T]])

  object Impl extends LoopDetector {
    def findCyclesForNode[T](node: T, graph: IncidenceMatrix[T]): Option[Cycles[T]] = {
      val loops = new mutable.HashSet[Loop[T]]()
      traceCycles(graph, loops)(node, Seq.empty, Set.empty)
      loops.toList match {
        case Nil =>
          None
        case l =>
          Some(Cycles(node, l))
      }

    }

    private def traceCycles[T](graph: IncidenceMatrix[T], loops: mutable.HashSet[Loop[T]])(current: T, path: Seq[T], seen: Set[T]): Unit = {
      if (seen.contains(current)) {
        loops.add(Loop(path :+ current))
        ()
      } else {
        graph.links.get(current) match {
          case Some(value) =>
            value.foreach {
              referenced =>
                traceCycles(graph, loops)(referenced, path :+ current, seen + current)
            }
          case None =>
        }
      }
    }

    def findLoopMember[T](graph: IncidenceMatrix[T]): Option[T] = {
      val untested = mutable.HashMap.from(graph.links)

      while (untested.nonEmpty) {
        val seen = new mutable.HashSet[T]()
        detectLoops(untested, seen, List(untested.head._1)) match {
          case Some(inLoop) =>
            return Some(inLoop)
          case None =>
            untested --= seen
        }
      }

      None
    }

    @scala.annotation.tailrec
    private def detectLoops[T](untested: mutable.HashMap[T, Set[T]], seen: mutable.HashSet[T], start: Seq[T]): Option[T] = {
      start.find(seen.contains) match {
        case Some(value) =>
          Some(value)
        case None =>
          seen ++= start
          val toTest = start.flatMap {
            (a: T) =>
              untested.getOrElse(a, Seq.empty)
          }
          if (toTest.isEmpty) {
            None
          } else {
            detectLoops(untested, seen, toTest)
          }
      }
    }
  }

}