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package izumi.fundamentals.graphs.tools.gc

import izumi.fundamentals.graphs.struct.IncidenceMatrix

import scala.annotation.tailrec
import scala.collection.mutable

// TODO: this class is not required for distage
class Tracer[N] {
  def trace(matrix: IncidenceMatrix[N], exclusions: Set[(N, N)], toTrace: Set[N]): Set[N] = {
    val reachable = mutable.HashSet[N]()
    reachable ++= toTrace
    trace(matrix, exclusions, toTrace, reachable)
    reachable.toSet
  }

  @tailrec
  private def trace(matrix: IncidenceMatrix[N], exclusions: Set[(N, N)], toTrace: Set[N], reachable: mutable.HashSet[N]): Unit = {
    val allReachable = toTrace
      .flatMap(s => matrix.links(s).map(p => (s, p)))

    val nextReachable = allReachable
      .diff(exclusions)
      .map(_._2)
      .diff(reachable)

    if (nextReachable.nonEmpty) {
      reachable ++= nextReachable
      trace(matrix, exclusions, nextReachable, reachable)
    }
  }
}