def apply()

in http-core/src/main/scala/org/apache/pekko/http/impl/engine/http2/PriorityTree.scala [56:160]
85 lines of code
10 McCabe index (conditional complexity)

  def apply(): PriorityTree = create(TreeMap(0 -> RootNode))

  private final val DefaultWeight = 16

  private val RootNode = PriorityInfo(0, 0, DefaultWeight, TreeSet.empty)
  private def create(nodes: TreeMap[Int, PriorityInfo]): PriorityTreeImpl =
    new PriorityTreeImpl(nodes)

  private class PriorityTreeImpl(nodes: TreeMap[Int, PriorityInfo]) extends PriorityTree {
    def rootNode: PriorityNode = node(0)

    def insertOrUpdate(streamId: Int, streamDependency: Int, weight: Int, exclusive: Boolean): PriorityTree =
      if (nodes.isDefinedAt(streamId)) update(streamId, streamDependency, weight, exclusive)
      else insert(streamId, streamDependency, weight, exclusive)

    private def insert(streamId: Int, streamDependency: Int, weight: Int, exclusive: Boolean): PriorityTreeImpl = {
      require(!nodes.isDefinedAt(streamId), s"Cannot insert node twice: $streamId")
      require(streamId != streamDependency, s"Stream cannot depend on itself: $streamId")

      if (nodes.isDefinedAt(streamDependency)) {
        val dependencyInfo = nodes(streamDependency)
        if (!exclusive)
          insertNode(PriorityInfo(streamId, streamDependency, weight, TreeSet.empty))
            .updateNode(streamDependency)(updateChildren(_ + streamId))
        else
          insertNode(PriorityInfo(streamId, streamDependency, weight, dependencyInfo.childrenIds))
            .updateNode(streamDependency)(updateChildren(_ => TreeSet(streamId)))
      } else
        insertNode(PriorityInfo(streamDependency, 0, DefaultWeight, TreeSet.empty)) // try again after creating intermediate
          .insert(streamId, streamDependency, weight, exclusive)
    }
    private def update(
        streamId: Int, newStreamDependency: Int, newWeight: Int, newlyExclusive: Boolean): PriorityTree = {
      require(nodes.isDefinedAt(streamId), s"Not must exist to be updated: $streamId")
      require(streamId != newStreamDependency, s"Stream cannot depend on itself: $streamId")

      if (nodes.isDefinedAt(newStreamDependency)) {
        val oldInfo = nodes(streamId)

        if (!newlyExclusive && newStreamDependency == dependencyOf(streamId)) // no dependency changes -> simple
          updateNode(streamId)(_.copy(weight = newWeight))
        else if (!dependsTransitivelyOn(newStreamDependency, streamId)) {
          remove(streamId)
            .insert(streamId, newStreamDependency, newWeight, newlyExclusive)
            .updateNode(streamId)(updateChildren(newChildren => newChildren ++ oldInfo.childrenIds))
        } else {
          this // FIXME: actually implement moving nodes down in a dependency chain
        }
      } else
        insert(newStreamDependency, 0, DefaultWeight, exclusive = false) // try again after creating intermediate
          .update(streamId, newStreamDependency, newWeight, newlyExclusive)
    }

    private def remove(streamId: Int): PriorityTreeImpl = {
      require(nodes.isDefinedAt(streamId), s"Node must exist to be removed")
      require(streamId != 0, "Cannot remove root node")

      val info = nodes(streamId)
      val dependencyInfo = nodes(info.streamDependency)

      create(
        (nodes - streamId) +
        (info.streamDependency -> dependencyInfo.copy(childrenIds = dependencyInfo.childrenIds - streamId)) ++
        info.childrenIds.unsorted.map(id =>
          id -> nodes(id).copy(streamDependency = info.streamDependency)))
    }

    private def dependsTransitivelyOn(child: Int, parent: Int): Boolean = {
      val realParent = dependencyOf(child)

      (child != 0) && (
        realParent == parent ||
        dependsTransitivelyOn(realParent, parent))
    }

    private def dependencyOf(streamId: Int): Int = nodes(streamId).streamDependency

    // lens-like "mutators"
    private def updateNodes(updater: TreeMap[Int, PriorityInfo] => TreeMap[Int, PriorityInfo]): PriorityTreeImpl =
      create(updater(nodes))

    private def updateNode(streamId: Int)(updater: PriorityInfo => PriorityInfo): PriorityTreeImpl =
      updateNodes { nodes =>
        nodes.updated(streamId, updater(nodes(streamId)))
      }
    private def updateChildren(
        updater: immutable.TreeSet[Int] => immutable.TreeSet[Int]): PriorityInfo => PriorityInfo = { old =>
      old.copy(childrenIds = updater(old.childrenIds))
    }
    private def insertNode(newNode: PriorityInfo): PriorityTreeImpl =
      updateNodes(_ + (newNode.streamId -> newNode))

    def print: String = {
      def printNode(node: PriorityNode): String = s"${node.streamId} [weight: ${node.weight}]"

      AsciiTreeLayout.toAscii[PriorityNode](rootNode, _.children, printNode)
    }

    private def node(_streamId: Int): PriorityNode = new PriorityNode {
      def streamId: Int = _streamId
      def weight: Int = nodes(streamId).weight
      def dependency: PriorityNode = node(nodes(streamId).streamDependency)
      def children: immutable.Seq[PriorityNode] = nodes(streamId).childrenIds.toVector.map(node)
    }
  }