object Dependencies extends sbt.Plugin with CommandSupport {

  override lazy val settings = Seq(commands += myCommand)

  lazy val myCommand =
    Command.args("dependencies", "<project>") { deps }

  def deps(state: State, args: Seq[String]): State = {
    val extracted = Project.extract(state)
    implicit val pr = extracted.currentRef
    implicit val bs = extracted.structure

    val evictedJars: Set[File] = {
      val s: Seq[File] =
      for(
        ep <- evicted.run.allEvictions;
        mr <- ep.evicteds;
        (artifact, file) <- mr.artifacts
      ) yield {
        if (file == null) sys.error(s"Artifact: ${artifact} could not be found at ${file.getAbsolutePath}")
        file
      }
      s.toSet
    }
}


trait CommandSupport {
  this: Plugin =>

  protected def fail(errorMessage: String)(implicit state: State): Nothing = {
    state.log.error(errorMessage)
    throw new IllegalArgumentException()
  }

  protected def log(implicit state: State) = state.log

  // our version of http://stackoverflow.com/questions/25246920
  protected implicit class RichSettingKey[A](key: SettingKey[A]) {
    def gimme(implicit pr: ProjectRef, bs: BuildStructure, s: State): A =
      gimmeOpt getOrElse { fail(s"Missing setting: ${key.key.label}") }
    def gimmeOpt(implicit pr: ProjectRef, bs: BuildStructure, s: State): Option[A] =
      key in pr get bs.data
  }

  protected implicit class RichTaskKey[A](key: TaskKey[A]) {
    def run(implicit pr: ProjectRef, bs: BuildStructure, s: State): A =
      runOpt.getOrElse { fail(s"Missing task key: ${key.key.label}") }
    def runOpt(implicit pr: ProjectRef, bs: BuildStructure, s: State): Option[A] =
      EvaluateTask(bs, key, s, pr).map(_._2) match {
        case Some(Value(v)) => Some(v)
        case _              => None
      }
  }
}