get compile times by file/dir in sbt

build.sbt

val compileTimes = inputKey[Unit]("Get compilation time for each folder")
compileTimes / aggregate := false,
compileTimes := {
  import complete.DefaultParsers._
  //  val ((depth, _), filter) = (NatBasic ~ " " ~ StringBasic).parsed
  val (depth, filter) = spaceDelimited("depth filter").parsed match {
    case Seq(depth, filter) => (depth.toInt, filter)
    case _ => sys.error("expected depth and filter arguments, eg '2 /'")
  }
  val s = state.value
  s.log.info(s"depth $depth, filter $filter")
  val extracted = Project.extract(s)
  extracted.runTask(Compile / compile, s)
  val source = (Compile / scalaSource).value
  val files = ((source / filter) ** "*.scala").get()
  s.log.info(s"Running on ${files.size} files")

  def pathParts(f: File) = f.relativeTo(source / filter).get.getPath.split(java.io.File.separatorChar)

  val times = files.groupBy(pathParts(_).take(depth).mkString("/")).toSeq.sortBy(_._1).map { case (path, files) =>
    def runCompile = {
      val start = System.currentTimeMillis()
      extracted.runTask(Compile / compile, s)
      val time = System.currentTimeMillis() - start
      s.log.info(s"compiled ${path} in $time ms")
      time
    }
    files.foreach(f => IO.append(f, " "))
    val time1 = runCompile
    files.foreach(f => IO.write(f, IO.readBytes(f).dropRight(1)))
    val time2 = runCompile
    (path, time1.min(time2))
  }
  s.log.info(times.sortBy(_._2).mkString("\n"))
}