shapeless-pathpattern.scala

import com.github.nscala_time.time.StaticInterval
import com.twitter.algebird.Monoid
import shapeless._, record._, syntax.singleton._
import shapeless.ops.record.SelectAll

/**  Represents a path component generator, basically a function keysTemplate.type -> Set[Seq[String]]
  * 
  */
trait PathPattern {

  val keysTemplate: HList
  val w: Witness
  
  def apply[L <: HList](keys: keysTemplate.type)
             (implicit sel: ops.record.Selector[keysTemplate.type , w.T]): Set[Seq[String]]
}

/** A path component generator taking a Joda Time Interval and generating a list of path components,
  * one for each hour contained in the interval
  */
case object HourlyIntervalPathPattern extends PathPattern {
  override val keysTemplate: HList = ("interval" ->> StaticInterval.lastHour) :: HNil
  override val w = Witness("interval")
  override def apply(keys: keysTemplate.type)
                    (implicit sel: ops.record.Selector[keysTemplate.type , w.T]): Set[Seq[String]] =
    _("interval").by[Set](1.hour).map(date =>
      Seq(date.toString("YYYY-MM-dd"), date.toString("HH")))
}

case class ConstantStringPathPattern(constant: String) extends PathPattern {
  override val keysTemplate: HList = (constant ->> "") :: HNil

  override def apply(keys: keysTemplate.type)
                      (implicit sel: ops.record.Selector[keysTemplate.type , w.T]): Set[Seq[String]] =
    Set(Seq(keys(constant)))
}

/** Make the path generators composable
  * The resulting generator takes a Shapeless records containing the keys of each sub-generators
  */
implicit object PathPatternMonoid extends Monoid[PathPattern] {
  override def zero: PathPattern = new PathPattern {
    override val keysTemplate = HNil
    override def apply(keys: keysTemplate.type)
                      (implicit sel: ops.record.Selector[keysTemplate.type , w.T])= Set(Seq(""))
  }

  override def plus(l: PathPattern, r: PathPattern): PathPattern = new PathPattern {
    override val keysTemplate = l.keysTemplate :: r.keysTemplate
    override def genPath (pKeys: keysTemplate.type)
                         (implicit sel: ops.record.Selector[keysTemplate.type , w.T])= for {
      p1 <- l(SelectAll[keysTemplate.type, l.keysTemplate.type](pKeys))
      p2 <- r(SelectAll[keysTemplate.type, r.keysTemplate.type](pKeys))
    } yield (p1 ++ p2)

  }
}

/** "Nice" syntax
  * 
  * @param pathPattern
  */
implicit class SlashablePathPattern(pathPattern: PathPattern) {
  def /(child: PathPattern)(implicit mon: Monoid[PathPattern]) = mon.plus(pathPattern, child)
}


val RegionHourlyPathPattern = ConstantStringPathPattern("region") / HourlyIntervalPathPattern
val paths = RegionHourlyPathPattern(("region" ->> "US") :: ("interval" ->> StaticInterval.lastHour) :: HNil)