package underscoreio
package enum

import shapeless._
import shapeless.labelled._
import shapeless.syntax.singleton._

// The intention of this code is to extract the names of the types at the leaves of an algebraic data type.
// E.g. for Color at the bottom of this file,
//
//  val repr = LabelledGeneric[Color]
//  val names = Enum.names[repr.Repr]
//
// should result in names being List("Red", "Green", "Blue") or some permutation thereof.
object Enum {
  trait NameExtractor[+A] {
    def extract: List[String]
  }

  object NameExtractor {
    implicit val hnilNameExtractor: NameExtractor[HNil] =
      new NameExtractor[HNil] {
        def extract: List[String] = List.empty[String]
      }

    implicit def hconsNameExtractor[Name <: Symbol, Head, Tail <: HList](
      implicit
        witness: Witness.Aux[Name],
        tailExtractor: NameExtractor[Tail]
    ): NameExtractor[KeyTag[Name, Head] :: Tail] =
      new NameExtractor[KeyTag[Name, Head] :: Tail] {
        def extract: List[String] = witness.value.toString :: tailExtractor.extract
      }

    implicit val cnilNameExtractor: NameExtractor[CNil] =
      new NameExtractor[CNil] {
        def extract: List[String] = List.empty[String]
      }

    implicit def cconsNameExtractor[Name <: Symbol, Head, Tail <: Coproduct](
      implicit
        witness: Witness.Aux[Name],
        tailExtractor: NameExtractor[Tail]
    ): NameExtractor[KeyTag[Name, Head] :+: Tail] =
      new NameExtractor[KeyTag[Name, Head] :+: Tail] {
        def extract: List[String] = witness.value.toString :: tailExtractor.extract
      }
  }

  def names[A](implicit extractor: NameExtractor[A]): List[String] = {
    extractor.extract
  }
}

sealed abstract class Color
final case object Red extends Color
final case object Green extends Color
final case object Blue extends Color

sealed abstract class Twice
final case object One extends Twice
final case object Two extends Twice

sealed abstract class Single
final case object Once extends Single

object Example {
  val color = LabelledGeneric[Color]
  val red = LabelledGeneric[Red.type]
  val single = LabelledGeneric[Single]
  //val names = Enum.names[single.Repr]
}