fold_by_parent.scala

package example

import cats._
import cats.implicits._

object GroupByAndSort extends App {
  val parents = List(
    Parent(5, "p1", Child("p1-c1", 0)),
    Parent(5, "p1", Child("p1-c2", 1)),
    Parent(4, "p2", Child("p1-c1", 0)),
    Parent(4, "p2", Child("p1-c2", 1)),
    Parent(0, "p3", Child("p3-c1", 0)),
    Parent(0, "p3", Child("p3-c2", 1))
  )

  final case class Parent(priority: Long, name: String, child: Child*)

  final case class Child(name: String, priority: Long)

  //monoidでもいい、その場合はcombineAllのoptionがはずせるけどParentの単位元が何か悩むことになる
  implicit val semigroup: Semigroup[Parent] = (x: Parent, y: Parent) =>
    //combineしていいのはnameもpriorityも一致しているときに限るので微妙
    Parent(
      name = x.name,
      priority = x.priority,
      child = x.child ++ y.child: _*
    )

  println(
    Semigroup
      .combineAllOption(
        parents
          .groupBy(g => (g.name, g.priority))
          .values
      )
      .map(_.sortBy(_.priority))
  )
  //List(
  // Parent(0,p3,ArraySeq(Child(p3-c1,0), Child(p3-c2,1))),
  // Parent(4,p2,ArraySeq(Child(p1-c1,0), Child(p1-c2,1))),
  // Parent(5,p1,ArraySeq(Child(p1-c1,0), Child(p1-c2,1))))

}