GroupSpec.scala

package adcal_1222

import adcal_1222.Amida._
import cats.instances.AllInstances
import cats.kernel.laws.discipline.GroupTests
import cats.{Eq, Group}
import org.scalacheck.{Arbitrary, Gen}
import org.specs2.Specification
import org.typelevel.discipline.specs2.Discipline

case class Amida(legs: List[Int]) {
  def combine(another: Amida): Amida = Amida(legs ++ another.legs)
  def inverse: Amida = Amida(legs.reverse)
  def permutation: Vector[Int] = legs.foldLeft((0 until Lines).toVector) { (acc, l) =>
    acc.updated(l, acc(l + 1)).updated(l + 1, acc(l))
  }
}
object Amida {
  val Lines = 4
  val Empty = Amida(List.empty[Int])
  implicit val amidaGroup: Group[Amida] = new Group[Amida] {
    def inverse(a: Amida): Amida           = a.inverse
    def empty: Amida                       = Amida.Empty
    def combine(x: Amida, y: Amida): Amida = x combine y
  }
}
class GroupSpec extends Specification with Discipline with AllInstances {
  import Gen.{chooseNum, listOfN}
  implicit val eqAmida:  Eq[Amida]        = _.permutation == _.permutation
  implicit val arbAmida: Arbitrary[Amida] = Arbitrary {
    for {
      len  <- chooseNum(0, 5)
      legs <- listOfN(len, chooseNum(0, Lines - 2))
    } yield Amida(legs)
  }
  private val check = checkAll("group of Amida", GroupTests[Amida].group)

  def is = s2"Amida satisfies group laws $check"
}