Up-to-date GameOfThrones Kata solution (see https://github.com/ludopradel/DojoGOT). The last test was fixed expect 51,20€ instead of 51,60€

BookShop.scala

object BookShop {
  case class Book(id: Int)

  val ONE_BOOK_PRICE = 8
  val TWO_DISTINCT_BOOKS_WITH_PROMO_PRICE = 15.2
  val THREE_DISTINCT_BOOKS_WITH_PROMO_PRICE = 21.6
  val FOUR_DISTINCT_BOOKS_WITH_PROMO_PRICE = 25.6
  val FIVE_DISTINCT_BOOKS_WITH_PROMO_PRICE = 30

  type Basket[T] = List[List[T]]

  val emptyBasket:Basket[Book] = List()

  def getPrice(bookList: List[Book]) : Double = {
    enumerateBasketCombinations(bookList).map(f => getBasketPrice(f)).sorted.head
  }

  def enumerateBasketCombinations(bookList: List[Book]): List[Basket[Book]] = {
    bookList match {
      case List() => List(emptyBasket)
      case _ =>
        val distinct = bookList.distinct
        enumerateListCombinations(distinct).flatMap(oneDistinctList => {
          val remainder = bookList diff oneDistinctList
          enumerateBasketCombinations(remainder).map(remainderBasketCombinations => oneDistinctList::remainderBasketCombinations)
        })
    }
  }

  def enumerateListCombinations(distinct: List[Book]): List[List[Book]] = {
    (1 to distinct.length).flatMap(i => distinct.combinations(i).toList).toList
  }

  def getBasketPrice(basket: Basket[Book]): Double = {
    basket.map(l => getDistinctPrice(l)).sum
  }

  def getDistinctPrice(distinctBookList: List[Book]) : Double = {
    distinctBookList.length match {
      case 1 => ONE_BOOK_PRICE
      case 2 => TWO_DISTINCT_BOOKS_WITH_PROMO_PRICE
      case 3 => THREE_DISTINCT_BOOKS_WITH_PROMO_PRICE
      case 4 => FOUR_DISTINCT_BOOKS_WITH_PROMO_PRICE
      case _ => FIVE_DISTINCT_BOOKS_WITH_PROMO_PRICE
    }
  }
}

GameOfThronesTest.scala

class GameOfThronesTest extends UnitSpec {
  "Book Shop" should {
    "return 8€ when we want one book" in {
      assert(BookShop.getPrice(List(Book(1))) == 8)
    }

    "return 16€ when we want the same book twice" in {
      assert(BookShop.getPrice(List(Book(1), Book(1))) == 16)
    }

    "return 15,20€ when we want two different books" in {
      assert(BookShop.getPrice(List(Book(1), Book(2))) == 15.2)
    }

    "return 23,20€ when we want three books with the same twice" in {
      assert(BookShop.getPrice(List(Book(1), Book(1), Book(3))) == 23.2)
    }

    "return 21,60€ when we want three different books" in {
      assert(BookShop.getPrice(List(Book(1), Book(2), Book(3))) == 21.6)
    }

    "return 43,20€ when we want three different books twice" in {
      assert(BookShop.getPrice(List(Book(1), Book(2), Book(3), Book(1), Book(2), Book(3))) == 43.2)
    }

    "return 25,60€ when we want four different books" in {
      assert(BookShop.getPrice(List(Book(1), Book(2), Book(3), Book(4))) == 25.6)
    }

    "return 30€ when we want five different books" in {
      assert(BookShop.getPrice(List(Book(1), Book(2), Book(3), Book(4), Book(5))) == 30)
    }

    "return 51,20€ when we want 1*2, 2*2, 3*2, 4 and 5" in {
      assert(BookShop.getPrice(List(Book(1), Book(1), Book(2), Book(2), Book(3), Book(3), Book(4), Book(5))) == 51.2)
    }
  }
}

Oui, j'ai vu ça après coup ...
Cette solution n'est pas optimum.
Elle correspond à ce qu'on était en train de dire à la fin du coding dojo.
En plus le test est mal décrit, c'est 3*2 au lieu de 3*3.