ScalaCheck生成BST

Question

我试图用ScalaCheck为BST创建一个Gen，但是当我调用.sample方法时，它给我的是java.lang.NullPointerException。我哪里错了？

sealed trait Tree

case class Node(left: Tree, right: Tree, v: Int) extends Tree

case object Leaf extends Tree

import org.scalacheck._
import Gen._
import Arbitrary.arbitrary

case class GenerateBST() {

  def genValue(left: Tree, right: Tree): Gen[Int] = (left, right) match {
    case (Node(_, _, min), Node(_, _, max)) => arbitrary[Int].suchThat(x => x > min && x < max)
    case (Node(_, _, min), Leaf) => arbitrary[Int].suchThat(x => x > min)
    case (Leaf, Node(_, _, max)) => arbitrary[Int].suchThat(x => x < max)
    case (Leaf, Leaf) => arbitrary[Int]
  }


  val genNode = for {
    left <- genTree
    right <- genTree
    v <- genValue(left, right)
  } yield Node(left, right, v)

  def genTree: Gen[Tree] = oneOf(const(Leaf), genNode)
}

GenerateBST().genTree.sample

Answer 1

由于您为递归数据类型递归定义生成器的方式，您需要在顶部使用Gen.lzy：

def genTree: Gen[Tree] = Gen.lzy(oneOf(const(Leaf), genNode))

作为一个不相关的附注，在生成器定义中使用suchThat通常只是最后的手段。这意味着sample经常会失败(使用固定版本的代码，大约有三分之一的时间)，更重要的是，如果有一天您想要创建导致Tree的任意函数，您将看到许多可怕的org.scalacheck.Gen$RetrievalError: couldn't generate value异常。

在这种情况下，您可以通过使用Gen.chooseNum并交换左右两侧(如果它们的顺序不正确)来很容易地避免suchThat：

sealed trait Tree
case class Node(left: Tree, right: Tree, v: Int) extends Tree
case object Leaf extends Tree

import org.scalacheck.{ Arbitrary, Gen }

object GenerateBST {
  def swapIfNeeded(l: Tree, r: Tree): (Tree, Tree) = (l, r) match {
    // If the two trees don't have space between them, we bump one and recheck:
    case (Node(_, _, x), n @ Node(_, _, y)) if math.abs(x - y) <= 1 =>
      swapIfNeeded(l, n.copy(v = y + 1))
    // If the values are in the wrong order, swap:
    case (Node(_, _, x), Node(_, _, y)) if x > y => (r, l)
    // Otherwise do nothing:
    case (_, _) => (l, r)
  }

  def genValue(left: Tree, right: Tree): Gen[Int] = (left, right) match {
    case (Node(_, _, min), Node(_, _, max)) => Gen.chooseNum(min + 1, max - 1)
    case (Node(_, _, min), Leaf) => Gen.chooseNum(min + 1, Int.MaxValue)
    case (Leaf, Node(_, _, max)) => Gen.chooseNum(Int.MinValue, max - 1)
    case (Leaf, Leaf) => Arbitrary.arbitrary[Int]
  }

  val genNode = for {
    l0 <- genTree
    r0 <- genTree
    (left, right) = swapIfNeeded(l0, r0)
    v <- genValue(left, right)
  } yield Node(left, right, v)

  def genTree: Gen[Tree] = Gen.lzy(Gen.oneOf(Gen.const(Leaf), genNode))
}

现在，您可以使用Arbitrary[Whatever => Tree]，而不必担心生成器故障：

scala> implicit val arbTree: Arbitrary[Tree] = Arbitrary(GenerateBST.genTree)
arbTree: org.scalacheck.Arbitrary[Tree] = org.scalacheck.ArbitraryLowPriority$$anon$1@606abb0e

scala> val f = Arbitrary.arbitrary[Int => Tree].sample.get
f: Int => Tree = org.scalacheck.GenArities$$Lambda$7109/289518656@13eefeaf

scala> f(1)
res0: Tree = Leaf

scala> f(2)
res1: Tree = Node(Leaf,Leaf,-20313200)

scala> f(3)
res2: Tree = Leaf

scala> f(4)
res3: Tree = Node(Node(Leaf,Leaf,-850041807),Leaf,-1)