Ensure preconditions are satisified when shrinking Commands actions

jvm/src/test/scala/org/scalacheck/commands/CommandsShrinkSpecification.scala

@@ -0,0 +1,137 @@
+package org.scalacheck.commands
+
+import org.scalacheck.Prop.forAll
+import org.scalacheck.rng.Seed
+import org.scalacheck.{Arbitrary, Gen, Prop, Properties, Shrink}
+import scala.reflect.ClassTag
+import scala.util.{Success, Try}
+
+object CommandsShrinkSpecification extends Properties("Commands Shrinking") {
+
+  property("Shrunk command sequences always satisfy preconditions") = {
+    forAll(Arbitrary.arbitrary[Long]) {
+      case seedValue =>
+        val seed = Seed(seedValue)
+        val parameters = Gen.Parameters.default.withInitialSeed(seed)
+        val result = BoundedQueueSpec.property(1)(parameters)
+
+        // If the property fails shrinking will kick in.  If shrinking does
+        // not guarantee that preconditions are satisfied and the shrunk
+        // command sequence contains the Dequeue command, we will invoke
+        // Nil.tail which will raise UnsupportedOperationException.
+        result.status match {
+          case Prop.Exception(e: UnsupportedOperationException) => false
+          case _ => true
+        }
+    }
+  }
+
+  class BoundedQueue[Element: ClassTag](val capacity: Int) {
+    require(capacity >= 0)
+
+    var buffer = new Array[Element](capacity + 1)
+    var reads = 0
+    var writes = 0
+
+    def size: Int = {
+      (writes + buffer.length - reads) % buffer.length
+    }
+
+    def enqueue(element: Element): Unit = {
+      require(size < capacity)
+      buffer(writes) = element
+      writes += 1
+      // writes %= buffer.length // deliberately broken to provoke failure
+    }
+
+    def dequeue(): Element = {
+      require(size > 0)
+      val index = reads
+      reads += 1
+      reads %= buffer.length
+      buffer(index)
+    }
+  }
+
+  object BoundedQueueSpec extends Commands {
+    type Element = Byte
+
+    type Sut = BoundedQueue[Element]
+
+    case class State(capacity: Int, elements: Seq[Element])
+
+    case object Capacity extends Command {
+      type Result = Int
+      def run(sut: Sut): Result = sut.capacity
+      def nextState(state: State): State = state
+      def preCondition(state: State): Boolean = true
+      def postCondition(state: State, result: Try[Result]): Prop =
+        result == Success(state.capacity)
+    }
+
+    case object Size extends Command {
+      type Result = Int
+      def run(sut: Sut): Result = sut.size
+      def nextState(state: State): State = state
+      def preCondition(state: State): Boolean = true
+      def postCondition(state: State, result: Try[Result]): Prop =
+        result == Success(state.elements.length)
+    }
+
+    case class Enqueue(element: Element) extends UnitCommand {
+      def run(sut: Sut): Unit =
+        sut.enqueue(element)
+      def nextState(state: State): State =
+        State(state.capacity, state.elements ++ List(element))
+      def preCondition(state: State): Boolean =
+        state.elements.length < state.capacity
+      def postCondition(state: State, success: Boolean): Prop =
+        success
+    }
+
+    case object Dequeue extends Command {
+      type Result = Element
+      def run(sut: Sut): Result =
+        sut.dequeue()
+      def nextState(state: State): State =
+        State(state.capacity, state.elements.tail)
+      def preCondition(state: State): Boolean =
+        state.elements.nonEmpty
+      def postCondition(state: State, result: Try[Result]): Prop =
+        result == Success(state.elements.head)
+    }
+
+    def genInitialState: Gen[State] =
+      Gen.choose(10, 20).map(State(_, Nil))
+
+    def genCommand(state: State): Gen[Command] =
+      Gen.oneOf(
+        Gen.const(Capacity),
+        Gen.const(Size),
+        Gen.const(Dequeue),
+        Arbitrary.arbitrary[Element].map(Enqueue(_))
+      ).retryUntil(_.preCondition(state), 100)
+
+    def newSut(state: State): Sut =
+      new BoundedQueue(state.capacity)
+
+    def initialPreCondition(state: State): Boolean =
+      state.capacity > 0 && state.elements.isEmpty
+
+    override def shrinkState: Shrink[State] =
+      Shrink.xmap[(Int, Seq[Element]), State](
+        { case (capacity, elems) => State(capacity, elems take capacity) },
+        { case State(capacity, elems) => (capacity, elems) }
+      ).suchThat(_.capacity >= 0)
+
+    def canCreateNewSut(
+      newState: State,
+      initSuts: Traversable[State],
+      runningSuts: Traversable[Sut]
+    ): Boolean =
+      true
+
+    def destroySut(sut: Sut): Unit =
+      ()
+  }
+}

src/main/scala/org/scalacheck/commands/Commands.scala

@@ -276,6 +276,34 @@ trait Commands {
 
     Shrink.shrinkWithOrig[State](as.s)(shrinkState) flatMap { state =>
       shrinkedCmds.map(_.copy(s = state))
+    } map { as => ensurePreconditions(as) }
+  }
+
+  private def ensurePreconditions(a: Actions): Actions = {
+    def filterCommandSequence(s: State, commands: Commands): Commands =
+      commands match {
+        case cmd :: cmds =>
+          if (cmd.preCondition(s))
+            cmd :: filterCommandSequence(cmd.nextState(s), cmds)
+          else
+            filterCommandSequence(s, cmds)
+        case Nil => Nil
+      }
+
+    val filteredSequentialCommands = filterCommandSequence(a.s, a.seqCmds)
+    val stateAfterSequentialCommands = filteredSequentialCommands
+      .foldLeft(a.s) { case (st, cmd) => cmd.nextState(st) }
+
+    val filteredParallelCommands = a.parCmds.map(commands => {
+      filterCommandSequence(stateAfterSequentialCommands, commands)
+    }).filter(_.nonEmpty)
+
+    filteredParallelCommands match {
+      case List(singleThreadedContinuation) =>
+        val seqCmds = filteredSequentialCommands ++ singleThreadedContinuation
+        Actions(a.s, seqCmds, Nil)
+      case _ =>
+        Actions(a.s, filteredSequentialCommands, filteredParallelCommands)
     }
   }