=str - 19149 - Corrects the behavior of FlowOps.sliding

Generatively attempts to verify that it behaves exactly like the
collections library

Removes the use of Vector.builder for an easier-to-follow Vector
implementation while waiting for this to be reimplemented as a
GraphStage.

Also, the Builder implementation relies on undefined behavior:

"Builder.result() — Produces a collection from the added elements.
The builder's contents are undefined after this operation." -
http://www.scala-lang.org/api/current/#scala.collection.mutable.Builder

akka-stream/src/main/scala/akka/stream/impl/fusing/Ops.scala

@@ -318,55 +318,32 @@ private[akka] final case class Grouped[T](n: Int) extends PushPullStage[T, immut
  * INTERNAL API
  */
 private[akka] final case class Sliding[T](n: Int, step: Int) extends PushPullStage[T, immutable.Seq[T]] {
-  private val buf = {
-    val b = Vector.newBuilder[T]
-    b.sizeHint(n)
-    b
-  }
-  var bufferedElements = 0
+  private var buf = Vector.empty[T]
 
   override def onPush(elem: T, ctx: Context[immutable.Seq[T]]): SyncDirective = {
-    buf += elem
-    bufferedElements += 1
-    if (bufferedElements < n) {
+    buf :+= elem
+    if (buf.size < n) {
+      ctx.pull()
+    } else if (buf.size == n) {
+      ctx.push(buf)
+    } else if (step > n) {
+      if (buf.size == step)
+        buf = Vector.empty
       ctx.pull()
-    } else if (bufferedElements == n) {
-      ctx.push(buf.result())
     } else {
-      if (step > n) {
-        if (bufferedElements == step) {
-          buf.clear()
-          buf.sizeHint(n)
-          bufferedElements = 0
-          ctx.pull()
-        } else {
-          ctx.pull()
-        }
-      } else {
-        val emit = buf.result()
-        buf.clear()
-        buf.sizeHint(n)
-        emit.drop(step).foreach(buf += _)
-        val updatedEmit = buf.result()
-        bufferedElements = updatedEmit.size
-        if (bufferedElements == n) ctx.push(updatedEmit)
-        else ctx.pull()
-      }
+      buf = buf.drop(step)
+      if (buf.size == n) ctx.push(buf)
+      else ctx.pull()
     }
   }
 
   override def onPull(ctx: Context[immutable.Seq[T]]): SyncDirective =
-    if (ctx.isFinishing) {
-      val emit = buf.result()
-      if (emit.size == n) {
-        ctx.finish()
-      } else {
-        ctx.pushAndFinish(emit)
-      }
-    } else ctx.pull()
+    if (!ctx.isFinishing) ctx.pull()
+    else if (buf.size >= n) ctx.finish()
+    else ctx.pushAndFinish(buf)
 
   override def onUpstreamFinish(ctx: Context[immutable.Seq[T]]): TerminationDirective =
-    if (buf.result().isEmpty) ctx.finish()
+    if (buf.isEmpty) ctx.finish()
     else ctx.absorbTermination()
 }