Realistically test behaviour of mergePreferred

Author: shagoon

Diff for: core/shared/src/test/scala/fs2/StreamMergeSuite.scala

@@ -243,14 +243,29 @@ class StreamMergeSuite extends Fs2Suite {
     }
   }
 
-  test("mergePreferred prefers") {
-    forAllF { (leftStream: Stream[Pure, Int], rightStream: Stream[Pure, Int]) =>
-      val leftTagged = leftStream.covary[IO]
-      val rightTagged = rightStream.covary[IO].delayBy(10.milli)
-      leftTagged
-        .mergePreferred(rightTagged)
-        .assertEmitsSameAs(leftStream ++ rightStream)
-    }
+  test("mergePreferred prefers this over that") {
+
+    val units = Stream.unit.covary[IO].repeat
+    val left = units.map(Left(_))
+    val right = units.map(Right(_))
+
+    val stream = left.mergePreferred(right)
+
+    stream
+      .take(10000)
+      .fold((0L, 0L)) {
+        case ((left, right), Left(_))  => (left + 1, right)
+        case ((left, right), Right(_)) => (left, right + 1)
+      }
+      .compile
+      .lastOrError
+      .map { case (left, right) =>
+        val relLeft = left.toDouble / (left + right).toDouble
+        // Tolerate up to 2% elements of the non preferred stream.
+        // Increase the value, if the test (ocassionally) reports false positives.
+        val delta = 0.02d
+        assertEqualsDouble(relLeft, 1.0d, delta)
+      }
   }
 
   test("mergePreferred fully consumes this") {