Add Source.mapParUnordered combinator

core/src/main/scala/ox/channels/SourceOps.scala

@@ -110,7 +110,34 @@ trait SourceOps[+T] { this: Source[T] =>
       closeScope.await()
     }
 
-  def mapParUnordered[U](parallelism: Int)(f: T => U)(using Ox, StageCapacity): Source[U] = ??? // TODO
+  def mapParUnordered[U](parallelism: Int)(f: T => U)(using Ox, StageCapacity): Source[U] =
+    val c = StageCapacity.newChannel[U]
+    val s = new Semaphore(parallelism)
+    forkDaemon {
+      supervised {
+        repeatWhile {
+          s.acquire()
+          receive() match
+            case ChannelClosed.Done => false
+            case e @ ChannelClosed.Error(r) =>
+              c.error(r)
+              throw e.toThrowable
+            case t: T @unchecked =>
+              fork {
+                try
+                  c.send(f(t))
+                  s.release()
+                catch
+                  case t: Throwable =>
+                    c.error(t)
+                    throw t
+              }
+              true
+        }
+      }
+      c.done()
+    }
+    c
 
   def take(n: Int)(using Ox, StageCapacity): Source[T] = transform(_.take(n))
 

core/src/test/scala/ox/channels/SourceOpsMapParUnorderedTest.scala

@@ -0,0 +1,137 @@
+package ox.channels
+
+import org.scalatest.concurrent.Eventually
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+import ox.util.Trail
+
+import java.util.concurrent.atomic.AtomicInteger
+
+class SourceOpsMapParUnorderedTest extends AnyFlatSpec with Matchers with Eventually {
+
+  behavior of "Source.mapParUnordered"
+
+  for (parallelism <- 1 to 10) {
+    it should s"map over a source with parallelism limit $parallelism" in scoped {
+      // given
+      val s = Source.fromIterable(1 to 10)
+      val running = new AtomicInteger(0)
+      val maxRunning = new AtomicInteger(0)
+
+      def f(i: Int) =
+        running.incrementAndGet()
+        try
+          Thread.sleep(100)
+          i * 2
+        finally running.decrementAndGet()
+
+      // update max running
+      fork {
+        var max = 0
+        forever {
+          max = math.max(max, running.get())
+          maxRunning.set(max)
+          Thread.sleep(10)
+        }
+      }
+
+      // when
+      val result = s.mapParUnordered(parallelism)(f).toList
+
+      // then
+      result should contain theSameElementsAs List(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)
+      maxRunning.get() shouldBe parallelism
+    }
+  }
+
+  it should s"map over a source with parallelism limit 10 (stress test)" in scoped {
+    for (i <- 1 to 100) {
+      info(s"iteration $i")
+
+      // given
+      val s = Source.fromIterable(1 to 10)
+
+      def f(i: Int) =
+        Thread.sleep(50)
+        i * 2
+
+      // when
+      val result = s.mapParUnordered(10)(f).toList
+
+      // then
+      result should contain theSameElementsAs List(2, 4, 6, 8, 10, 12, 14, 16, 18, 20)
+    }
+  }
+
+  it should "propagate errors" in scoped {
+    // given
+    val s = Source.fromIterable(1 to 10)
+    val started = new AtomicInteger()
+
+    // when
+    val s2 = s.mapParUnordered(3) { i =>
+      started.incrementAndGet()
+      if i > 4 then throw new Exception("boom")
+      i * 2
+    }
+
+    // then
+    try
+      s2.toList
+      fail("should have thrown")
+    catch
+      case ChannelClosedException.Error(Some(reason)) if reason.getMessage == "boom" =>
+        started.get() should be >= 4
+        started.get() should be <= 7 // 4 successful + at most 3 taking up all the permits
+  }
+
+  it should "cancel other running forks when there's an error" in scoped {
+    // given
+    val trail = Trail()
+    val s = Source.fromIterable(1 to 10)
+
+    // when
+    val s2 = s.mapParUnordered(2) { i =>
+      if i == 4 then
+        Thread.sleep(100)
+        trail.add("exception")
+        throw new Exception("boom")
+      else
+        Thread.sleep(200)
+        trail.add(s"done")
+        i * 2
+    }
+
+    // then
+    List(s2.receive(), s2.receive()) should contain only (2, 4)
+    s2.receive() should matchPattern { case ChannelClosed.Error(Some(reason)) if reason.getMessage == "boom" => }
+    s2.isError shouldBe true
+
+    // checking if the forks aren't left running
+    Thread.sleep(200)
+    trail.get shouldBe Vector("done", "done", "exception")
+  }
+
+  it should "emit downstream as soon as a value is ready, regardless of the incoming order" in scoped {
+    // given
+    val s = Source.fromIterable(1 to 5)
+    val delays = Map(
+      1 -> 100,
+      2 -> 10,
+      3 -> 50,
+      4 -> 500,
+      5 -> 200
+    )
+    val expectedElements = delays.toList.sortBy(_._2).map(_._1)
+
+    // when
+    val s2 = s.mapParUnordered(5) { i =>
+      Thread.sleep(delays(i))
+      i
+    }
+
+    // then
+    s2.toList should contain theSameElementsInOrderAs expectedElements
+  }
+}