feat: introduce intersperse function

The intersperse function behaves as `List.mkString` e.g.:
* called with an `inject` element only it produces

  Source.fromValues("f", "b").intersperse(", ")  // ("f", ", ", "b")

* called with `start`, `inject` and `end` it produces

  Source.fromValues("f", "b").intersperse("[", ", ", "]")  // ("[", "f", ", ", "b", "]")

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

@@ -2,7 +2,7 @@ package ox.channels
 
 import ox.*
 
-import java.util.concurrent.{ArrayBlockingQueue, ConcurrentLinkedQueue, CountDownLatch, LinkedBlockingQueue, Semaphore}
+import java.util.concurrent.{CountDownLatch, Semaphore}
 import scala.collection.mutable
 import scala.concurrent.duration.FiniteDuration
 
@@ -34,6 +34,57 @@ trait SourceOps[+T] { this: Source[T] =>
     }
     c2
 
+  /** Intersperses stream with provided element.
+    *
+    * @param inject
+    *   An element to be injected between the stream elements.
+    * @return
+    *   A source, onto which elements will be injected.
+    * @example
+    *   {{{
+    *   Source.empty[String].intersperse(", ")             // ()
+    *   Source.fromValues("foo").intersperse(", ")         // ("foo")
+    *   Source.fromValues("foo", "bar").intersperse(", ")  // ("foo", ", ", "bar")
+    *   }}}
+    */
+  def intersperse[U >: T](inject: U)(using Ox, StageCapacity): Source[U] =
+    intersperse(None, inject, None)
+
+  /** Intersperses stream with start, end and provided element.
+    *
+    * @param start
+    *   An element to be prepended to the stream.
+    * @param inject
+    *   An element to be injected between the stream elements.
+    * @param end
+    *   An element to be appended to the end of the stream.
+    * @return
+    *   A source, onto which elements will be injected.
+    * @example
+    *   {{{
+    *   Source.empty[String].intersperse("[", ", ", "]")             // ("[", "]")
+    *   Source.fromValues("foo").intersperse("[", ", ", "]")         // ("[", "foo", "]")
+    *   Source.fromValues("foo", "bar").intersperse("[", ", ", "]")  // ("[", "foo", ", ", "bar", "]")
+    *   }}}
+    */
+  def intersperse[U >: T](start: U, inject: U, end: U)(using Ox, StageCapacity): Source[U] =
+    intersperse(Some(start), inject, Some(end))
+
+  private def intersperse[U >: T](start: Option[U], inject: U, end: Option[U])(using Ox, StageCapacity): Source[U] =
+    val c = StageCapacity.newChannel[U]
+    forkDaemon {
+      var firstEmitted = false
+      start.foreach(c.send(_))
+      repeatWhile {
+        receive() match
+          case ChannelClosed.Done               => end.foreach(c.send(_)); c.done(); false
+          case ChannelClosed.Error(e)           => c.error(e); false
+          case v: U @unchecked if !firstEmitted => firstEmitted = true; c.send(v).isValue
+          case v: U @unchecked                  => c.send(inject); c.send(v).isValue
+      }
+    }
+    c
+
   /** Applies the given mapping function `f` to each element received from this source, and sends the results to the returned channel. At
     * most `parallelism` invocations of `f` are run in parallel.
     *

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

@@ -0,0 +1,107 @@
+package ox.channels
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsIntersperseTest extends AnyFlatSpec with Matchers {
+  behavior of "Source.intersperse"
+
+  it should "intersperse with inject only over an empty source" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.done()
+      }
+
+      val s = c.intersperse(", ")
+
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+
+  it should "intersperse with inject only over a source with one element" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.send("foo")
+        c.done()
+      }
+
+      val s = c.intersperse(", ")
+
+      s.receive() shouldBe "foo"
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+
+  it should "intersperse with inject only over a source with multiple elements" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.send("foo")
+        c.send("bar")
+        c.done()
+      }
+
+      val s = c.intersperse(", ")
+
+      s.receive() shouldBe "foo"
+      s.receive() shouldBe ", "
+      s.receive() shouldBe "bar"
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+
+  it should "intersperse with start, inject and end over an empty source" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.done()
+      }
+
+      val s = c.intersperse("[", ", ", "]")
+
+      s.receive() shouldBe "["
+      s.receive() shouldBe "]"
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+
+  it should "intersperse with start, inject and end over a source with one element" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.send("foo")
+        c.done()
+      }
+
+      val s = c.intersperse("[", ", ", "]")
+
+      s.receive() shouldBe "["
+      s.receive() shouldBe "foo"
+      s.receive() shouldBe "]"
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+
+  it should "intersperse with start, inject and end over a source with multiple elements" in {
+    scoped {
+      val c = Channel[String]()
+      fork {
+        c.send("foo")
+        c.send("bar")
+        c.done()
+      }
+
+      val s = c.intersperse("[", ", ", "]")
+
+      s.receive() shouldBe "["
+      s.receive() shouldBe "foo"
+      s.receive() shouldBe ", "
+      s.receive() shouldBe "bar"
+      s.receive() shouldBe "]"
+      s.receive() shouldBe ChannelClosed.Done
+    }
+  }
+}