feat: implement head and headOption operators

The `headOption` operator returns the first element in `Source` wrapped
in `Some` or `None` in case when source is empty or failed e.g.:

  Source.empty[Int].headOption()       // None
  Source.fromValues(1, 2).headOption() // Some(1)

The `head` operator returns the first element in `Source` or throws
`NoSuchElementException` in case when it is either empty or `receive()`
operation fails without error. In case when `receive()` fails with
exception then this exception is re-thrown e.g.:

  Source.empty[Int].head()       // throws NoSuchElementException("cannot obtain head from the empty source")
  Source.fromValues(1, 2).head() // 1

Note that neither `head` nor `headOption` are idempotent operations.

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

@@ -5,6 +5,7 @@ import ox.*
 import java.util.concurrent.{CountDownLatch, Semaphore}
 import scala.collection.{IterableOnce, mutable}
 import scala.concurrent.duration.FiniteDuration
+import scala.util.Try
 
 trait SourceOps[+T] { this: Source[T] =>
   // view ops (lazy)
@@ -55,6 +56,21 @@ trait SourceOps[+T] { this: Source[T] =>
   def intersperse[U >: T](inject: U)(using Ox, StageCapacity): Source[U] =
     intersperse(None, inject, None)
 
+  private def intersperse[U >: T](start: Option[U], inject: U, end: Option[U])(using Ox, StageCapacity): Source[U] =
+    val c = StageCapacity.newChannel[U]
+    forkDaemon {
+      start.foreach(c.send)
+      var firstEmitted = false
+      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); true
+          case v: U @unchecked                  => c.send(inject); c.send(v); true
+      }
+    }
+    c
+
   /** Intersperses this source with start, end and provided elements and forwards it to the returned channel.
     *
     * @param start
@@ -80,21 +96,6 @@ trait SourceOps[+T] { this: Source[T] =>
   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 {
-      start.foreach(c.send)
-      var firstEmitted = false
-      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); true
-          case v: U @unchecked                  => c.send(inject); c.send(v); true
-      }
-    }
-    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.
     *
@@ -366,6 +367,15 @@ trait SourceOps[+T] { this: Source[T] =>
   def interleave[U >: T](other: Source[U], segmentSize: Int = 1, eagerComplete: Boolean = false)(using Ox, StageCapacity): Source[U] =
     Source.interleaveAll(List(this, other), segmentSize, eagerComplete)
 
+  /** Accumulates all elements received from the channel into a list. Blocks until the channel is done.
+    * @throws ChannelClosedException
+    *   when there is an upstream error.
+    */
+  def toList: List[T] =
+    val b = List.newBuilder[T]
+    foreach(b += _)
+    b.result()
+
   /** Invokes the given function for each received element. Blocks until the channel is done.
     * @throws ChannelClosedException
     *   when there is an upstream error.
@@ -378,15 +388,6 @@ trait SourceOps[+T] { this: Source[T] =>
         case t: T @unchecked        => f(t); true
     }
 
-  /** Accumulates all elements received from the channel into a list. Blocks until the channel is done.
-    * @throws ChannelClosedException
-    *   when there is an upstream error.
-    */
-  def toList: List[T] =
-    val b = List.newBuilder[T]
-    foreach(b += _)
-    b.result()
-
   /** Passes each received element from this channel to the given sink. Blocks until the channel is done.
     * @throws ChannelClosedException
     *   when there is an upstream error, or when the sink is closed.
@@ -513,13 +514,62 @@ trait SourceOps[+T] { this: Source[T] =>
       }
     }
     c
+
+  /** Returns the first element from this source wrapped in `Some` or `None` when the source is empty or fails during the receive operation.
+    * Note that `headOption` is not an idempotent operation on source as it receives elements from it.
+    *
+    * @return
+    *   A `Some(first element)` if source is not empty or None` otherwise.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   scoped {
+    *     Source.empty[Int].headOption()  // None
+    *     val s = Source.fromValues(1, 2)
+    *     s.headOption()                  // Some(1)
+    *     s.headOption()                  // Some(2)
+    *   }
+    *   }}}
+    */
+  def headOption(): Option[T] = Try(head()).toOption
+
+  /** Returns the first element from this source or throws `NoSuchElementException` when the source is empty or `receive()` operation fails
+    * without error. In case when the `receive()` operation fails with exception that exception is re-thrown. Note that `headOption` is not
+    * an idempotent operation on source as it receives elements from it.
+    *
+    * @return
+    *   A first element if source is not empty or throws otherwise.
+    * @throws NoSuchElementException
+    *   When source is empty or `receive()` failed without error.
+    * @throws exception
+    *   When `receive()` failed with exception then this exception is re-thrown.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   scoped {
+    *     Source.empty[Int].head()        // throws NoSuchElementException("cannot obtain head from the empty source")
+    *     val s = Source.fromValues(1, 2)
+    *     s.head()                        // 1
+    *     s.head()                        // 2
+    *   }
+    *   }}}
+    */
+  def head(): T =
+    supervised {
+      receive() match
+        case ChannelClosed.Done     => throw new NoSuchElementException("cannot obtain head from an empty source")
+        case ChannelClosed.Error(r) => throw r.getOrElse(new NoSuchElementException("getting head failed"))
+        case t: T @unchecked        => t
+    }
 }
 
 trait SourceCompanionOps:
   def fromIterable[T](it: Iterable[T])(using Ox, StageCapacity): Source[T] = fromIterator(it.iterator)
 
-  def fromValues[T](ts: T*)(using Ox, StageCapacity): Source[T] = fromIterator(ts.iterator)
-
   def fromIterator[T](it: => Iterator[T])(using Ox, StageCapacity): Source[T] =
     val c = StageCapacity.newChannel[T]
     forkDaemon {
@@ -531,6 +581,8 @@ trait SourceCompanionOps:
     }
     c
 
+  def fromValues[T](ts: T*)(using Ox, StageCapacity): Source[T] = fromIterator(ts.iterator)
+
   def fromFork[T](f: Fork[T])(using Ox, StageCapacity): Source[T] =
     val c = StageCapacity.newChannel[T]
     forkDaemon {
@@ -643,11 +695,6 @@ trait SourceCompanionOps:
     }
     c
 
-  def empty[T]: Source[T] =
-    val c = DirectChannel()
-    c.done()
-    c
-
   /** Sends a given number of elements (determined byc `segmentSize`) from each source in `sources` to the returned channel and repeats. The
     * order of elements in all sources is preserved.
     *
@@ -729,6 +776,11 @@ trait SourceCompanionOps:
         }
         c
 
+  def empty[T]: Source[T] =
+    val c = DirectChannel()
+    c.done()
+    c
+
   /** Creates a source that fails immediately with the given [[java.lang.Throwable]]
     *
     * @param t
@@ -740,3 +792,13 @@ trait SourceCompanionOps:
     val c = DirectChannel[T]()
     c.error(t)
     c
+
+  /** Creates a source that fails immediately
+    *
+    * @return
+    *   A source that would fail immediately
+    */
+  def failed[T](): Source[T] =
+    val c = DirectChannel[T]()
+    c.error(None)
+    c

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

@@ -0,0 +1,30 @@
+package ox.channels
+
+import org.scalatest.OptionValues
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsHeadOptionTest extends AnyFlatSpec with Matchers with OptionValues {
+  behavior of "Source.headOption"
+
+  it should "return None for the empty source" in supervised {
+    Source.empty[Int].headOption() shouldBe None
+  }
+
+  it should "return None for the failed source" in supervised {
+    Source
+      .failed(new RuntimeException("source is broken"))
+      .headOption() shouldBe None
+  }
+
+  it should "return Some element for the non-empty source" in supervised {
+    Source.fromValues(1, 2).headOption().value shouldBe 1
+  }
+
+  it should "be not idempotent operation" in supervised {
+    val s = Source.fromValues(1, 2)
+    s.headOption().value shouldBe 1
+    s.headOption().value shouldBe 2
+  }
+}

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

@@ -0,0 +1,39 @@
+package ox.channels
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsHeadTest extends AnyFlatSpec with Matchers {
+  behavior of "Source.head"
+
+  it should "throw NoSuchElementException for the empty source" in supervised {
+    the[NoSuchElementException] thrownBy {
+      Source.empty[Int].head()
+    } should have message "cannot obtain head from an empty source"
+  }
+
+  it should "re-throw exception that was thrown during element retrieval" in supervised {
+    the[RuntimeException] thrownBy {
+      Source
+        .failed(new RuntimeException("source is broken"))
+        .head()
+    } should have message "source is broken"
+  }
+
+  it should "throw NoSuchElementException for source failed without exception" in supervised {
+    the[NoSuchElementException] thrownBy {
+      Source.failed[Int]().head()
+    } should have message "getting head failed"
+  }
+
+  it should "return first value from non empty source" in supervised {
+    Source.fromValues(1, 2).head() shouldBe 1
+  }
+
+  it should "be not idempotent operation" in supervised {
+    val s = Source.fromValues(1, 2)
+    s.head() shouldBe 1
+    s.head() shouldBe 2
+  }
+}