feat: implement fold operator

The `fold` operation returns combined value retrieved from running function
`f` on all source elements in a cumulative manner where result of the previous
call is used as an input value to the next e.g.:

  Source.empty[Int].fold(0)((acc, n) => acc + n)       // 0
  Source.fromValues(2, 3).fold(5)((acc, n) => acc - n) // 0

Note that in case when `receive()` operation fails then
ChannelClosedException.Error exception is thrown.

Author: geminicaprograms

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

@@ -623,6 +623,42 @@ trait SourceOps[+T] { this: Source[T] =>
     *   }}}
     */
   def last(): T = lastOption().getOrElse(throw new NoSuchElementException("cannot obtain last from an empty source"))
+
+  /** Uses `zero` as the current value and applies function `f` on it and a value received from a source. The returned value is used as the
+    * next current value and `f` is applied again with the value received from a source. The operation is repeated until the source is
+    * drained.
+    *
+    * @param zero
+    *   An initial value to be used as the first argument to function `f` call.
+    * @param f
+    *   A binary function (a function that takes two arguments) that is applied to the current value and value received from a source.
+    * @return
+    *   Combined value retrieved from running function `f` on all source elements in a cumulative manner where result of the previous call
+    *   is used as an input value to the next.
+    * @throws ChannelClosedException.Error
+    *   When [[receive]] fails.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   supervised {
+    *     Source.empty[Int].fold(0)((acc, n) => acc + n)       // 0
+    *     Source.fromValues(2, 3).fold(5)((acc, n) => acc - n) // 0
+    *   }
+    *   }}}
+    */
+  def fold[U](zero: U)(f: (U, T) => U): U =
+    supervised {
+      var current = zero
+      repeatWhile {
+        receive() match
+          case ChannelClosed.Done     => false
+          case e: ChannelClosed.Error => throw e.toThrowable
+          case t: T @unchecked        => current = f(current, t); true
+      }
+      current
+    }
 }
 
 trait SourceCompanionOps:

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

@@ -0,0 +1,39 @@
+package ox.channels
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsFoldTest extends AnyFlatSpec with Matchers {
+  behavior of "Source.fold"
+
+  it should "throw ChannelClosedException.Error with exception and message that was thrown during retrieval" in supervised {
+    the[ChannelClosedException.Error] thrownBy {
+      Source
+        .failed[Int](new RuntimeException("source is broken"))
+        .fold(0)((acc, n) => acc + n)
+    } should have message "java.lang.RuntimeException: source is broken"
+  }
+
+  it should "throw ChannelClosedException.Error for source failed without exception" in supervised {
+    the[ChannelClosedException.Error] thrownBy {
+      Source
+        .failedWithoutReason[Int]()
+        .fold(0)((acc, n) => acc + n)
+    }
+  }
+
+  it should "return `zero` value from fold on the empty source" in supervised {
+    Source.empty[Int].fold(0)((acc, n) => acc + n) shouldBe 0
+  }
+
+  it should "return fold on non-empty source" in supervised {
+    Source.fromValues(1, 2).fold(0)((acc, n) => acc + n) shouldBe 3
+  }
+
+  it should "drain the source" in supervised {
+    val s = Source.fromValues(1)
+    s.fold(0)((acc, n) => acc + n) shouldBe 1
+    s.receive() shouldBe ChannelClosed.Done
+  }
+}