From 77a9a8c493856988269233584d850c7bb9824cc9 Mon Sep 17 00:00:00 2001
From: Jacek Centkowski <jacek.centkowski@softwaremill.com>
Date: Mon, 16 Oct 2023 18:07:37 +0200
Subject: [PATCH] feat: implement `dropWhile` function

Drops elements from the source as long as the predicate is satisfied.
Note the if predicate fails then subsequent elements are no longer
dropped even if they could still satisfy it. Examples:

  Source.empty[Int].dropWhile(_ > 3).toList          // List()
  Source.fromValues(1, 2, 3).dropWhile(_ < 3).toList // List(3)
  Source.fromValues(1, 2, 1).dropWhile(_ < 2).toList // List(2, 1)
---
 .../main/scala/ox/channels/SourceOps.scala    | 19 +++++++++++
 .../ox/channels/SourceOpsDropWhileTest.scala  | 34 +++++++++++++++++++
 2 files changed, 53 insertions(+)
 create mode 100644 core/src/test/scala/ox/channels/SourceOpsDropWhileTest.scala

diff --git a/core/src/main/scala/ox/channels/SourceOps.scala b/core/src/main/scala/ox/channels/SourceOps.scala
index 6984efc4..216c14e3 100644
--- a/core/src/main/scala/ox/channels/SourceOps.scala
+++ b/core/src/main/scala/ox/channels/SourceOps.scala
@@ -159,6 +159,25 @@ trait SourceOps[+T] { this: Source[T] =>
     */
   def drop(n: Int)(using Ox, StageCapacity): Source[T] = transform(_.drop(n))
 
+  /** Drops elements from the source as long as predicate `f` is satisfied (returns `true`). Note that once the predicate `f` is not
+    * satisfied (returns `false`) no elements are dropped from the source even if they could still satisfy it.
+    *
+    * @param f
+    *   A predicate function.
+    * @example
+    *   {{{
+    *   import ox.*
+    *   import ox.channels.Source
+    *
+    *   scoped {
+    *     Source.empty[Int].dropWhile(_ > 3).toList          // List()
+    *     Source.fromValues(1, 2, 3).dropWhile(_ < 3).toList // List(3)
+    *     Source.fromValues(1, 2, 1).dropWhile(_ < 2).toList // List(2, 1)
+    *   }
+    *   }}}
+    */
+  def dropWhile(f: T => Boolean)(using Ox, StageCapacity): Source[T] = transform(_.dropWhile(f))
+
   def filter(f: T => Boolean)(using Ox, StageCapacity): Source[T] = transform(_.filter(f))
 
   def transform[U](f: Iterator[T] => Iterator[U])(using Ox, StageCapacity): Source[U] =
diff --git a/core/src/test/scala/ox/channels/SourceOpsDropWhileTest.scala b/core/src/test/scala/ox/channels/SourceOpsDropWhileTest.scala
new file mode 100644
index 00000000..f52e5403
--- /dev/null
+++ b/core/src/test/scala/ox/channels/SourceOpsDropWhileTest.scala
@@ -0,0 +1,34 @@
+package ox.channels
+
+import org.scalatest.flatspec.AnyFlatSpec
+import org.scalatest.matchers.should.Matchers
+import ox.*
+
+class SourceOpsDropWhileTest extends AnyFlatSpec with Matchers {
+  behavior of "Source.drop"
+
+  it should "not drop from the empty source" in scoped {
+    val s = Source.empty[Int]
+    s.dropWhile(_ > 0).toList shouldBe List.empty
+  }
+
+  it should "drop elements from the source while predicate is true" in scoped {
+    val s = Source.fromValues(1, 2, 3)
+    s.dropWhile(_ < 3).toList shouldBe List(3)
+  }
+
+  it should "drop elements from the source until predicate is true and then emit subsequent ones" in scoped {
+    val s = Source.fromValues(1, 2, 3, 2)
+    s.dropWhile(_ < 3).toList shouldBe List(3, 2)
+  }
+
+  it should "not drop elements from the source if predicate is false" in scoped {
+    val s = Source.fromValues(1, 2, 3)
+    s.dropWhile(_ > 3).toList shouldBe List(1, 2, 3)
+  }
+
+  it should "not drop elements from the source when predicate is false for first or more elements" in scoped {
+    val s = Source.fromValues(1, 4, 5)
+    s.dropWhile(_ > 3).toList shouldBe List(1, 4, 5)
+  }
+}