Import splitWith from strawman-contrib

Taken from: https://github.com/scala/collection-strawman/issues/241

Author: markus1189

src/main/scala/scala/collection/decorators/SeqDecorator.scala

@@ -41,4 +41,48 @@ class SeqDecorator[C, S <: HasSeqOps[C]](coll: C)(implicit val seq: S) {
   def intersperse[B >: seq.A, That](start: B, sep: B, end: B)(implicit bf: BuildFrom[C, B, That]): That =
     bf.fromSpecificIterable(coll)(new View.IntersperseSurround(seq(coll), start, sep, end))
 
+  /** Splits this collection into groups according to the given predicate.
+    *
+    * @param p the predicate used to discriminate elements
+    * @return A nested collection with groups of elements,
+    *         opening new groups whenever the predicate
+    *         changes the return type
+    *
+    * @example {{{
+    * // Example 1: Split a list of integers into groups that are even / odd
+    * List(1, 2, 4, 6, 7).splitWith(i => i % 2 == 0) => List(List(1), List(2, 4, 6), List(7))
+    *
+    * // Example 2: Split a list of chars into groups that are upper case or lower case
+    * List('a', 'b', 'C', 'D', 'e', 'f').splitWith(_.isUpper) => List(List('a', 'b'), List('C', 'D'), List('e', 'f'))
+    * }}}
+    */
+  def splitWith[Group, That](p: seq.A => Boolean)(implicit bfGroup: BuildFrom[C, seq.A, Group], bfThat: BuildFrom[C, Group, That]): That = {
+    def newGroupBuilder() = bfGroup.newBuilder(coll)
+
+    val groups: mutable.Builder[Group, That] = bfThat.newBuilder(coll)
+    val it: Iterator[seq.A] = seq(coll).iterator
+
+    var currentGroup = newGroupBuilder()
+    var lastTestResult = Option.empty[Boolean]
+
+    while (it.hasNext) {
+      val elem = it.next()
+      val currentTest = p(elem)
+
+      lastTestResult match {
+        case None =>
+          currentGroup.addOne(elem)
+        case Some(lastTest) if currentTest == lastTest =>
+          currentGroup.addOne(elem)
+        case Some(_) =>
+          groups.addOne(currentGroup.result())
+          currentGroup = newGroupBuilder().addOne(elem)
+      }
+
+      lastTestResult = Some(currentTest)
+    }
+
+    groups.addOne(currentGroup.result()).result()
+  }
+
 }

src/test/scala/scala/collection/decorators/SeqDecoratorTest.scala

@@ -16,6 +16,13 @@ class SeqDecoratorTest {
     assertEquals(List(0, 1, 2), List(1).intersperse(0, 5, 2))
   }
 
+  @Test def splitWith(): Unit = {
+    assertEquals(List(List()), List().splitWith(_ => true))
+    assertEquals(List(List()), List().splitWith(_ => false))
+    assertEquals(List(List(1), List(2, 4, 6), List(7)), List(1, 2, 4, 6, 7).splitWith(i => i % 2 == 0))
+    assertEquals(List(List('a', 'b'), List('C', 'D'), List('e', 'f')), List('a', 'b', 'C', 'D', 'e', 'f').splitWith(_.isUpper))
+  }
+
   // This test just checks that there is no compilation error
   @Test def genericDecorator(): Unit = {
     val vector = Vector(1, 2, 3)