adds splitBy extension method on scala.collection.Iterable

Author: jeantil

src/main/scala/scala/collection/decorators/IterableDecorator.scala

@@ -33,4 +33,46 @@ class IterableDecorator[C, I <: IsIterable[C]](coll: C)(implicit val it: I) {
   def lazyFoldRight[B](z: B)(op: it.A => Either[B, B => B]): B =
     it(coll).iterator.lazyFoldRight(z)(op)
 
+
+  /**
+    * Constructs an iterator where consecutive elements are accumulated as
+    * long as the output of f for each element doesn't change.
+    * <pre>
+    * Vector(1,2,2,3,3,3,2,2)
+    * .splitBy(identity)
+    * </pre>
+    * produces
+    * <pre>
+    * Iterator(Vector(1),
+    * Vector(2,2),
+    * Vector(3,3,3),
+    * Vector(2,2))
+    * </pre>
+    *
+    * @param f the function to compute a key for an element
+    * @tparam K the type of the computed key
+    * @return an iterator of sequences of the consecutive elements with the
+    *         same key in the original iterator
+    */
+  def splitBy[K, That, CC](f: it.A => K)(implicit bf: BuildFrom[C, it.A, That], bff: BuildFrom[C, That, CC]): CC = {
+    val builder = bff.newBuilder(coll)
+
+    val iterator = it(coll).iterator
+    var init = bf.newBuilder(coll)
+    if (iterator.hasNext) {
+      var ref = iterator.next();
+      init += ref
+      while (iterator.hasNext) {
+        val el = iterator.next();
+        if (f(el) != f(ref)) {
+          builder += init.result()
+          init = bf.newBuilder(coll)
+          ref = el
+        }
+        init += el
+      }
+      builder += init.result()
+    }
+    builder.result();
+  }
 }

src/test/scala/scala/collection/decorators/IterableDecoratorTest.scala

@@ -2,7 +2,8 @@ package scala.collection
 package decorators
 
 import org.junit.{Assert, Test}
-import scala.collection.immutable.{LazyList, List, Range, Map}
+
+import scala.collection.immutable.{LazyList, List, Map, Range}
 
 class IterableDecoratorTest {
 
@@ -36,4 +37,44 @@ class IterableDecoratorTest {
     Assert.assertEquals(2, result2)
   }
 
+  @Test
+  def splitByShouldHonorEmptyIterator(): Unit = {
+    val split = Vector.empty[Int].splitBy(identity)
+    Assert.assertEquals(Vector.empty, split)
+  }
+
+  @Test
+  def splitByShouldReturnSingleSeqWhenSingleElement(): Unit = {
+    val value = Vector("1")
+    val split = value.splitBy(identity)
+    Assert.assertEquals(Vector(value), split)
+  }
+
+  @Test
+  def splitByShouldReturnSingleSeqWhenAllElHaveTheSameKey(): Unit = {
+    val value = Vector("1", "1", "1")
+    val split = value.splitBy(identity)
+    Assert.assertEquals(Vector(value), split)
+  }
+
+  @Test
+  def splitByShouldReturnVectorOfVectorOrConsecutiveElementsWithTheSameKey(): Unit = {
+    val value = Vector("1", "2", "2", "3", "3", "3", "2", "2")
+    val split: Vector[Vector[String]] = value.splitBy(identity)
+    Assert.assertEquals(Vector(Vector("1"), Vector("2", "2"), Vector("3", "3", "3"), Vector("2", "2")), split)
+  }
+
+  @Test
+  def splitByShouldReturnListOfListOfConsecutiveElementsWithTheSameKey(): Unit = {
+    val value = List("1", "2", "2", "3", "3", "3", "2", "2")
+    val split: List[List[String]] = value.splitBy(identity)
+    Assert.assertEquals(List(List("1"), List("2", "2"), List("3", "3", "3"), List("2", "2")), split)
+  }
+
+  @Test
+  def splitByShouldReturnSetOfSetOfConsecutiveElementsWithTheSameKey(): Unit = {
+    val value = Set("1", "2", "2", "3", "3", "3", "2", "2")
+    val split: Set[Set[String]] = value.splitBy(identity)
+    Assert.assertEquals(Set(Set("1"), Set("2"), Set("3")), split)
+  }
 }