Add groupMapReduce method to IterableOnceOps

src/main/scala/scala/collection/next/IterableOnceExtensions.scala

@@ -0,0 +1,41 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.collection
+package next
+
+object IterableOnceExtensions {
+  implicit class NextIterableOnceOpsExtensions[A](private val iter: IterableOnceOps[A, Any, Any]) extends AnyVal {
+    /**
+     * Partitions this IterableOnce into a map according to a discriminator function `key`. All the values that
+     * have the same discriminator are then transformed by the `value` function and then reduced into a
+     * single value with the `reduce` function.
+     *
+     * {{{
+     *   def occurrences[A](as: IterableOnce[A]): Map[A, Int] =
+     *     as.iterator.groupMapReduce(identity)(_ => 1)(_ + _)
+     * }}}
+     *
+     * @note This will force the evaluation of the Iterator.
+     */
+    def groupMapReduce[K, B](key: A => K)(f: A => B)(reduce: (B, B) => B): immutable.Map[K, B] = {
+      val m = mutable.Map.empty[K, B]
+      iter.foreach { elem =>
+        m.updateWith(key = key(elem)) {
+          case Some(b) => Some(reduce(b, f(elem)))
+          case None    => Some(f(elem))
+        }
+      }
+      m.to(immutable.Map)
+    }
+  }
+}

src/test/scala/scala/collection/next/TestIterableOnceExtensions.scala

@@ -0,0 +1,86 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala.collection.next
+
+import org.junit.Assert._
+import org.junit.Test
+import scala.collection.IterableOnceOps
+import scala.collection.generic.IsIterableOnce
+
+import IterableOnceExtensions._
+
+final class TestIterableOnceExtensions {
+  import TestIterableOnceExtensions.LowerCaseString
+
+  @Test
+  def iteratorGroupMapReduce(): Unit = {
+    def occurrences[A](coll: IterableOnce[A]): Map[A, Int] =
+      coll.iterator.groupMapReduce(identity)(_ => 1)(_ + _)
+
+    val xs = Seq('a', 'b', 'b', 'c', 'a', 'a', 'a', 'b')
+    val expected = Map('a' -> 4, 'b' -> 3, 'c' -> 1)
+    assertEquals(expected, occurrences(xs))
+  }
+
+  @Test
+  def iterableOnceOpsGroupMapReduce(): Unit = {
+    def occurrences[A](coll: IterableOnceOps[A, Any, Any]): Map[A, Int] =
+      coll.groupMapReduce(identity)(_ => 1)(_ + _)
+
+    val xs = Seq('a', 'b', 'b', 'c', 'a', 'a', 'a', 'b')
+    val expected = Map('a' -> 4, 'b' -> 3, 'c' -> 1)
+    assertEquals(expected, occurrences(xs))
+  }
+
+  @Test
+  def anyLikeIterableOnceGroupMapReduce(): Unit = {
+    def occurrences[Repr](coll: Repr)(implicit it: IsIterableOnce[Repr]): Map[it.A, Int] =
+      it(coll).iterator.groupMapReduce(identity)(_ => 1)(_ + _)
+
+    val xs = "abbcaaab"
+    val expected = Map('a' -> 4, 'b' -> 3, 'c' -> 1)
+    assertEquals(expected, occurrences(xs))
+  }
+
+  @Test
+  def customIterableOnceOpsGroupMapReduce(): Unit = {
+    def occurrences(coll: LowerCaseString): Map[Char, Int] =
+      coll.groupMapReduce(identity)(_ => 1)(_ + _)
+
+    val xs = LowerCaseString("abBcAaAb")
+    val expected = Map('a' -> 4, 'b' -> 3, 'c' -> 1)
+    assertEquals(expected, occurrences(xs))
+  }
+}
+
+object TestIterableOnceExtensions {
+  final case class LowerCaseString(source: String) extends IterableOnce[Char] with IterableOnceOps[Char, Iterable, String] {
+    override def iterator: Iterator[Char] = source.iterator.map(_.toLower)
+
+    override def scanLeft[B](z: B)(op: (B, Char) => B): Iterable[B] = ???
+    override def filter(p: Char => Boolean): String = ???
+    override def filterNot(pred: Char => Boolean): String = ???
+    override def take(n: Int): String = ???
+    override def takeWhile(p: Char => Boolean): String = ???
+    override def drop(n: Int): String = ???
+    override def dropWhile(p: Char => Boolean): String = ???
+    override def slice(from: Int, until: Int): String = ???
+    override def map[B](f: Char => B): Iterable[B] = ???
+    override def flatMap[B](f: Char => IterableOnce[B]): Iterable[B] = ???
+    override def flatten[B](implicit asIterable: Char => IterableOnce[B]): Iterable[B] = ???
+    override def collect[B](pf: PartialFunction[Char,B]): Iterable[B] = ???
+    override def zipWithIndex: Iterable[(Char, Int)] = ???
+    override def span(p: Char => Boolean): (String, String) = ???
+    override def tapEach[U](f: Char => U): String = ???
+  }
+}