Add implementation of distinctBy (#602)

Author: alexklibisz

compat/src/main/scala-2.11_2.12/scala/collection/compat/PackageShared.scala

@@ -564,6 +564,19 @@ class TraversableLikeExtensionMethods[A, Repr](private val self: c.GenTraversabl
     }
     map.toMap
   }
+
+  def distinctBy[B, That](f: A => B)(implicit cbf: CanBuildFrom[Repr, A, That]): That = {
+    val builder = cbf()
+    val keys    = collection.mutable.Set.empty[B]
+    for (element <- self) {
+      val key = f(element)
+      if (!keys.contains(key)) {
+        builder += element
+        keys += key
+      }
+    }
+    builder.result()
+  }
 }
 
 class TrulyTraversableLikeExtensionMethods[El1, Repr1](

compat/src/test/scala/test/scala/collection/CollectionTest.scala

@@ -186,4 +186,11 @@ class CollectionTest {
     assertTrue(List(1, 2, 3).lengthIs >= 2)
     assertTrue(List(1, 2, 3).lengthIs > 2)
   }
+
+  @Test
+  def testDistinctBy(): Unit = {
+    assertEquals(List(1, 2, 3).distinctBy(_   % 2 == 0), List(1, 2))
+    assertEquals(List(3, 1, 2).distinctBy(_   % 2 == 0), List(3, 2))
+    assertEquals(List.empty[Int].distinctBy(_ % 2 == 0), List.empty)
+  }
 }