use classtag

Author: Martijn Hoekstra

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

@@ -12,73 +12,45 @@
 
 package scala.collection.compat
 
-import scala.collection.LinearSeq
+import scala.reflect.ClassTag
 import scala.collection.generic.CanBuildFrom
 import scala.collection.mutable.Builder
 import scala.collection.{immutable => i, mutable => m}
 
 /* builder optimized for a single ++= call, which returns identity on result if possible
  * and defers to the underlying builder if not.
  */
-private final class IdentityPreservingSeqBuilder[A](that: Builder[A, Seq[A]])
-    extends Builder[A, Seq[A]] {
-  var collection: Seq[A] = null
+private final class IdentityPreservingBuilder[A, CC[X] <: TraversableOnce[X]](that: Builder[A, CC[A]])(implicit ct: ClassTag[CC[A]])
+    extends Builder[A, CC[A]] {
+  var collection: CC[A] = null.asInstanceOf[CC[A]]
   var ruined = false
 
   final override def ++=(elems: TraversableOnce[A]): this.type =
-      if(!ruined && collection == null && elems.isInstanceOf[Seq[_]]) {
-        collection = elems.asInstanceOf[Seq[A]]
-        this
-      }
-      else {
-        ruined = true
-        if (collection != null) that ++= collection
-        that ++= elems
-        collection = null
-        this
-      }
-
-  final def +=(elem: A): this.type = {
-    collection = null
-    ruined = true
-    that += elem
-    this
-  }
-  final def clear(): Unit = {
-    collection = null
-    if (ruined) that.clear()
-  }
-  final def result(): Seq[A] = if(ruined) that.result() else if (collection eq null) Nil else collection
-}
-
-private final class IdentityPreservingLinearSeqBuilder[A](that: Builder[A, LinearSeq[A]]) extends Builder[A, LinearSeq[A]] {
-  var collection: LinearSeq[A] = null
-  var ruined = false
-
-  final override def ++=(elems: TraversableOnce[A]): this.type =
-      if(!ruined && collection == null && elems.isInstanceOf[LinearSeq[_]]) {
-        collection = elems.asInstanceOf[LinearSeq[A]]
-        this
-      }
-      else {
-        ruined = true
-        if (collection != null) that ++= collection
-        that ++= elems
-        collection = null
-        this
+      elems match {
+        case ct(ca) if (collection == null && !ruined) =>  {
+          collection = ca
+          this
+        }
+        case _ => {
+          ruined = true
+          if (collection != null) that ++= collection
+          that ++= elems
+          collection = null.asInstanceOf[CC[A]]
+          this
+        }
       }
 
   final def +=(elem: A): this.type = {
-    collection = null
+    collection = null.asInstanceOf[CC[A]]
     ruined = true
     that += elem
     this
   }
   final def clear(): Unit = {
-    collection = null
+    collection = null.asInstanceOf[CC[A]]
     if (ruined) that.clear()
   }
-  final def result(): LinearSeq[A] = if(ruined) that.result() else if (collection eq null) Nil else collection
+  final def result(): CC[A] = if(ruined || (collection == null)) that.result() else collection
 }
 
 private[compat] object CompatImpl {

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

@@ -46,10 +46,10 @@ private[compat] trait PackageShared {
 
   implicit def genericCompanionToCBF[A, CC[X] <: GenTraversable[X]](
       fact: GenericCompanion[CC]): CanBuildFrom[Any, A, CC[A]] = {
-    val builder = if (fact == Seq) new IdentityPreservingSeqBuilder[A](Seq.newBuilder[A]).asInstanceOf[m.Builder[A, CC[A]]]
-                  else if (fact == LinearSeq) new IdentityPreservingLinearSeqBuilder[A](LinearSeq.newBuilder[A]).asInstanceOf[m.Builder[A, CC[A]]]
+    val builder = if (fact == Seq) new IdentityPreservingBuilder[A, Seq](Seq.newBuilder[A])
+                  else if (fact == LinearSeq) new IdentityPreservingBuilder[A, LinearSeq](LinearSeq.newBuilder[A])
                   else fact.newBuilder[A]
-    simpleCBF(builder)
+    simpleCBF(builder.asInstanceOf[m.Builder[A, CC[A]]])
   }
 
   implicit def sortedSetCompanionToCBF[A: Ordering,