Fix #58 (SI-4528) BasicTransformer has no longer exponential complexity.

src/main/scala/scala/xml/transform/BasicTransformer.scala

@@ -32,11 +32,10 @@ abstract class BasicTransformer extends Function1[Node, Node] {
    *  otherwise a new sequence of concatenated results.
    */
   def transform(ns: Seq[Node]): Seq[Node] = {
-    val (xs1, xs2) = ns span (n => unchanged(n, transform(n)))
-
-    if (xs2.isEmpty) ns
-    else xs1 ++ transform(xs2.head) ++ transform(xs2.tail)
-  }
+    val changed = ns flatMap transform
+    if (changed.length != ns.length || (changed, ns).zipped.exists(_ != _)) changed
+    else ns
+}
 
   def transform(n: Node): Seq[Node] = {
     if (n.doTransform) n match {

src/test/scala/scala/xml/ReuseNodesTest.scala

@@ -0,0 +1,106 @@
+package scala.xml
+
+import scala.xml.transform._
+import org.junit.Test
+import org.junit.Assert.assertTrue
+import org.junit.Assert.assertEquals
+import org.junit.experimental.theories.Theories
+import org.junit.experimental.theories.Theory
+import org.junit.experimental.theories.DataPoints
+import org.junit.runner.RunWith
+/**
+ * This test verify that after the tranform, the resultant xml node
+ * uses as many old nodes as possible. 
+ * 
+ * Three transformers class for case - 
+ * One for orginal, one for modified, and one proposed which shows 
+ * all are equivalent when it comes to reusing as many nodes as possible
+ */
+object ReuseNodesTest {
+
+  class OriginalTranformr(rules: RewriteRule*) extends RuleTransformer(rules:_*) {
+    override def transform(ns: Seq[Node]): Seq[Node] = {
+      val xs = ns.toStream map transform
+      val (xs1, xs2) = xs zip ns span { case (x, n) => unchanged(n, x) }
+
+      if (xs2.isEmpty) ns
+      else (xs1 map (_._2)) ++ xs2.head._1 ++ transform(ns drop (xs1.length + 1))
+    }
+    override def transform(n:Node): Seq[Node] = super.transform(n)
+  }
+
+  class ModifiedTranformr(rules: RewriteRule*) extends RuleTransformer(rules:_*) {
+    override def transform(ns: Seq[Node]): Seq[Node] = {
+      val changed = ns flatMap transform
+
+      if (changed.length != ns.length || (changed, ns).zipped.exists(_ != _)) changed
+      else ns
+    }
+    override def transform(n:Node): Seq[Node] = super.transform(n)
+  }
+
+  class AlternateTranformr(rules: RewriteRule*) extends RuleTransformer(rules:_*) {
+    override def transform(ns: Seq[Node]): Seq[Node] = {
+      val xs = ns.toStream map transform
+      val (xs1, xs2) = xs zip ns span { case (x, n) => unchanged(n, x) }
+
+      if (xs2.isEmpty) ns
+      else (xs1 map (_._2)) ++ xs2.head._1 ++ transform(ns drop (xs1.length + 1))
+    }
+    override def transform(n:Node): Seq[Node] = super.transform(n)
+  }
+
+  def rewriteRule = new RewriteRule {
+    override def transform(n: Node): NodeSeq = n match {
+      case n if n.label == "change" => Elem(
+           n.prefix, "changed", n.attributes, n.scope, n.child.isEmpty, n.child : _*)
+      case _ => n
+    }
+  }
+
+  @DataPoints 
+  def tranformers() = Array(
+      new OriginalTranformr(rewriteRule),
+      new ModifiedTranformr(rewriteRule),
+      new AlternateTranformr(rewriteRule))
+}
+
+@RunWith(classOf[Theories])
+class ReuseNodesTest {
+
+  @Theory
+  def transformReferentialEquality(rt:RuleTransformer) = { 
+    val original = <p><lost/></p>
+    val tranformed = rt.transform(original)
+    assertTrue(original eq tranformed)
+  }
+
+  @Theory
+  def transformReferentialEqualityOnly(rt:RuleTransformer) = { 
+    val original = <changed><change><lost/><a><b><c/></b></a></change><a><b><c/></b></a></changed>
+    val transformed = rt.transform(original)
+    recursiveAssert(original,transformed)
+  }
+
+  def recursiveAssert(original:Seq[Node], transformed:Seq[Node]):Unit = {
+    (original.toList,transformed.toList) match {
+      case (Nil, Nil) => {}
+      case (x::xs,y::ys) => { 
+        recursiveAssert(x,y)
+        recursiveAssert(xs,ys)
+      }
+    } 
+  }
+
+  def recursiveAssert(original:Node, transformed:Node):Unit = {
+    transformed.label match { 
+      case "changed" => // do nothing expect this node to be changed
+        recursiveAssert(original.child,transformed.child)
+      case _ => {
+        assertTrue(original eq transformed)
+        // No need to check for childrens, node being immuatable 
+        // childs can't be differnt if parents are refertially equal
+      }
+    }
+  }
+}