adds memoization for f(hd)

Author: jeantil

src/main/scala/scala/collection/decorators/IteratorDecorator.scala

@@ -97,6 +97,7 @@ class IteratorDecorator[A](val `this`: Iterator[A]) extends AnyVal {
   def splitBy[K](f: A => K): Iterator[immutable.Seq[A]] =
     new AbstractIterator[immutable.Seq[A]] {
       private var hd: A = _
+      private var hdKey: K = _
       private var hdDefined: Boolean = false
 
       override def hasNext: Boolean = hdDefined || `this`.hasNext
@@ -107,20 +108,24 @@ class IteratorDecorator[A](val `this`: Iterator[A]) extends AnyVal {
           if (hdDefined) {
             seq += hd
           } else {
-            hd = `this`.next()
+            val init = `this`.next()
+            hd = init
+            hdKey = f(init)
             hdDefined = true
-            seq += hd
+            seq += init
           }
           var hadSameKey = true
           while (`this`.hasNext && hadSameKey) {
             val el = `this`.next()
             hdDefined = true
-            if (f(el) == f(hd)) {
+            val key = f(el)
+            if (key == hdKey) {
               seq += el
             } else {
               hadSameKey = false
+              hdKey = key
+              hd = el
             }
-            hd = el
           }
           if (hadSameKey) {
             hdDefined = false