Avoid extra hasNext in trailing

Once trailing was advanced, hasNext was
needlessly delegated to the underlying
iterator on every invocation.

Author: som-snytt

src/library/scala/collection/Iterator.scala

@@ -290,8 +290,6 @@ object Iterator {
   }
 }
 
-import Iterator.empty
-
 /** Iterators are data structures that allow to iterate over a sequence
  *  of elements. They have a `hasNext` method for checking
  *  if there is a next element available, and a `next` method
@@ -357,6 +355,8 @@ import Iterator.empty
 trait Iterator[+A] extends TraversableOnce[A] {
   self =>
 
+  import Iterator.empty
+
   def seq: Iterator[A] = this
 
   /** Tests whether this iterator can provide another element.
@@ -760,34 +760,31 @@ trait Iterator[+A] extends TraversableOnce[A] {
        *   -1 not yet accessed
        *   0 single element waiting in leading
        *   1 defer to self
+       *   2 self.hasNext already
+       *   3 exhausted
        */
       private[this] var status = -1
-      def hasNext = {
-        if (status > 0) self.hasNext
-        else {
-          if (status == 0) true
-          else if (myLeading.finish()) {
-            status = 0
-            true
-          }
-          else {
-            status = 1
-            myLeading = null
-            self.hasNext
-          }
-        }
+      def hasNext = status match {
+        case 3 => false
+        case 2 => true
+        case 1 => if (self.hasNext) { status = 2 ; true } else { status = 3 ; false }
+        case 0 => true
+        case _ =>
+          if (myLeading.finish()) { status = 0 ; true } else { status = 1 ; myLeading = null ; hasNext }
       }
       def next() = {
         if (hasNext) {
-          if (status > 0) self.next()
-          else {
+          if (status == 0) {
             status = 1
-            val ans = myLeading.trailer
+            val res = myLeading.trailer
             myLeading = null
-            ans
+            res
+          } else {
+            status = 1
+            self.next()
           }
         }
-        else Iterator.empty.next()
+        else empty.next()
       }
 
       override def toString = "unknown-if-empty iterator"

test/junit/scala/collection/IteratorTest.scala

@@ -325,4 +325,28 @@ class IteratorTest {
     assertSameElements(List(10,11,13), scan)
     assertSameElements(List(10,-1,-1,-11,11,-2,-2,-13,13,-3), results)
   }
+  @Test def `scan trailing avoids extra hasNext`(): Unit = {
+    val it = new AbstractIterator[Int] {
+      var i = 0
+      var checkedAt = -1
+      def hasNext =
+        if (checkedAt == i) false
+        else {
+          checkedAt = i
+          true
+        }
+      def next() = {
+        i += 1
+        i
+      }
+    }
+    val (lo, hi) = it.span(_ < 3)
+    assertTrue(lo.hasNext)
+    assertEquals(1, lo.next())
+    assertTrue(hi.hasNext)
+    assertEquals(3, hi.next())
+    assertTrue(hi.hasNext)
+    assertTrue(hi.hasNext) // no longer delegated
+    assertTrue(hi.hasNext)
+  }
 }