Use iterator, moveNext(), current.

sdk/lib/core/iterator.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * The [Iterator] class provides methods to iterate over an object. It
  * is transparently used by the for-in construct to test for the end
  * of the iteration, and to get the elements.
  *
  * If the object iterated over is changed during the iteration, the
  * behavior is unspecified.
+ *
+ * The [Iterator] is initially positioned before the first element. Before
+ * accessing the first element the iterator must thus be advanced ([moveNext])
+ * to point to the first element. If there is no element left, then [moveNext]
+ * returns false.
  */
 abstract class Iterator<E> {
   /**
-   * Gets the next element in the iteration. Throws a
-   * [StateError] if no element is left.
+   * Moves to the next element. Returns true if [current] contains the next
+   * element. Returns false, if no element was left. It the latter case
+   * trying to read [current] will throw a [StateError].
+   *
+   * It is safe to invoke [moveNext] even when the iterator is already
+   * positioned after the last element. In this case [moveNext] has no effect.
    */
-  E next();
+  bool moveNext();
 
   /**
-   * Returns whether the [Iterator] has elements left.
+   * Returns the current element.
+   *
+   * Throws a [StateError] if the iterator has not yet been moved to the first
+   * element, or if the iterator has been moved after the last element of the
+   * [Iterable].
    */
-  bool get hasNext;
+  E get current;
 }
+
+class HasNextIterator<E> {
+  static const int _HAS_NEXT_AND_NEXT_IN_CURRENT = 0;
+  static const int _NO_NEXT = 1;
+  static const int _NOT_MOVED_YET = 2;
+
+  Iterator _iterator;
+  int _state = _NOT_MOVED_YET;
+
+  HasNextIterator(this._iterator);
+
+  bool get hasNext {
+    switch(_state) {
+      case _NOT_MOVED_YET:
+        bool currentIsValid = _iterator.moveNext();
+        if (currentIsValid) {
+          _state = _HAS_NEXT_AND_NEXT_IN_CURRENT;
+        } else {
+          _state = _NO_NEXT;
+        }
+        return currentIsValid;
+      case _HAS_NEXT_AND_NEXT_IN_CURRENT:
+        return true;
+      case _NO_NEXT:
+        return false;
+    }
+  }
+
+  E next() {
+    if (!hasNext) throw new StateError("No more elements");
+    assert(_state == _HAS_NEXT_AND_NEXT_IN_CURRENT);
+    return _iterator.current;
+  }
+}