Make List.skip, List.take and List.reversed return Iterables, not Lists.

sdk/lib/_collection_dev/list.dart

Index: sdk/lib/_collection_dev/list.dart
diff --git a/sdk/lib/_collection_dev/list.dart b/sdk/lib/_collection_dev/list.dart
index 7af0fbbe6f773a68bfea405f75adcc3140ffc9da..0260290dc1dd423987a17037d675354d6c6f9024 100644
--- a/sdk/lib/_collection_dev/list.dart
+++ b/sdk/lib/_collection_dev/list.dart
@@ -101,23 +101,23 @@ abstract class ListBase<E> extends Collection<E> implements List<E> {
     return new MappedList(this, f);
   }
 
-  List<E> take(int n) {
-    return new ListView(this, 0, n);
+  Iterable<E> take(int n) {
+    return new SubListIterable(this, 0, n);
   }
 
-  List<E> skip(int n) {
-    return new ListView(this, n, null);
+  Iterable<E> skip(int n) {
+    return new SubListIterable(this, n, null);
   }
 
-  String toString() => ToString.collectionToString(this);
+  Iterable<E> get reversed => new ReversedListIterable(this);
 
-  List<E> get reversed {
-    return new ReversedListView(this, 0, null);
-  }
+  String toString() => ToString.collectionToString(this);
 }
 
 /**
  * Abstract class implementing the non-length changing operations of [List].
+ *
+ * All modifications are performed using [[]=].
  */
 abstract class FixedLengthListBase<E> extends ListBase<E> {
   void operator[]=(int index, E value);
@@ -286,35 +286,6 @@ abstract class UnmodifiableListBase<E> extends ListBase<E> {
   }
 }
 
-/**
- * Iterates over a [List] in growing index order.
- */
-class ListIterator<E> implements Iterator<E> {
-  final List<E> _list;
-  final int _length;
-  int _position;
-  E _current;
-
-  ListIterator(List<E> list)
-      : _list = list, _position = -1, _length = list.length;
-
-  bool moveNext() {
-    if (_list.length != _length) {
-      throw new ConcurrentModificationError(_list);
-    }
-    int nextPosition = _position + 1;
-    if (nextPosition < _length) {
-      _position = nextPosition;
-      _current = _list[nextPosition];
-      return true;
-    }
-    _current = null;
-    return false;
-  }
-
-  E get current => _current;
-}
-
 class MappedList<S, T> extends UnmodifiableListBase<T> {
   final List<S> _list;
   // TODO(ahe): Restore type when feature is implemented in dart2js
@@ -332,218 +303,17 @@ class EmptyList<E> extends FixedLengthListBase<E> {
   int get length => 0;
   E operator[](int index) { throw new RangeError.value(index); }
   void operator []=(int index, E value) { throw new RangeError.value(index); }
-  List<E> skip(int count) => this;
-  List<E> take(int count) => this;
-  List<E> get reversed => this;
+  Iterable<E> skip(int count) => const EmptyIterable();
+  Iterable<E> take(int count) => const EmptyIterable();
+  Iterable<E> get reversed => const EmptyIterable();
   void sort([int compare(E a, E b)]) {}
 }
 
-/**
- * A fixed-length view of a sub-range of another [List].
- *
- * The range is described by start and end points relative
- * to the other List's start or end.
- *
- * The range changes dynamically as the underlying list changes
- * its length.
- */
-abstract class SubListView<E> extends UnmodifiableListBase<E> {
-  final List<E> _list;
-  final int _start;
-  final int _end;
-
-  /**
-   * Create a sub-list view.
-   *
-   * Both [_start] and [_end] can be given as positions
-   * relative to the start of [_list] (a non-negative integer)
-   * or relative to the end of [_list] (a negative integer or
-   * null, with null being at the end of the list).
-   */
-  SubListView(this._list, this._start, this._end);
-
-  int _absoluteIndex(int relativeIndex) {
-    if (relativeIndex == null) return _list.length;
-    if (relativeIndex < 0) {
-      int result = _list.length + relativeIndex;
-      if (result < 0) return 0;
-      return result;
-    }
-    if (relativeIndex > _list.length) {
-      return _list.length;
-    }
-    return relativeIndex;
-  }
-
-  int get length {
-    int result = _absoluteIndex(_end) - _absoluteIndex(_start);
-    if (result >= 0) return result;
-    return 0;
-  }
-
-  _createListView(int start, int end) {
-    if (start == null) return new EmptyList<E>();
-    if (end != null) {
-      if (start < 0) {
-        if (end <= start) return new EmptyList<E>();
-      } else {
-        if (end >= 0 && end <= start) return new EmptyList<E>();
-      }
-    }
-    return new ListView(_list, start, end);
-  }
-
-  _createReversedListView(int start, int end) {
-    if (start == null) return new EmptyList<E>();
-    if (end != null) {
-      if (start < 0) {
-        if (end <= start) return new EmptyList<E>();
-      } else {
-        if (end >= 0 && end <= start) return new EmptyList<E>();
-      }
-    }
-    return new ReversedListView(_list, start, end);
-  }
-}
-
-
-/**
- * A fixed-length view of a sub-range of a [List].
- */
-class ListView<E> extends SubListView<E> {
-
-  ListView(List<E> list, int start, int end) : super(list, start, end);
-
-  E operator[](int index) {
-    int start = _absoluteIndex(_start);
-    int end = _absoluteIndex(_end);
-    int length = end - start;
-    if (index < 0 || index >= length) {
-      throw new RangeError.range(index, 0, length);
-    }
-    return _list[start + index];
-  }
-
-  List<E> skip(int count) {
-    if (count is! int || count < 0) {
-      throw new ArgumentError(count);
-    }
-    if (_start == null) {
-      return new EmptyList<E>();
-    }
-    int newStart = _start + count;
-    if (_start < 0 && newStart >= 0) {
-      return new EmptyList<E>();
-    }
-    return _createListView(newStart, _end);
-  }
-
-  List<E> take(int count) {
-    if (count is! int || count < 0) {
-      throw new ArgumentError(count);
-    }
-    if (_start == null) {
-      return new EmptyList<E>();
-    }
-    int newEnd = _start + count;
-    if (_start < 0 && newEnd >= 0) {
-      newEnd = null;
-    }
-    return _createListView(_start, newEnd);
-  }
-
-  List<E> get reversed => new ReversedListView(_list, _start, _end);
-}
-
-/**
- * Reversed view of a [List], or a slice of a list.
- *
- * The view is fixed-length and becomes invalid if the underlying
- * list changes its length below the slice used by this reversed list.
- *
- * Start index and end index can be either positive, negative or null.
- * Positive means an index relative to the start of the list,
- * negative means an index relative to the end of the list, and null
- * means at the end of the list (since there is no -0 integer).
- */
-class ReversedListView<E> extends SubListView<E> {
-
-  ReversedListView(List<E> list, int start, int end)
-      : super(list, start, end);
-
-  E operator[](int index) {
-    int start = _absoluteIndex(_start);
-    int end = _absoluteIndex(_end);
-    int length = end - start;
-    if (index < 0 || index >= length) {
-      throw new RangeError.range(index, 0, length);
-    }
-    return _list[end - index - 1];
-  }
-
-  List<E> skip(int count) {
-    if (count is! int || count < 0) {
-      throw new ArgumentError(count);
-    }
-    if (_end == null) {
-      return _createReversedListView(_start, -count);
-    }
-    int newEnd = _end - count;
-    if (_end >= 0 && newEnd < 0) {
-      return new EmptyList<E>();
-    }
-    return _createReversedListView(_start, newEnd);
-  }
-
-  List<E> take(int count) {
-    if (count is! int || count < 0) {
-      throw new ArgumentError(count);
-    }
-    int newStart;
-    if (_end == null) {
-      newStart = -count;
-    } else {
-      newStart = _end - count;
-      if (_end >= 0 && newStart < 0) {
-        return new EmptyList<E>();
-      }
-    }
-    return _createReversedListView(newStart, _end);
-  }
+class ReversedListIterable<E> extends ListIterable<E> {
+  Iterable<E> _source;
+  ReversedListIterable(this._source);
 
-  Iterator<E> get iterator => new ReverseListIterator<E>(
-      _list, _absoluteIndex(_start), _absoluteIndex(_end));
-
-  List<E> get reversed {
-    return new ListView(_list, _start, _end);
-  }
-}
-
-/**
- * An [Iterator] over a slice of a list that access elements in reverse order.
- */
-class ReverseListIterator<E> implements Iterator<E> {
-  final List<E> _list;
-  final int _start;
-  final int _originalLength;
-  int _index;
-  E _current;
-
-  ReverseListIterator(List<E> list, int start, int end)
-      : _list = list,
-        _start = start,
-        _index = end,
-        _originalLength = list.length;
-
-  bool moveNext() {
-    if (_list.length != _originalLength) {
-      throw new ConcurrentModificationError(_list);
-    }
-    if (_index <= _start) return false;
-    _index -= 1;
-    _current = _list[_index];
-    return true;
-  }
+  int get length => _source.length;
 
-  E get current => _current;
+  E elementAt(int index) => _source.elementAt(_source.length - 1 - index);
 }