Add List.reversed to give a reverse fixed-length view of a list.

sdk/lib/collection_dev/list.dart

 // Copyright (c) 2013, 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.
 
 part of dart.collection.dev;
 
 /**
- * Skeleton class for an unmodifiable [List].
+ * Class implementing the read-operations on [List].
+ *
+ * Implements all read-only operations, except [:operator[]:] and [:length:],
+ * in terms of those two operations.
  */
-abstract class NonExtensibleListMixin<E>
-    extends Iterable<E> implements List<E> {
-
+abstract class ListBase<E> extends Iterable<E> implements List<E> {
   Iterator<E> get iterator => new ListIterator(this);
 
   void forEach(f(E element)) {
     for (int i = 0; i < this.length; i++) f(this[i]);
   }
 
   bool contains(E value) {
     for (int i = 0; i < length; i++) {
       if (this[i] == value) return true;
     }
@@ skipping 62 matching lines @@
   }
 
   List<E> getRange(int start, int length) {
     List<E> result = <E>[];
     for (int i = 0; i < length; i++) {
       result.add(this[start + i]);
     }
     return result;
   }
 
+  String toString() => Collections.collectionToString(this);
+}
+
+/**
+ * Abstract class implementing the non-length changing operations of [List].
+ */
+abstract class FixedLengthListBase<E> extends ListBase<E> {
+  void operator[]=(int index, E value);
+
+  List<E> get reversed => new ReversedListView<E>(this, 0, null);
+
+  void sort([Comparator<E> compare]) {
+    coreSort(this, compare);
+  }
+
+  void setRange(int start, int length, List<E> from, [int startFrom]) {
+    if (length < 0) throw new ArgumentError("length: $length");
+    if (startFrom == null) startFrom = 0;
+    for (int i = 0; i < length; i++) {
+      this[start + i] = from[startFrom + i];
+    }
+  }
+
+  void set length(int newLength) {
+    throw new UnsupportedError(
+        "Cannot change the length of a fixed-length list");
+  }
+
+  void add(E value) {
+    throw new UnsupportedError(
+        "Cannot add to a fixed-length list");
+  }
+
+  void addLast(E value) {
+    throw new UnsupportedError(
+        "Cannot add to a fixed-length list");
+  }
+
+  void addAll(Iterable<E> iterable) {
+    throw new UnsupportedError(
+        "Cannot add to a fixed-length list");
+  }
+
+  void remove(E element) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void removeAll(Iterable elements) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void retainAll(Iterable elements) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void removeMatching(bool test(E element)) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void clear() {
+    throw new UnsupportedError(
+        "Cannot clear a fixed-length list");
+  }
+
+  E removeAt(int index) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  E removeLast() {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void removeRange(int start, int length) {
+    throw new UnsupportedError(
+        "Cannot remove from a fixed-length list");
+  }
+
+  void insertRange(int start, int length, [E initialValue]) {
+    throw new UnsupportedError(
+        "Cannot insert range in a fixed-length list");
+  }
+}
+
+/**
+ * An unmodifiable [List].
+ */
+abstract class UnmodifiableListBase<E> extends ListBase<E> {
+
   void operator []=(int index, E value) {
     throw new UnsupportedError(
         "Cannot modify an unmodifiable list");
   }
 
   void set length(int newLength) {
     throw new UnsupportedError(
         "Cannot change the length of an unmodifiable list");
   }
 
@@ skipping 24 matching lines @@
 
   void retainAll(Iterable elements) {
     throw new UnsupportedError(
         "Cannot remove from an unmodifiable list");
   }
 
   void removeMatching(bool test(E element)) {
     throw new UnsupportedError(
         "Cannot remove from an unmodifiable list");
   }
+
   void sort([Comparator<E> compare]) {
     throw new UnsupportedError(
         "Cannot modify an unmodifiable list");
   }
 
   void clear() {
     throw new UnsupportedError(
         "Cannot clear an unmodifiable list");
   }
 
@@ skipping 24 matching lines @@
 }
 
 /**
  * Iterates over a [List] in growing index order.
  */
 class ListIterator<E> implements Iterator<E> {
   final List<E> _list;
   int _position;
   E _current;
 
-  ListIterator(this._list) : _position = -1;
+  ListIterator(List<E> list) : _list = list, _position = -1;
 
   bool moveNext() {
     int nextPosition = _position + 1;
     if (nextPosition < _list.length) {
       _current = _list[nextPosition];
       _position = nextPosition;
       return true;
     }
     _position = _list.length;
     _current = null;
     return false;
   }
 
   E get current => _current;
 }
 
-class MappedList<S, T> extends NonExtensibleListMixin<T> {
+class MappedList<S, T> extends UnmodifiableListBase<T> {
   final List<S> _list;
   // TODO(ahe): Restore type when feature is implemented in dart2js
   // checked mode. http://dartbug.com/7733
   final /* _Transformation<S, T> */ _f;
 
   MappedList(this._list, T this._f(S element));
 
   T operator[](int index) => _f(_list[index]);
   int get length => _list.length;
 }
 
-/**
- * An immutable view of a [List].
- */
-class ListView<E> extends NonExtensibleListMixin<E> {
+/** An empty fixed-length list. */
+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;
+  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 _offset;
-  final int _length;
+  final int _start;
+  final int _end;
 
   /**
-   * If the given length is `null` then the ListView's length is bound by
-   * the backed [list].
+   * 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).
    */
-  ListView(List<E> list, this._offset, this._length) : _list = list {
-    if (_offset is! int || _offset < 0) {
-      throw new ArgumentError(_offset);
-    }
-    if (_length != null &&
-        (_length is! int || _length < 0)) {
-      throw new ArgumentError(_length);
-    }
+  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 originalLength = _list.length;
-    int skipLength = originalLength - _offset;
-    if (skipLength < 0) return 0;
-    if (_length == null || _length > skipLength) return skipLength;
-    return _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 skipIndex = index + _offset;
-    if (index < 0 ||
-        (_length != null && index >= _length) ||
-        index + _offset >= _list.length) {
-      throw new RangeError.value(index);
-    }
-    return _list[index + _offset];
-  }
-
-  ListView<E> skip(int skipCount) {
-    if (skipCount is! int || skipCount < 0) {
-      throw new ArgumentError(skipCount);
-    }
-    return new ListView(_list, _offset + skipCount, _length);
-  }
-
-  ListView<E> take(int takeCount) {
-    if (takeCount is! int || takeCount < 0) {
-      throw new ArgumentError(takeCount);
-    }
-    int newLength = takeCount;
-    if (_length != null && takeCount > _length) newLength = _length;
-    return new ListView(_list, _offset, newLength);
-  }
-}
+    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);
+  }
+
+  Iterable<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;
+  }
+
+  E get current => _current;
+}