Make Rectangle and MutableRectangle constructors handle negative lengths.

sdk/lib/math/rectangle.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.math;
 
 /**
  * A base class for representing two-dimensional axis-aligned rectangles.
  *
  * This rectangle uses a left-handed Cartesian coordinate system, with x
  * directed to the right and y directed down, as per the convention in 2D
  * computer graphics.
  *
  * See also:
  *    [W3C Coordinate Systems Specification](http://www.w3.org/TR/SVG/coords.html#InitialCoordinateSystem).
+ *
+ * The rectangle is the set of points with representable coordinates greater
+ * than or equal to left/top, and with distance to left/top no greater than
+ * width/height (to the limit of the precission of the coordinates).
  */
 abstract class _RectangleBase<T extends num> {
   const _RectangleBase();
 
   /** The x-coordinate of the left edge. */
   T get left;
   /** The y-coordinate of the top edge. */
   T get top;
   /** The `width` of the rectangle. */
   T get width;
@@ skipping 98 matching lines @@
 /**
  * A class for representing two-dimensional rectangles whose properties are
  * immutable.
  */
 class Rectangle<T extends num> extends _RectangleBase<T> {
   final T left;
   final T top;
   final T width;
   final T height;
 
-  const Rectangle(this.left, this.top, this.width, this.height);
+  const Rectangle(left, top, width, height)
+      : left = (width >= 0) ? left : left + width,
+        width = (width >= 0) ? width : -width,
+        top = (height >= 0) ? top : top + height,
+        height = (height >= 0) ? height : -height;

[srdjan, 2014/01/28 16:47:41]

This looks very unreadable to me (same names for arguments and instance
variables). Shouldn't the arguments be typed? (see style guide)

[Lasse Reichstein Nielsen, 2014/01/28 17:49:59]

Yes, they should be typed "T", and I should write "this.left" on the left-hand
side of the '='.

 
   factory Rectangle.fromPoints(Point<T> a, Point<T> b) {
     T left = min(a.x, b.x);
     T width = max(a.x, b.x) - left;
     T top = min(a.y, b.y);
     T height = max(a.y, b.y) - top;
     return new Rectangle<T>(left, top, width, height);
   }
 }
 
 /**
  * A class for representing two-dimensional axis-aligned rectangles with mutable
  * properties.
  */
-class MutableRectangle<T extends num>  extends _RectangleBase<T>
-    implements Rectangle<T> {
+class MutableRectangle<T extends num> extends _RectangleBase<T>
+                                      implements Rectangle<T> {
   T left;
   T top;
   T width;
   T height;
 
-  MutableRectangle(this.left, this.top, this.width, this.height);
+  MutableRectangle(left, top, width, height)
+      : left = (width >= 0) ? left : left + width,
+        width = (width >= 0) ? width : -width,
+        top = (height >= 0) ? top : top + height,
+        height = (height >= 0) ? height : -height;
 
   factory MutableRectangle.fromPoints(Point<T> a, Point<T> b) {
     T left = min(a.x, b.x);
     T width = max(a.x, b.x) - left;
     T top = min(a.y, b.y);
     T height = max(a.y, b.y) - top;
     return new MutableRectangle<T>(left, top, width, height);
   }
 }