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. */
+  /** The width of the rectangle. */
   T get width;
-  /** The `height` of the rectangle. */
+  /** The height of the rectangle. */
   T get height;
 
   /** The x-coordinate of the right edge. */
   T get right => left + width;
   /** The y-coordinate of the bottom edge. */
   T get bottom => top + height;
 
   String toString() {
     return 'Rectangle ($left, $top) $width x $height';
   }
@@ skipping 87 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);
+  /**
+   * Create a rectangle spanned by `(left, top)` and `(left+width, top+height)`.
+   *
+   * The rectangle contains the points
+   * with x-coordinate between `left` and `left + width`, and
+   * with y-coordiante between `top` and `top + height`, both inclusive.
+   *
+   * The `width` and `height` should be non-negative.
+   * If `width` is negative, it is truncated to zero.

[srdjan, 2014/01/30 15:28:45]

I think clamped to zero is a better term than truncated to zero.

[Lasse Reichstein Nielsen, 2014/01/31 08:51:53]

True, and it matches the naming for Uint8ClampedList.
Will change.

+   * Similar for `height`.

[srdjan, 2014/01/30 15:28:45]

Please repeat the comment, instead of saying similar.

[Lasse Reichstein Nielsen, 2014/01/31 08:51:53]

Done.

+   *
+   * If `width` and `height` are zero, the "rectangle" comprises only the single
+   * point `(left, top)`.
+   */
+  const Rectangle(this.left, this.top, T width, T height)
+      : this.width = (width >= 0) ? width : -width * 0,

[Emily Fortuna, 2014/01/30 18:35:13]

Why not:
this.width = (width >= 0) ? width : 0,  ...

instead?

I do like the clamped at zero idea better though. It makes it a bit more obvious
that the data you passed is "invalid"

[srdjan, 2014/01/30 18:46:40]

The problem is that width is of 'T extends num', and 0 is only integer. I.e., if
T is double, 0 is not assignable to width. Tricky. Only removing generics or
clamping seems to be cleaner.

[Emily Fortuna, 2014/01/30 18:56:23]

Ah yes. Touché. (can we maybe at a little comment here mentioning that for
someone like me that was scanning it quickly and didn't realize that subtlety
the first time?)

[Lasse Reichstein Nielsen, 2014/01/31 08:51:53]

I'll add a statuc "clampToZero" function and use that, and then document the
function (there isn't room to document it here anyway).

+        this.height = (height >= 0) ? height : -height * 0;
 
+  /*
+   * Create a rectangle spanned by the points [a] and [b];
+   *
+   * The rectangle contains the points
+   * with x-coordinate between `a.x` and `b.x`, and
+   * with y-coordiante between `a.y` and `b.y`, both inclusive.
+   *
+   * If the distance between `a.x` and `b.x` is not representable
+   * (which can happen if one or both is a double),
+   * the actual right edge might be slightly off from `max(a.x, b.x)`.
+   * Similar for the y-coordinates and the bottom edge.
+   */
   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> {
+
+  /**
+   * The x-coordinate of the left edge.
+   *
+   * Setting the value will move the rectangle without changing its width.
+   */
   T left;
+  /**
+   * The y-coordinate of the left edge.
+   *
+   * Setting the value will move the rectangle without changing its height.
+   */
   T top;
-  T width;
-  T height;
+  T _width;
+  T _height;
 
-  MutableRectangle(this.left, this.top, this.width, this.height);
+  /**
+   * Create a mutable rectangle spanned by `(left, top)` and
+   * `(left+width, top+height)`.
+   *
+   * The rectangle contains the points
+   * with x-coordinate between `left` and `left + width`, and
+   * with y-coordiante between `top` and `top + height`, both inclusive.
+   *
+   * The `width` and `height` should be non-negative.
+   * If `width` is negative, it is truncated to zero.
+   * Similar for `height`.

[srdjan, 2014/01/30 15:28:45]

dito

[Lasse Reichstein Nielsen, 2014/01/31 08:51:53]

Done.

+   *
+   *
+   * If `width` and `height` are zero, the "rectangle" comprises only the single
+   * point `(left, top)`.
+   */
+  MutableRectangle(this.left, this.top, T width, T height)
+      : this._width = (width >= 0) ? width : -width * 0,

[Lasse Reichstein Nielsen, 2014/01/30 13:42:34]

-width * 0  is integer zero if width is int and double 0.0 if width is double.
The '-' is to avoid -0.0.

[srdjan, 2014/01/30 15:28:45]

This is not as evident from code itself, please add it as comment. Why do you
need to avoid -0.0?

[Lasse Reichstein Nielsen, 2014/01/31 08:51:53]

I don't *need* to avoid -0.0, but I want to if I can (which I can, so I do :).
Here I get to choose between picking -0.0 and 0.0 as the value, so I just pick
0.0, even if it costs one extra operation.

+        this._height = (height >= 0) ? height : -height * 0;
 
+  /*
+   * Create a mutable rectangle spanned by the points [a] and [b];
+   *
+   * The rectangle contains the points
+   * with x-coordinate between `a.x` and `b.x`, and
+   * with y-coordiante between `a.y` and `b.y`, both inclusive.
+   *
+   * If the distance between `a.x` and `b.x` is not representable
+   * (which can happen if one or both is a double),
+   * the actual right edge might be slightly off from `max(a.x, b.x)`.
+   * Similar for the y-coordinates and the bottom edge.
+   */
   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);
   }
+
+  T get width => _width;
+
+ /**
+   * Sets the width of the rectangle.
+   *
+   * The width must be non-negative.
+   * If a negative width is supplied, it is truncated to zero.
+   *
+   * Setting the value will change the right edge of the rectangle,
+   * but will not change [left].
+   */
+  void set width(T width) {
+    if (width < 0) width = -width * 0;  // Preserves int/double-ness of width.
+    _width = width;
+  }
+
+  T get height => _height;
+
+  /**
+   * Sets the height of the rectangle.
+   *
+   * The height must be non-negative.
+   * If a negative height is supplied, it is truncated to zero.
+   *
+   * Setting the value will change the bottom edge of the rectangle,
+   * but will not change [top].
+   */
+  void set height(T height) {
+    if (height < 0) height = -height * 0;
+    _height = height;
+  }
 }