[SVG] Fix root element length values handling.

Source/core/svg/SVGLengthContext.cpp

 /*
  * Copyright (C) 2004, 2005, 2006 Nikolas Zimmermann <zimmermann@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007 Rob Buis <buis@kde.org>
  * Copyright (C) 2007 Apple Inc. All rights reserved.
  * Copyright (C) Research In Motion Limited 2011. All rights reserved.
  *
  * This library is free software; you can redistribute it and/or
  * modify it under the terms of the GNU Library General Public
  * License as published by the Free Software Foundation; either
  * version 2 of the License, or (at your option) any later version.
@@ skipping 142 matching lines @@
     case LengthTypePC:
         return value * 6 / cssPixelsPerInch;
     }
 
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 float SVGLengthContext::convertValueFromUserUnitsToPercentage(float value, SVGLengthMode mode, ExceptionState& es) const
 {
-    float width = 0;
-    float height = 0;
-    if (!determineViewport(width, height)) {
+    FloatSize viewportSize;

[pdr., 2013/11/11 21:03:14]

Nit: I think we prefer to return the FloatSize and then check isEmpty. Up to
you, but I think it's easier to understand than out parameters.

[f(malita), 2013/11/11 21:14:29]

Doh, good point - if we only have one out-param now, why not just return it?
Possibly an extra copy, but I'm guessing RVO takes care of it.

Will do.

[f(malita), 2013/11/11 21:50:05]

On second thought, this causes a subtle change in semantics: we can no longer
differentiate between "cannot compute the viewport" (for which
SVGLengthContext::convertValueFromUserUnitsToPercentag throws an exception) and
"a valid, empty viewport" (no exception). Maybe it's OK to conflate the two, but
I'm thinking we shouldn't do it in this patch. What say you?

+    if (!determineViewport(viewportSize)) {
         es.throwUninformativeAndGenericDOMException(NotSupportedError);
         return 0;
     }
 
     switch (mode) {
     case LengthModeWidth:
-        return value / width * 100;
+        return value / viewportSize.width() * 100;
     case LengthModeHeight:
-        return value / height * 100;
+        return value / viewportSize.height() * 100;
     case LengthModeOther:
-        return value / (sqrtf((width * width + height * height) / 2)) * 100;
+        return value / sqrtf(viewportSize.diagonalLengthSquared() / 2) * 100;

[pdr., 2013/11/11 21:03:14]

square root of length squared? Please refactor for speed :)

[f(malita), 2013/11/11 21:14:29]

The name is misleading, there's no redundant squaring (what it's trying to say
is diagonalLengthBeforeSqrtf: w^2 + h^2) :)

[pdr., 2013/11/11 21:16:31]

Gah, I just spaced out here. You're right. Carry on :)

     };
 
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 float SVGLengthContext::convertValueFromPercentageToUserUnits(float value, SVGLengthMode mode, ExceptionState& es) const
 {
-    float width = 0;
-    float height = 0;
-    if (!determineViewport(width, height)) {
+    FloatSize viewportSize;
+    if (!determineViewport(viewportSize)) {
         es.throwUninformativeAndGenericDOMException(NotSupportedError);
         return 0;
     }
 
     switch (mode) {
     case LengthModeWidth:
-        return value * width;
+        return value * viewportSize.width();
     case LengthModeHeight:
-        return value * height;
+        return value * viewportSize.height();
     case LengthModeOther:
-        return value * sqrtf((width * width + height * height) / 2);
+        return value * sqrtf(viewportSize.diagonalLengthSquared() / 2);
     };
 
     ASSERT_NOT_REACHED();
     return 0;
 }
 
 static inline RenderStyle* renderStyleForLengthResolving(const SVGElement* context)
 {
     if (!context)
         return 0;
@@ skipping 63 matching lines @@
     if (!style) {
         es.throwUninformativeAndGenericDOMException(NotSupportedError);
         return 0;
     }
 
     // Use of ceil allows a pixel match to the W3Cs expected output of coords-units-03-b.svg
     // if this causes problems in real world cases maybe it would be best to remove this
     return value * ceilf(style->fontMetrics().xHeight());
 }
 
-bool SVGLengthContext::determineViewport(float& width, float& height) const
+bool SVGLengthContext::determineViewport(FloatSize& viewportSize) const
 {
     if (!m_context)
         return false;
 
     // If an overriden viewport is given, it has precedence.
     if (!m_overridenViewport.isEmpty()) {
-        width = m_overridenViewport.width();
-        height = m_overridenViewport.height();
+        viewportSize = m_overridenViewport.size();
         return true;
     }
 
-    // SVGLengthContext should NEVER be used to resolve width/height values for <svg> elements,
-    // as they require special treatment, due the relationship with the CSS width/height properties.
-    ASSERT(m_context->document().documentElement() != m_context);
+    // Root <svg> element lengths are resolved against the top level viewport.
+    if (m_context->isOutermostSVGSVGElement()) {
+        viewportSize = toSVGSVGElement(m_context)->currentViewportSize();
+        return true;
+    }
 
     // Take size from nearest viewport element.
     SVGElement* viewportElement = m_context->viewportElement();
     if (!viewportElement || !viewportElement->isSVGSVGElement())
         return false;
 
     const SVGSVGElement* svg = static_cast<const SVGSVGElement*>(viewportElement);
-    FloatSize viewportSize = svg->currentViewBoxRect().size();
+    viewportSize = svg->currentViewBoxRect().size();
     if (viewportSize.isEmpty())
         viewportSize = svg->currentViewportSize();
 
-    width = viewportSize.width();
-    height = viewportSize.height();
     return true;
 }
 
 }