Fix edge case bugs of canvas arc.

Source/core/html/canvas/CanvasPathMethods.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  * Copyright (C) 2012, 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 112 matching lines @@
     FloatPoint p2 = FloatPoint(x2, y2);
 
     if (!m_path.hasCurrentPoint())
         m_path.moveTo(p1);
     else if (p1 == m_path.currentPoint() || p1 == p2 || !r)
         lineTo(x1, y1);
     else
         m_path.addArcTo(p1, p2, r);
 }
 
-void CanvasPathMethods::arc(float x, float y, float r, float sa, float ea, bool anticlockwise, ExceptionCode& ec)
+static float adjustEndAngle(float startAngle, float endAngle, bool anticlockwise)
+{
+    float twoPi = 2 * piFloat;
+    /* http://www.whatwg.org/specs/web-apps/current-work/multipage/the-canvas-element.html#dom-context-2d-arc
+     * If the anticlockwise argument is false and endAngle-startAngle is equal to or greater than 2π, or,
+     * if the anticlockwise argument is true and startAngle-endAngle is equal to or greater than 2π,
+     * then the arc is the whole circumference of this ellipse.
+     */
+    if (!anticlockwise && endAngle - startAngle >= twoPi)
+        return startAngle + twoPi + fmodf(endAngle - startAngle, twoPi);
+    if (anticlockwise && startAngle - endAngle >= twoPi)
+        return startAngle - twoPi - fmodf(startAngle - endAngle, twoPi);
+
+    /*
+     * Otherwise, the arc is the path along the circumference of this ellipse from the start point to the end point,
+     * going anti-clockwise if the anticlockwise argument is true, and clockwise otherwise.
+     * Since the points are on the ellipse, as opposed to being simply angles from zero,
+     * the arc can never cover an angle greater than 2π radians.
+     */
+    if (!anticlockwise && startAngle > endAngle)
+        return startAngle + (twoPi - fmodf(startAngle - endAngle, twoPi));

[alph, 2013/07/10 13:06:29]

I might think of a rare corner case when e.g.
anticlockwise = false;
startAngle = twoPi;
endAngle = 0;
One would expect it to degenerate to a single dot, as both start and end angles
point to the same location of the circle. Your code draws the entire circle.
This is however is a kind of bifurcation point, so if you decrease startEngle a
bit it should really draw the (almost) entire circle. Though I'm not sure if
it's worth fixing it. 

[dshwang, 2013/07/10 13:36:42]

Wow, you have eagle eye. If you want to follow the spec completely, we should
fix. So I'll fix. :)

+    if (anticlockwise && startAngle < endAngle)
+        return startAngle - (twoPi - fmodf(endAngle - startAngle, twoPi));
+
+    return endAngle;
+}
+
+void CanvasPathMethods::arc(float x, float y, float radius, float startAngle, float endAngle, bool anticlockwise, ExceptionCode& ec)
 {
     ec = 0;
-    if (!std::isfinite(x) || !std::isfinite(y) || !std::isfinite(r) || !std::isfinite(sa) || !std::isfinite(ea))
+    if (!std::isfinite(x) || !std::isfinite(y) || !std::isfinite(radius) || !std::isfinite(startAngle) || !std::isfinite(endAngle))
         return;
 
-    if (r < 0) {
+    if (radius < 0) {
         ec = IndexSizeError;
         return;
     }
 
-    if (!r || sa == ea) {
-        // The arc is empty but we still need to draw the connecting line.
-        lineTo(x + r * cosf(sa), y + r * sinf(sa));
-        return;
-    }
-
     if (!isTransformInvertible())
         return;
 
-    // If 'sa' and 'ea' differ by more than 2Pi, just add a circle starting/ending at 'sa'.
-    if (anticlockwise && sa - ea >= 2 * piFloat) {
-        m_path.addArc(FloatPoint(x, y), r, sa, sa - 2 * piFloat, anticlockwise);
-        return;
-    }
-    if (!anticlockwise && ea - sa >= 2 * piFloat) {
-        m_path.addArc(FloatPoint(x, y), r, sa, sa + 2 * piFloat, anticlockwise);
+    if (!radius || startAngle == endAngle) {
+        // The arc is empty but we still need to draw the connecting line.
+        lineTo(x + radius * cosf(startAngle), y + radius * sinf(startAngle));
         return;
     }
 
-    m_path.addArc(FloatPoint(x, y), r, sa, ea, anticlockwise);
+    float adjustedEndAngle = adjustEndAngle(startAngle, endAngle, anticlockwise);
+    m_path.addArc(FloatPoint(x, y), radius, startAngle, adjustedEndAngle, anticlockwise);
 }
 
 void CanvasPathMethods::rect(float x, float y, float width, float height)
 {
     if (!isTransformInvertible())
         return;
 
     if (!std::isfinite(x) || !std::isfinite(y) || !std::isfinite(width) || !std::isfinite(height))
         return;
 
     if (!width && !height) {
         m_path.moveTo(FloatPoint(x, y));
         return;
     }
 
     m_path.addRect(FloatRect(x, y, width, height));
 }
 }