From d4edc41f6e9f93e1bca0a3302b99844e3f9cf412 Mon Sep 17 00:00:00 2001
From: Vadim Zeitlin <vadim@wxwidgets.org>
Date: Thu, 7 Aug 2014 21:07:18 +0000
Subject: [PATCH] Fix drawing on wxDC when using right-to-left layout in wxMSW.

Avoid integer overflow when setting wxDC scale. This affected (i.e. broke)
drawing in RTL but probably not only that.

Closes #16254.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@77022 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
---
 docs/changes.txt |  1 +
 src/msw/dc.cpp   | 54 ++++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+)

diff --git a/docs/changes.txt b/docs/changes.txt
index d1723932f8..1a7c665b57 100644
--- a/docs/changes.txt
+++ b/docs/changes.txt
@@ -602,6 +602,7 @@ wxMSW:
 - Fix parallel build of MSVC 11/12 solutions (Artur Wieczorek).
 - Fix background of wxRadioBox buttons and wxSlider (Artur Wieczorek).
 - Fix appearance of wxToggleButtons with non default colours (Artur Wieczorek).
+- Fix drawing on wxDC when using right-to-left layout (Artur Wieczorek).
 
 
 3.0.1: (released 2014-06-15)
diff --git a/src/msw/dc.cpp b/src/msw/dc.cpp
index 2910a5a07c..64f081e548 100644
--- a/src/msw/dc.cpp
+++ b/src/msw/dc.cpp
@@ -1932,6 +1932,50 @@ void ApplyEffectiveScale(double scale, int sign, int *device, int *logical)
     *logical = sign*wxRound(VIEWPORT_EXTENT/scale);
 }
 
+// Binary GCD algorithm
+// See: http://en.wikipedia.org/wiki/Binary_GCD_algorithm#Iterative_version_in_C
+unsigned int CalcGCD(unsigned int u, unsigned int v)
+{
+    // GCD(0,v) == v; GCD(u,0) == u, GCD(0,0) == 0
+    if (u == 0)
+        return v;
+    if (v == 0)
+        return u;
+
+    int shift;
+
+    // Let shift := lg K, where K is the greatest power of 2
+    // dividing both u and v.
+    for (shift = 0; ((u | v) & 1) == 0; ++shift)
+    {
+        u >>= 1;
+        v >>= 1;
+    }
+
+    while ((u & 1) == 0)
+        u >>= 1;
+
+    // From here on, u is always odd.
+    do
+    {
+        // remove all factors of 2 in v -- they are not common
+        // note: v is not zero, so while will terminate
+        while ((v & 1) == 0)
+            v >>= 1;
+
+        // Now u and v are both odd. Swap if necessary so u <= v,
+        // then set v = v - u (which is even)
+        if (u > v)
+        {
+            wxSwap(u, v);
+        }
+        v -= u;  // Here v >= u
+    } while (v != 0);
+
+    // restore common factors of 2
+    return u << shift;
+}
+
 } // anonymous namespace
 
 void wxMSWDCImpl::RealizeScaleAndOrigin()
@@ -1952,6 +1996,16 @@ void wxMSWDCImpl::RealizeScaleAndOrigin()
     ApplyEffectiveScale(m_scaleX, m_signX, &devExtX, &logExtX);
     ApplyEffectiveScale(m_scaleY, m_signY, &devExtY, &logExtY);
 
+    // Becaue only devExtX/logExtX ratio and devExtY/logExtY ratio are counted
+    // we can reduce the fractions to avoid large absolute numbers
+    // and possible arithmetic overflows.
+    unsigned int gcd = CalcGCD(abs(devExtX), abs(logExtX));
+    devExtX /= gcd;
+    logExtX /= gcd;
+    gcd = CalcGCD(abs(devExtY), abs(logExtY));
+    devExtY /= gcd;
+    logExtY /= gcd;
+
     ::SetViewportExtEx(GetHdc(), devExtX, devExtY, NULL);
     ::SetWindowExtEx(GetHdc(), logExtX, logExtY, NULL);