Add wxGCD() helper function.

It is needed in wxMSW code and it looks like it could be useful to the library
users, so make it public.

See #16254.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@77019 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/utilscmn.cpp

@@ -52,6 +52,7 @@
 #include "wx/mimetype.h"
 #include "wx/config.h"
 #include "wx/versioninfo.h"
+#include "wx/math.h"
 
 #if defined(__WXWINCE__) && wxUSE_DATETIME
     #include "wx/datetime.h"
@@ -967,10 +968,58 @@ void wxQsort(void* pbase, size_t total_elems,
   }
 }
 
+// ----------------------------------------------------------------------------
+// wxGCD
+// Compute the greatest common divisor of two positive integers
+// using binary GCD algorithm.
+// See:
+//     http://en.wikipedia.org/wiki/Binary_GCD_algorithm#Iterative_version_in_C
+// ----------------------------------------------------------------------------
+
+unsigned int wxGCD(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;
+}
+
 #endif // wxUSE_BASE
 
-
-
 // ============================================================================
 // GUI-only functions from now on
 // ============================================================================