Math: added Constants::tau().

Or two pi, 6.28.

src/Magnum/Math/Constants.h

@@ -46,10 +46,18 @@ template<class T> struct Constants {
     /**
      * @brief Pi
      *
-     * @see @ref Deg, @ref Rad
+     * @see @ref tau(), @ref Deg, @ref Rad
      */
     static constexpr T pi();
 
+    /**
+     * @brief Tau
+     *
+     * Or two pi.
+     * @see @ref pi(), @ref Deg, @ref Rad
+     */
+    static constexpr T tau();
+
     static constexpr T sqrt2();  /**< @brief Square root of 2 */
     static constexpr T sqrt3();  /**< @brief Square root of 3 */
     #endif
@@ -61,6 +69,7 @@ template<> struct Constants<Double> {
     Constants() = delete;
 
     static constexpr Double pi()    { return 3.141592653589793; }
+    static constexpr Double tau()   { return 6.283185307179586; }
     static constexpr Double sqrt2() { return 1.414213562373095; }
     static constexpr Double sqrt3() { return 1.732050807568877; }
 };
@@ -69,6 +78,7 @@ template<> struct Constants<Float> {
     Constants() = delete;
 
     static constexpr Float pi()    { return 3.141592654f; }
+    static constexpr Float tau()   { return 6.283185307f; }
     static constexpr Float sqrt2() { return 1.414213562f; }
     static constexpr Float sqrt3() { return 1.732050808f; }
 };

src/Magnum/Math/Test/ConstantsTest.cpp

@@ -48,14 +48,20 @@ void ConstantsTest::constantsFloat() {
     constexpr Float b = Constants<Float>::sqrt3();
     CORRADE_COMPARE(Math::pow<2>(a), 2.0f);
     CORRADE_COMPARE(Math::pow<2>(b), 3.0f);
+
+    constexpr Float c = Constants<Float>::pi();
+    CORRADE_COMPARE(2.0f*c, Constants<Float>::tau());
 }
 
 void ConstantsTest::constantsDouble() {
     #ifndef MAGNUM_TARGET_GLES
-    constexpr Double c = Constants<Double>::sqrt2();
-    constexpr Double d = Constants<Double>::sqrt3();
-    CORRADE_COMPARE(Math::pow<2>(c), 2.0);
-    CORRADE_COMPARE(Math::pow<2>(d), 3.0);
+    constexpr Double a = Constants<Double>::sqrt2();
+    constexpr Double b = Constants<Double>::sqrt3();
+    CORRADE_COMPARE(Math::pow<2>(a), 2.0);
+    CORRADE_COMPARE(Math::pow<2>(b), 3.0);
+
+    constexpr Double c = Constants<Double>::pi();
+    CORRADE_COMPARE(2.0*c, Constants<Double>::tau());
     #else
     CORRADE_SKIP("Double precision is not supported when targeting OpenGL ES.");
     #endif