core: fix the sign of a negative real number raised to an even power, and

add the corresponding unit test.

src/real.cpp

@@ -692,8 +692,14 @@ real pow(real const &x, real const &y)
         return exp(y * log(x));
     else /* x < 0 */
     {
+        /* Odd integer exponent */
+        if (y == (round(y >> 1) << 1))
+            return exp(y * log(-x));
+
+        /* Even integer exponent */
         if (y == round(y))
             return -exp(y * log(-x));
+
         /* FIXME: negative nth root */
         return real::R_0;
     }

test/unit/real.cpp

@@ -326,6 +326,17 @@ LOLUNIT_FIXTURE(RealTest)
             LOLUNIT_ASSERT_EQUAL(b2, a2);
         }
     }
+
+    LOLUNIT_TEST(Pow)
+    {
+        double a1 = pow(-real::R_2, real::R_2);
+        double b1 = 4.0;
+        LOLUNIT_ASSERT_DOUBLES_EQUAL(a1, b1, 1.0e-13);
+
+        double a2 = pow(-real::R_2, real::R_3);
+        double b2 = -8.0;
+        LOLUNIT_ASSERT_DOUBLES_EQUAL(a2, b2, 1.0e-13);
+    }
 };
 
 } /* namespace lol */