polynomial: accuracy improvement on double root in order 3

src/lol/math/polynomial.h

@@ -231,7 +231,7 @@ struct polynomial
             // if root of the derivative is also root of the current polynomial, we have a double root.
             for (auto root : polynomial<T>{c, T(2)*b, T(3)*a}.roots())
                 if (eval(root) == T(0))
-                    return array<T> { solutions[0] - k,
+                    return array<T> { (solutions[0] + solutions[2]) / T(2) - k,
                                       solutions[1] - k };
 
             // we have 3 or 1 root depending on delta sign

src/t/math/polynomial.cpp

@@ -288,7 +288,7 @@ lolunit_declare_fixture(polynomial_test)
 
         // Should have 2 solutions only, but precision leads to 3 solutions
         lolunit_assert_equal(roots1.count(), 2);
-        lolunit_assert_doubles_equal(roots1[0], -1, 1e-3);
+        lolunit_assert_doubles_equal(roots1[0], -1, 1e-6);
         lolunit_assert_doubles_equal(roots1[1], -2, 1e-6);
     }