math.c: adjust cbrt

* math.c (math_cbrt): refine the approximation result on boundary
  values by an iteration of Newton-Raphson method.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63592 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

math.c

@@ -689,7 +689,14 @@ rb_math_sqrt(VALUE x)
 static VALUE
 math_cbrt(VALUE unused_obj, VALUE x)
 {
-    return DBL2NUM(cbrt(Get_Double(x)));
+    double f = Get_Double(x);
+    double r = cbrt(f);
+#if defined __GLIBC__
+    if (isfinite(r)) {
+	r = (2.0 * r + (f / r / r)) / 3.0;
+    }
+#endif
+    return DBL2NUM(r);
 }
 
 /*

test/ruby/test_math.rb

@@ -202,6 +202,7 @@ class TestMath < Test::Unit::TestCase
     check(3, Math.cbrt(27))
     check(-0.1, Math.cbrt(-0.001))
     assert_nothing_raised { assert_infinity(Math.cbrt(1.0/0)) }
+    assert_operator(Math.cbrt(1.0 - Float::EPSILON), :<=, 1.0)
   end
 
   def test_frexp