bpo-39288: Add math.nextafter(x, y) (GH-17937)

Return the next floating-point value after x towards y.

1 files changed, 54 insertions, 0 deletions

diff --git a/Lib/test/test_math.py b/Lib/test/test_math.py
index 5c35c8c..b64fd41 100644
--- a/Lib/test/test_math.py
+++ b/Lib/test/test_math.py
@@ -2033,6 +2033,60 @@ class IsCloseTests(unittest.TestCase):
             self.assertIs(type(comb(IntSubclass(5), IntSubclass(k))), int)
             self.assertIs(type(comb(MyIndexable(5), MyIndexable(k))), int)
 
+    def assertEqualSign(self, x, y):
+        """Similar to assertEqual(), but compare also the sign.
+
+        Function useful to check to signed zero.
+        """
+        self.assertEqual(x, y)
+        self.assertEqual(math.copysign(1.0, x), math.copysign(1.0, y))
+
+    @requires_IEEE_754
+    def test_nextafter(self):
+        # around 2^52 and 2^63
+        self.assertEqual(math.nextafter(4503599627370496.0, -INF),
+                         4503599627370495.5)
+        self.assertEqual(math.nextafter(4503599627370496.0, INF),
+                         4503599627370497.0)
+        self.assertEqual(math.nextafter(9223372036854775808.0, 0.0),
+                         9223372036854774784.0)
+        self.assertEqual(math.nextafter(-9223372036854775808.0, 0.0),
+                         -9223372036854774784.0)
+
+        # around 1.0
+        self.assertEqual(math.nextafter(1.0, -INF),
+                         float.fromhex('0x1.fffffffffffffp-1'))
+        self.assertEqual(math.nextafter(1.0, INF),
+                         float.fromhex('0x1.0000000000001p+0'))
+
+        # x == y: y is returned
+        self.assertEqual(math.nextafter(2.0, 2.0), 2.0)
+        self.assertEqualSign(math.nextafter(-0.0, +0.0), +0.0)
+        self.assertEqualSign(math.nextafter(+0.0, -0.0), -0.0)
+
+        # around 0.0
+        smallest_subnormal = sys.float_info.min * sys.float_info.epsilon
+        self.assertEqual(math.nextafter(+0.0, INF), smallest_subnormal)
+        self.assertEqual(math.nextafter(-0.0, INF), smallest_subnormal)
+        self.assertEqual(math.nextafter(+0.0, -INF), -smallest_subnormal)
+        self.assertEqual(math.nextafter(-0.0, -INF), -smallest_subnormal)
+        self.assertEqualSign(math.nextafter(smallest_subnormal, +0.0), +0.0)
+        self.assertEqualSign(math.nextafter(-smallest_subnormal, +0.0), -0.0)
+        self.assertEqualSign(math.nextafter(smallest_subnormal, -0.0), +0.0)
+        self.assertEqualSign(math.nextafter(-smallest_subnormal, -0.0), -0.0)
+
+        # around infinity
+        largest_normal = sys.float_info.max
+        self.assertEqual(math.nextafter(INF, 0.0), largest_normal)
+        self.assertEqual(math.nextafter(-INF, 0.0), -largest_normal)
+        self.assertEqual(math.nextafter(largest_normal, INF), INF)
+        self.assertEqual(math.nextafter(-largest_normal, -INF), -INF)
+
+        # NaN
+        self.assertTrue(math.isnan(math.nextafter(NAN, 1.0)))
+        self.assertTrue(math.isnan(math.nextafter(1.0, NAN)))
+        self.assertTrue(math.isnan(math.nextafter(NAN, NAN)))
+
 
 def test_main():
     from doctest import DocFileSuite