gh-94906: Support multiple steps in math.nextafter (#103881)

This PR updates `math.nextafter` to add a new `steps` argument. The behaviour is as though `math.nextafter` had been called `steps` times in succession.

---------

Co-authored-by: Mark Dickinson <mdickinson@enthought.com>

1 files changed, 17 insertions, 3 deletions

diff --git a/Lib/test/test_math.py b/Lib/test/test_math.py
index f282434..2bda610 100644
--- a/Lib/test/test_math.py
+++ b/Lib/test/test_math.py
@@ -2296,11 +2296,20 @@ class MathTests(unittest.TestCase):
                          float.fromhex('0x1.fffffffffffffp-1'))
         self.assertEqual(math.nextafter(1.0, INF),
                          float.fromhex('0x1.0000000000001p+0'))
+        self.assertEqual(math.nextafter(1.0, -INF, steps=1),
+                         float.fromhex('0x1.fffffffffffffp-1'))
+        self.assertEqual(math.nextafter(1.0, INF, steps=1),
+                         float.fromhex('0x1.0000000000001p+0'))
+        self.assertEqual(math.nextafter(1.0, -INF, steps=3),
+                         float.fromhex('0x1.ffffffffffffdp-1'))
+        self.assertEqual(math.nextafter(1.0, INF, steps=3),
+                         float.fromhex('0x1.0000000000003p+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)
+        for steps in range(1, 5):
+            self.assertEqual(math.nextafter(2.0, 2.0, steps=steps), 2.0)
+            self.assertEqualSign(math.nextafter(-0.0, +0.0, steps=steps), +0.0)
+            self.assertEqualSign(math.nextafter(+0.0, -0.0, steps=steps), -0.0)
 
         # around 0.0
         smallest_subnormal = sys.float_info.min * sys.float_info.epsilon
@@ -2325,6 +2334,11 @@ class MathTests(unittest.TestCase):
         self.assertIsNaN(math.nextafter(1.0, NAN))
         self.assertIsNaN(math.nextafter(NAN, NAN))
 
+        self.assertEqual(1.0, math.nextafter(1.0, INF, steps=0))
+        with self.assertRaises(ValueError):
+            math.nextafter(1.0, INF, steps=-1)
+
+
     @requires_IEEE_754
     def test_ulp(self):
         self.assertEqual(math.ulp(1.0), sys.float_info.epsilon)