pythongh-94906: Support multiple steps in math.nextafter

Author: hauntsaninja

Doc/library/math.rst

@@ -229,9 +229,9 @@ Number-theoretic and representation functions
    of *x* and are floats.
 
 
-.. function:: nextafter(x, y)
+.. function:: nextafter(x, y, /, *, steps=1)
 
-   Return the next floating-point value after *x* towards *y*.
+   Return the floating-point value *steps* steps after *x* towards *y*.
 
    If *x* is equal to *y*, return *y*.
 
@@ -244,6 +244,9 @@ Number-theoretic and representation functions
 
    See also :func:`math.ulp`.
 
+   .. versionchanged:: 3.12
+      Added the *steps* argument.
+
    .. versionadded:: 3.9
 
 .. function:: perm(n, k=None)

Lib/test/test_math.py

@@ -2036,11 +2036,20 @@ def test_nextafter(self):
                          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
@@ -2065,6 +2074,12 @@ def test_nextafter(self):
         self.assertIsNaN(math.nextafter(1.0, NAN))
         self.assertIsNaN(math.nextafter(NAN, NAN))
 
+        with self.assertRaises(ValueError):
+            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)

Modules/clinic/mathmodule.c.h

@@ -3781,14 +3781,17 @@ math.nextafter
     x: double
     y: double
     /
+    *
+    steps: int = 1
 
-Return the next floating-point value after x towards y.
+Return the floating-point value the given number of steps after x towards y.
 [clinic start generated code]*/
 
 static PyObject *
-math_nextafter_impl(PyObject *module, double x, double y)
-/*[clinic end generated code: output=750c8266c1c540ce input=02b2d50cd1d9f9b6]*/
+math_nextafter_impl(PyObject *module, double x, double y, int steps)
+/*[clinic end generated code: output=14190eb869199e5a input=e87d3b26a7611ff4]*/
 {
+    int i;
 #if defined(_AIX)
     if (x == y) {
         /* On AIX 7.1, libm nextafter(-0.0, +0.0) returns -0.0.
@@ -3802,7 +3805,14 @@ math_nextafter_impl(PyObject *module, double x, double y)
         return PyFloat_FromDouble(y);
     }
 #endif
-    return PyFloat_FromDouble(nextafter(x, y));
+    if (steps < 1) {
+        PyErr_SetString(PyExc_ValueError, "steps must be >= 1");
+        return NULL;
+    }
+    for (i = 0; i < steps; i++) {
+        x = nextafter(x, y);
+    }
+    return PyFloat_FromDouble(x);
 }