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, 47 insertions, 8 deletions

diff --git a/Modules/clinic/mathmodule.c.h b/Modules/clinic/mathmodule.c.h
index bc5bbce..c16c1b0 100644
--- a/Modules/clinic/mathmodule.c.h
+++ b/Modules/clinic/mathmodule.c.h
@@ -826,25 +826,59 @@ exit:
 }
 
 PyDoc_STRVAR(math_nextafter__doc__,
-"nextafter($module, x, y, /)\n"
+"nextafter($module, x, y, /, *, steps=None)\n"
 "--\n"
 "\n"
-"Return the next floating-point value after x towards y.");
+"Return the floating-point value the given number of steps after x towards y.\n"
+"\n"
+"If steps is not specified or is None, it defaults to 1.\n"
+"\n"
+"Raises a TypeError, if x or y is not a double, or if steps is not an integer.\n"
+"Raises ValueError if steps is negative.");
 
 #define MATH_NEXTAFTER_METHODDEF    \
-    {"nextafter", _PyCFunction_CAST(math_nextafter), METH_FASTCALL, math_nextafter__doc__},
+    {"nextafter", _PyCFunction_CAST(math_nextafter), METH_FASTCALL|METH_KEYWORDS, math_nextafter__doc__},
 
 static PyObject *
-math_nextafter_impl(PyObject *module, double x, double y);
+math_nextafter_impl(PyObject *module, double x, double y, PyObject *steps);
 
 static PyObject *
-math_nextafter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
+math_nextafter(PyObject *module, PyObject *const *args, Py_ssize_t nargs, PyObject *kwnames)
 {
     PyObject *return_value = NULL;
+    #if defined(Py_BUILD_CORE) && !defined(Py_BUILD_CORE_MODULE)
+
+    #define NUM_KEYWORDS 1
+    static struct {
+        PyGC_Head _this_is_not_used;
+        PyObject_VAR_HEAD
+        PyObject *ob_item[NUM_KEYWORDS];
+    } _kwtuple = {
+        .ob_base = PyVarObject_HEAD_INIT(&PyTuple_Type, NUM_KEYWORDS)
+        .ob_item = { &_Py_ID(steps), },
+    };
+    #undef NUM_KEYWORDS
+    #define KWTUPLE (&_kwtuple.ob_base.ob_base)
+
+    #else  // !Py_BUILD_CORE
+    #  define KWTUPLE NULL
+    #endif  // !Py_BUILD_CORE
+
+    static const char * const _keywords[] = {"", "", "steps", NULL};
+    static _PyArg_Parser _parser = {
+        .keywords = _keywords,
+        .fname = "nextafter",
+        .kwtuple = KWTUPLE,
+    };
+    #undef KWTUPLE
+    PyObject *argsbuf[3];
+    Py_ssize_t noptargs = nargs + (kwnames ? PyTuple_GET_SIZE(kwnames) : 0) - 2;
     double x;
     double y;
+    PyObject *steps = Py_None;
 
-    if (!_PyArg_CheckPositional("nextafter", nargs, 2, 2)) {
+    args = _PyArg_UnpackKeywords(args, nargs, NULL, kwnames, &_parser, 2, 2, 0, argsbuf);
+    if (!args) {
         goto exit;
     }
     if (PyFloat_CheckExact(args[0])) {
@@ -867,7 +901,12 @@ math_nextafter(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
             goto exit;
         }
     }
-    return_value = math_nextafter_impl(module, x, y);
+    if (!noptargs) {
+        goto skip_optional_kwonly;
+    }
+    steps = args[2];
+skip_optional_kwonly:
+    return_value = math_nextafter_impl(module, x, y, steps);
 
 exit:
     return return_value;
@@ -911,4 +950,4 @@ math_ulp(PyObject *module, PyObject *arg)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=a6437a3ba18c486a input=a9049054013a1b77]*/
+/*[clinic end generated code: output=91a0357265a2a553 input=a9049054013a1b77]*/