pythongh-111545: Add Py_HashDouble() function

Add tests: Modules/_testcapi/hash.c and
Lib/test/test_capi/test_hash.py.

Author: vstinner

Doc/c-api/hash.rst

@@ -5,12 +5,16 @@ PyHash API
 
 See also the :c:member:`PyTypeObject.tp_hash` member.
 
+Types
+^^^^^
+
 .. c:type:: Py_hash_t
 
    Hash value type: signed integer.
 
    .. versionadded:: 3.2
 
+
 .. c:type:: Py_uhash_t
 
    Hash value type: unsigned integer.
@@ -41,6 +45,29 @@ See also the :c:member:`PyTypeObject.tp_hash` member.
    .. versionadded:: 3.4
 
 
+Functions
+^^^^^^^^^
+
+.. c:function:: Py_hash_t Py_HashDouble(double value)
+
+   Hash a C double number.
+
+   * If *value* is positive infinity, return :data:`sys.hash_info.inf
+     <sys.hash_info>`.
+   * If *value* is negative infinity, return :data:`-sys.hash_info.inf
+     <sys.hash_info>`.
+   * If *value* is not-a-number (NaN), return :data:`sys.hash_info.nan
+     <sys.hash_info>` (``0``).
+   * Otherwise, return the hash value of the finite *value* number.
+
+   .. note::
+      Return the hash value ``0`` for the floating point numbers ``-0.0`` and
+      ``+0.0``, and for not-a-number (NaN). ``Py_IS_NAN(value)`` can be used to
+      check if *value* is not-a-number.
+
+   .. versionadded:: 3.13
+
+
 .. c:function:: PyHash_FuncDef* PyHash_GetFuncDef(void)
 
    Get the hash function definition.

Doc/whatsnew/3.13.rst

@@ -1281,6 +1281,21 @@ New Features
 * Add :c:func:`Py_HashPointer` function to hash a pointer.
   (Contributed by Victor Stinner in :gh:`111545`.)
 
+* Add :c:func:`Py_HashDouble` function to hash a C double number. Existing code
+  using the private ``_Py_HashDouble()`` function can be updated to::
+
+        Py_hash_t hash_double(PyObject *obj, double value)
+        {
+            if (!Py_IS_NAN(value)) {
+                return Py_HashDouble(value);
+            }
+            else {
+                return Py_HashPointer(obj);
+            }
+        }
+
+  (Contributed by Victor Stinner in :gh:`111545`.)
+
 
 Porting to Python 3.13
 ----------------------

Include/cpython/pyhash.h

@@ -37,3 +37,4 @@ typedef struct {
 PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
 
 PyAPI_FUNC(Py_hash_t) Py_HashPointer(const void *ptr);
+PyAPI_FUNC(Py_hash_t) Py_HashDouble(double value);

Lib/test/test_capi/test_hash.py

@@ -1,3 +1,4 @@
+import math
 import sys
 import unittest
 from test.support import import_helper
@@ -77,3 +78,43 @@ def python_hash_pointer(x):
         # Py_HashPointer((void*)(uintptr_t)-1) doesn't return -1 but -2
         VOID_P_MAX = -1 & (2 ** (8 * SIZEOF_VOID_P) - 1)
         self.assertEqual(hash_pointer(VOID_P_MAX), -2)
+
+    def test_hash_double(self):
+        # Test Py_HashDouble()
+        hash_double = _testcapi.hash_double
+
+        # test some integers
+        integers = [
+            *range(1, 30),
+            2**30 - 1,
+            2 ** 233,
+            int(sys.float_info.max),
+        ]
+        for x in integers:
+            with self.subTest(x=x):
+                self.assertEqual(hash_double(float(x)), hash(x))
+                self.assertEqual(hash_double(float(-x)), hash(-x))
+
+        # test positive and negative zeros
+        self.assertEqual(hash_double(float(0.0)), 0)
+        self.assertEqual(hash_double(float(-0.0)), 0)
+
+        # test +inf and -inf
+        inf = float("inf")
+        self.assertEqual(hash_double(inf), sys.hash_info.inf)
+        self.assertEqual(hash_double(-inf), -sys.hash_info.inf)
+
+        # special float values: compare with Python hash() function
+        special_values = (
+            math.nextafter(0.0, 1.0),  # smallest positive subnormal number
+            sys.float_info.min,        # smallest positive normal number
+            sys.float_info.epsilon,
+            sys.float_info.max,        # largest positive finite number
+        )
+        for x in special_values:
+            with self.subTest(x=x):
+                self.assertEqual(hash_double(x), hash(x))
+                self.assertEqual(hash_double(-x), hash(-x))
+
+        # test not-a-number (NaN)
+        self.assertEqual(hash_double(float('nan')), 0)

Misc/NEWS.d/next/C API/2023-12-06-15-32-30.gh-issue-111545.kSOygi.rst

@@ -0,0 +1,2 @@
+Add :c:func:`Py_HashDouble` function to hash a C double number. Patch by
+Victor Stinner.

Modules/_testcapi/hash.c

@@ -1,6 +1,17 @@
 #include "parts.h"
 #include "util.h"
 
+
+static PyObject *
+long_from_hash(Py_hash_t hash)
+{
+    assert(hash != -1);
+
+    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
+    return PyLong_FromLongLong(hash);
+}
+
+
 static PyObject *
 hash_getfuncdef(PyObject *Py_UNUSED(module), PyObject *Py_UNUSED(args))
 {
@@ -54,14 +65,27 @@ hash_pointer(PyObject *Py_UNUSED(module), PyObject *arg)
     }
 
     Py_hash_t hash = Py_HashPointer(ptr);
-    Py_BUILD_ASSERT(sizeof(long long) >= sizeof(hash));
-    return PyLong_FromLongLong(hash);
+    return long_from_hash(hash);
+}
+
+
+static PyObject *
+hash_double(PyObject *Py_UNUSED(module), PyObject *args)
+{
+    double value;
+    if (!PyArg_ParseTuple(args, "d", &value)) {
+        return NULL;
+    }
+
+    Py_hash_t hash = Py_HashDouble(value);
+    return long_from_hash(hash);
 }
 
 
 static PyMethodDef test_methods[] = {
     {"hash_getfuncdef", hash_getfuncdef, METH_NOARGS},
     {"hash_pointer", hash_pointer, METH_O},
+    {"hash_double", hash_double, METH_VARARGS},
     {NULL},
 };
 

Python/pyhash.c

@@ -84,17 +84,20 @@ static Py_ssize_t hashstats[Py_HASH_STATS_MAX + 1] = {0};
    */
 
 Py_hash_t
-_Py_HashDouble(PyObject *inst, double v)
+Py_HashDouble(double v)
 {
     int e, sign;
     double m;
     Py_uhash_t x, y;
 
     if (!Py_IS_FINITE(v)) {
-        if (Py_IS_INFINITY(v))
-            return v > 0 ? _PyHASH_INF : -_PyHASH_INF;
-        else
-            return _Py_HashPointer(inst);
+        if (Py_IS_INFINITY(v)) {
+            return (v > 0 ? _PyHASH_INF : -_PyHASH_INF);
+        }
+        else {
+            assert(Py_IS_NAN(v));
+            return 0;
+        }
     }
 
     m = frexp(v, &e);
@@ -129,6 +132,19 @@ _Py_HashDouble(PyObject *inst, double v)
     return (Py_hash_t)x;
 }
 
+Py_hash_t
+_Py_HashDouble(PyObject *obj, double value)
+{
+    assert(obj != NULL);
+
+    if (!Py_IS_NAN(value)) {
+        return Py_HashDouble(value);
+    }
+    else {
+        return Py_HashPointer(obj);
+    }
+}
+
 Py_hash_t
 Py_HashPointer(const void *ptr)
 {