gh-111389: expose _PyHASH_INF/BITS/MODULUS/IMAG macros as public

Doc/c-api/hash.rst

@@ -3,7 +3,8 @@
 PyHash API
 ----------
 
-See also the :c:member:`PyTypeObject.tp_hash` member.
+See also the :c:member:`PyTypeObject.tp_hash` member and :ref:`numeric-hash`
+for more details about hashing of numeric types.
 
 .. c:type:: Py_hash_t
 
@@ -17,6 +18,21 @@ See also the :c:member:`PyTypeObject.tp_hash` member.
 
    .. versionadded:: 3.2
 
+.. c:macro:: PyHASH_MODULUS
+
+   The Mersenne prime ``P = 2**n -1``, used for numeric hash scheme.
+
+.. c:macro:: PyHASH_BITS
+
+   The exponent ``n`` of ``P``.
+
+.. c:macro:: PyHASH_INF
+
+   The hash value returned for a positive infinity.
+
+.. c:macro:: PyHASH_IMAG
+
+   The multiplier used for the imaginary part of a complex number.
 
 .. c:type:: PyHash_FuncDef
 

Include/pyhash.h

@@ -4,6 +4,28 @@
 extern "C" {
 #endif
 
+/* Prime multiplier used in string and various other hashes. */
+#define _PyHASH_MULTIPLIER 1000003UL  /* 0xf4243 */
+
+/* Parameters used for the numeric hash implementation.  See notes for
+   _Py_HashDouble in Python/pyhash.c.  Numeric hashes are based on
+   reduction modulo the prime 2**_PyHASH_BITS - 1. */
+
+#if SIZEOF_VOID_P >= 8
+#  define _PyHASH_BITS 61
+#else
+#  define _PyHASH_BITS 31
+#endif
+
+#define PyHASH_BITS _PyHASH_BITS
+
+#define _PyHASH_MODULUS (((size_t)1 << _PyHASH_BITS) - 1)
+#define PyHASH_MODULUS _PyHASH_MODULUS
+#define _PyHASH_INF 314159
+#define PyHASH_INF _PyHASH_INF
+#define _PyHASH_IMAG _PyHASH_MULTIPLIER
+#define PyHASH_IMAG _PyHASH_IMAG
+
 /* Cutoff for small string DJBX33A optimization in range [1, cutoff).
  *
  * About 50% of the strings in a typical Python application are smaller than

Misc/NEWS.d/next/C API/2023-11-15-09-24-51.gh-issue-111418.FYYetY.rst

@@ -0,0 +1,2 @@
+Add :c:macro:`PyHASH_MODULUS`, :c:macro:`PyHASH_BITS`, :c:macro:`PyHASH_INF`
+and :c:macro:`PyHASH_IMAG` C macroses.  Patch by Sergey B Kirpichev.