cpython/Include/pyhash.h

#ifndef Py_HASH_H
#define Py_HASH_H
#ifdef __cplusplus
extern "C" {
#endif

#ifndef Py_LIMITED_API
/* hash function definition */
typedef struct {
    Py_hash_t (*const hash)(const void *, Py_ssize_t);
    const char *name;
    const int hash_bits;
    const int seed_bits;
} PyHash_FuncDef;

PyAPI_FUNC(PyHash_FuncDef*) PyHash_GetFuncDef(void);
#endif


/* Cutoff for small string DJBX33A optimization in range [1, cutoff).
 *
 * About 50% of the strings in a typical Python application are smaller than
 * 6 to 7 chars. However DJBX33A is vulnerable to hash collision attacks.
 * NEVER use DJBX33A for long strings!
 *
 * A Py_HASH_CUTOFF of 0 disables small string optimization. 32 bit platforms
 * should use a smaller cutoff because it is easier to create colliding
 * strings. A cutoff of 7 on 64bit platforms and 5 on 32bit platforms should
 * provide a decent safety margin.
 */
#ifndef Py_HASH_CUTOFF
#  define Py_HASH_CUTOFF 0
#elif (Py_HASH_CUTOFF > 7 || Py_HASH_CUTOFF < 0)
#  error Py_HASH_CUTOFF must in range 0...7.
#endif /* Py_HASH_CUTOFF */


/* Hash algorithm selection
 *
 * The values for Py_HASH_* are hard-coded in the
 * configure script.
 *
 * - FNV and SIPHASH* are available on all platforms and architectures.
 * - With EXTERNAL embedders can provide an alternative implementation with::
 *
 *     PyHash_FuncDef PyHash_Func = {...};
 *
 * XXX: Figure out __declspec() for extern PyHash_FuncDef.
 */
#define Py_HASH_EXTERNAL 0
#define Py_HASH_SIPHASH24 1
#define Py_HASH_FNV 2
#define Py_HASH_SIPHASH13 3

#ifndef Py_HASH_ALGORITHM
#  ifndef HAVE_ALIGNED_REQUIRED
#    define Py_HASH_ALGORITHM Py_HASH_SIPHASH13
#  else
#    define Py_HASH_ALGORITHM Py_HASH_FNV
#  endif /* uint64_t && uint32_t && aligned */
#endif /* Py_HASH_ALGORITHM */

#ifdef __cplusplus
}
#endif
#endif  // !Py_HASH_H