Include/cpython/setobject.h

#ifndef Py_CPYTHON_SETOBJECT_H
#  error "this header file must not be included directly"
#endif

/* There are three kinds of entries in the table:

1. Unused:  key == NULL and hash == 0
2. Dummy:   key == dummy and hash == -1
3. Active:  key != NULL and key != dummy and hash != -1

The hash field of Unused slots is always zero.

The hash field of Dummy slots are set to -1
meaning that dummy entries can be detected by
either entry->key==dummy or by entry->hash==-1.
*/

#define PySet_MINSIZE 8

typedef struct {
    PyObject *key;
    Py_hash_t hash;             /* Cached hash code of the key */
} setentry;

/* The SetObject data structure is shared by set and frozenset objects.

Invariant for sets:
 - hash is -1

Invariants for frozensets:
 - data is immutable.
 - hash is the hash of the frozenset or -1 if not computed yet.

*/

typedef struct {
    PyObject_HEAD

    Py_ssize_t fill;            /* Number active and dummy entries*/
    Py_ssize_t used;            /* Number active entries */

    /* The table contains mask + 1 slots, and that's a power of 2.
     * We store the mask instead of the size because the mask is more
     * frequently needed.
     */
    Py_ssize_t mask;

    /* The table points to a fixed-size smalltable for small tables
     * or to additional malloc'ed memory for bigger tables.
     * The table pointer is never NULL which saves us from repeated
     * runtime null-tests.
     */
    setentry *table;
    Py_hash_t hash;             /* Only used by frozenset objects */
    Py_ssize_t finger;          /* Search finger for pop() */

    setentry smalltable[PySet_MINSIZE];
    PyObject *weakreflist;      /* List of weak references */
} PySetObject;

#define _PySet_CAST(so) \
    (assert(PyAnySet_Check(so)), _Py_CAST(PySetObject*, so))

static inline Py_ssize_t PySet_GET_SIZE(PyObject *so) {
#ifdef Py_GIL_DISABLED
    return _Py_atomic_load_ssize_relaxed(&(_PySet_CAST(so)->used));
#else
    return _PySet_CAST(so)->used;
#endif
}
#define PySet_GET_SIZE(so) PySet_GET_SIZE(_PyObject_CAST(so))