include/python3.10/internal/pycore_long.h

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

#ifndef Py_BUILD_CORE
#  error "this header requires Py_BUILD_CORE define"
#endif

#include "pycore_interp.h"        // PyInterpreterState.small_ints
#include "pycore_pystate.h"       // _PyThreadState_GET()

/*
 * Default int base conversion size limitation: Denial of Service prevention.
 *
 * Chosen such that this isn't wildly slow on modern hardware and so that
 * everyone's existing deployed numpy test suite passes before
 * https://github.com/numpy/numpy/issues/22098 is widely available.
 *
 * $ python -m timeit -s 's = "1"*4300' 'int(s)'
 * 2000 loops, best of 5: 125 usec per loop
 * $ python -m timeit -s 's = "1"*4300; v = int(s)' 'str(v)'
 * 1000 loops, best of 5: 311 usec per loop
 * (zen2 cloud VM)
 *
 * 4300 decimal digits fits a ~14284 bit number.
 */
#define _PY_LONG_DEFAULT_MAX_STR_DIGITS 4300
/*
 * Threshold for max digits check.  For performance reasons int() and
 * int.__str__() don't checks values that are smaller than this
 * threshold.  Acts as a guaranteed minimum size limit for bignums that
 * applications can expect from CPython.
 *
 * % python -m timeit -s 's = "1"*640; v = int(s)' 'str(int(s))'
 * 20000 loops, best of 5: 12 usec per loop
 *
 * "640 digits should be enough for anyone." - gps
 * fits a ~2126 bit decimal number.
 */
#define _PY_LONG_MAX_STR_DIGITS_THRESHOLD 640

#if ((_PY_LONG_DEFAULT_MAX_STR_DIGITS != 0) && \
   (_PY_LONG_DEFAULT_MAX_STR_DIGITS < _PY_LONG_MAX_STR_DIGITS_THRESHOLD))
# error "_PY_LONG_DEFAULT_MAX_STR_DIGITS smaller than threshold."
#endif

// Don't call this function but _PyLong_GetZero() and _PyLong_GetOne()
static inline PyObject* __PyLong_GetSmallInt_internal(int value)
{
    PyInterpreterState *interp = _PyInterpreterState_GET();
    assert(-_PY_NSMALLNEGINTS <= value && value < _PY_NSMALLPOSINTS);
    size_t index = _PY_NSMALLNEGINTS + value;
    PyObject *obj = (PyObject*)interp->small_ints[index];
    // _PyLong_GetZero(), _PyLong_GetOne() and get_small_int() must not be
    // called before _PyLong_Init() nor after _PyLong_Fini().
    assert(obj != NULL);
    return obj;
}

// Return a borrowed reference to the zero singleton.
// The function cannot return NULL.
static inline PyObject* _PyLong_GetZero(void)
{ return __PyLong_GetSmallInt_internal(0); }

// Return a borrowed reference to the one singleton.
// The function cannot return NULL.
static inline PyObject* _PyLong_GetOne(void)
{ return __PyLong_GetSmallInt_internal(1); }

#ifdef __cplusplus
}
#endif
#endif /* !Py_INTERNAL_LONG_H */