githubmirror/cpython

Modules/itertoolsmodule.c
4 248 строк116 KB
Miss Islington (bot)
[3.14] gh-153908: Fix data race in `itertools.count.__repr__` (GH-153917) (GH-153952)
18 июл 2026, 16:29
18 июл 2026, 16:29950e7fb
#include "Python.h"
#include "pycore_call.h"              // _PyObject_CallNoArgs()
#include "pycore_ceval.h"             // _PyEval_GetBuiltin()
#include "pycore_critical_section.h"  // Py_BEGIN_CRITICAL_SECTION()
#include "pycore_long.h"              // _PyLong_GetZero()
#include "pycore_moduleobject.h"      // _PyModule_GetState()
#include "pycore_typeobject.h"        // _PyType_GetModuleState()
#include "pycore_object.h"            // _PyObject_GC_TRACK()
#include "pycore_tuple.h"             // _PyTuple_ITEMS()

#include <stddef.h>                   // offsetof()

/* Itertools module written and maintained
   by Raymond D. Hettinger <python@rcn.com>
*/

typedef struct {
    PyTypeObject *accumulate_type;
    PyTypeObject *batched_type;
    PyTypeObject *chain_type;
    PyTypeObject *combinations_type;
    PyTypeObject *compress_type;
    PyTypeObject *count_type;
    PyTypeObject *cwr_type;
    PyTypeObject *cycle_type;
    PyTypeObject *dropwhile_type;
    PyTypeObject *filterfalse_type;
    PyTypeObject *groupby_type;
    PyTypeObject *_grouper_type;
    PyTypeObject *islice_type;
    PyTypeObject *pairwise_type;
    PyTypeObject *permutations_type;
    PyTypeObject *product_type;
    PyTypeObject *repeat_type;
    PyTypeObject *starmap_type;
    PyTypeObject *takewhile_type;
    PyTypeObject *tee_type;
    PyTypeObject *teedataobject_type;
    PyTypeObject *ziplongest_type;
} itertools_state;

static inline itertools_state *
get_module_state(PyObject *mod)
{
    void *state = _PyModule_GetState(mod);
    assert(state != NULL);
    return (itertools_state *)state;
}

static inline itertools_state *
get_module_state_by_cls(PyTypeObject *cls)
{
    void *state = _PyType_GetModuleState(cls);
    assert(state != NULL);
    return (itertools_state *)state;
}

static struct PyModuleDef itertoolsmodule;

static inline itertools_state *
find_state_by_type(PyTypeObject *tp)
{
    PyObject *mod = PyType_GetModuleByDef(tp, &itertoolsmodule);
    assert(mod != NULL);
    return get_module_state(mod);
}

/*[clinic input]
module itertools
class itertools.groupby "groupbyobject *" "clinic_state()->groupby_type"
class itertools._grouper "_grouperobject *" "clinic_state()->_grouper_type"
class itertools.teedataobject "teedataobject *" "clinic_state()->teedataobject_type"
class itertools._tee "teeobject *" "clinic_state()->tee_type"
class itertools.batched "batchedobject *" "clinic_state()->batched_type"
class itertools.cycle "cycleobject *" "clinic_state()->cycle_type"
class itertools.dropwhile "dropwhileobject *" "clinic_state()->dropwhile_type"
class itertools.takewhile "takewhileobject *" "clinic_state()->takewhile_type"
class itertools.starmap "starmapobject *" "clinic_state()->starmap_type"
class itertools.chain "chainobject *" "clinic_state()->chain_type"
class itertools.combinations "combinationsobject *" "clinic_state()->combinations_type"
class itertools.combinations_with_replacement "cwr_object *" "clinic_state()->cwr_type"
class itertools.permutations "permutationsobject *" "clinic_state()->permutations_type"
class itertools.accumulate "accumulateobject *" "clinic_state()->accumulate_type"
class itertools.compress "compressobject *" "clinic_state()->compress_type"
class itertools.filterfalse "filterfalseobject *" "clinic_state()->filterfalse_type"
class itertools.count "countobject *" "clinic_state()->count_type"
class itertools.pairwise "pairwiseobject *" "clinic_state()->pairwise_type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=aa48fe4de9d4080f]*/

#define clinic_state() (find_state_by_type(type))
#define clinic_state_by_cls() (get_module_state_by_cls(base_tp))
#include "clinic/itertoolsmodule.c.h"
#undef clinic_state_by_cls
#undef clinic_state


/* batched object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *it;
    Py_ssize_t batch_size;
    bool strict;
} batchedobject;

#define batchedobject_CAST(op)  ((batchedobject *)(op))

/*[clinic input]
@classmethod
itertools.batched.__new__ as batched_new
    iterable: object
    n: Py_ssize_t
    *
    strict: bool = False

Batch data into tuples of length n. The last batch may be shorter than n.

Loops over the input iterable and accumulates data into tuples
up to size n.  The input is consumed lazily, just enough to
fill a batch.  The result is yielded as soon as a batch is full
or when the input iterable is exhausted.

    >>> for batch in batched('ABCDEFG', 3):
    ...     print(batch)
    ...
    ('A', 'B', 'C')
    ('D', 'E', 'F')
    ('G',)

If "strict" is True, raises a ValueError if the final batch is shorter
than n.

[clinic start generated code]*/

static PyObject *
batched_new_impl(PyTypeObject *type, PyObject *iterable, Py_ssize_t n,
                 int strict)
/*[clinic end generated code: output=c6de11b061529d3e input=7814b47e222f5467]*/
{
    PyObject *it;
    batchedobject *bo;

    if (n < 1) {
        /* We could define the n==0 case to return an empty iterator
           but that is at odds with the idea that batching should
           never throw-away input data.
        */
        PyErr_SetString(PyExc_ValueError, "n must be at least one");
        return NULL;
    }
    it = PyObject_GetIter(iterable);
    if (it == NULL) {
        return NULL;
    }

    /* create batchedobject structure */
    bo = (batchedobject *)type->tp_alloc(type, 0);
    if (bo == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    bo->batch_size = n;
    bo->it = it;
    bo->strict = (bool) strict;
    return (PyObject *)bo;
}

static void
batched_dealloc(PyObject *op)
{
    batchedobject *bo = batchedobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(bo);
    PyObject_GC_UnTrack(bo);
    Py_XDECREF(bo->it);
    tp->tp_free(bo);
    Py_DECREF(tp);
}

static int
batched_traverse(PyObject *op, visitproc visit, void *arg)
{
    batchedobject *bo = batchedobject_CAST(op);
    Py_VISIT(Py_TYPE(bo));
    Py_VISIT(bo->it);
    return 0;
}

static PyObject *
batched_next(PyObject *op)
{
    batchedobject *bo = batchedobject_CAST(op);
    Py_ssize_t i;
    Py_ssize_t n = FT_ATOMIC_LOAD_SSIZE_RELAXED(bo->batch_size);
    PyObject *it = bo->it;
    PyObject *item;
    PyObject *result;

    if (n < 0) {
        return NULL;
    }
    result = PyTuple_New(n);
    if (result == NULL) {
        return NULL;
    }
    iternextfunc iternext = *Py_TYPE(it)->tp_iternext;
    PyObject **items = _PyTuple_ITEMS(result);
    for (i=0 ; i < n ; i++) {
        item = iternext(it);
        if (item == NULL) {
            goto null_item;
        }
        items[i] = item;
    }
    return result;

 null_item:
    if (PyErr_Occurred()) {
        if (!PyErr_ExceptionMatches(PyExc_StopIteration)) {
            /* Input raised an exception other than StopIteration */
            FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
#ifndef Py_GIL_DISABLED
            Py_CLEAR(bo->it);
#endif
            Py_DECREF(result);
            return NULL;
        }
        PyErr_Clear();
    }
    if (i == 0) {
        FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
#ifndef Py_GIL_DISABLED
        Py_CLEAR(bo->it);
#endif
        Py_DECREF(result);
        return NULL;
    }
    if (bo->strict) {
        FT_ATOMIC_STORE_SSIZE_RELAXED(bo->batch_size, -1);
#ifndef Py_GIL_DISABLED
        Py_CLEAR(bo->it);
#endif
        Py_DECREF(result);
        PyErr_SetString(PyExc_ValueError, "batched(): incomplete batch");
        return NULL;
    }
    _PyTuple_Resize(&result, i);
    return result;
}

static PyType_Slot batched_slots[] = {
    {Py_tp_dealloc, batched_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)batched_new__doc__},
    {Py_tp_traverse, batched_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, batched_next},
    {Py_tp_alloc, PyType_GenericAlloc},
    {Py_tp_new, batched_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec batched_spec = {
    .name = "itertools.batched",
    .basicsize = sizeof(batchedobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = batched_slots,
};


/* pairwise object ***********************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *it;
    PyObject *old;
    PyObject *result;
} pairwiseobject;

#define pairwiseobject_CAST(op) ((pairwiseobject *)(op))

/*[clinic input]
@classmethod
itertools.pairwise.__new__ as pairwise_new
    iterable: object
    /
Return an iterator of overlapping pairs taken from the input iterator.

    s -> (s0,s1), (s1,s2), (s2, s3), ...

[clinic start generated code]*/

static PyObject *
pairwise_new_impl(PyTypeObject *type, PyObject *iterable)
/*[clinic end generated code: output=9f0267062d384456 input=6e7c3cddb431a8d6]*/
{
    PyObject *it;
    pairwiseobject *po;

    it = PyObject_GetIter(iterable);
    if (it == NULL) {
        return NULL;
    }
    po = (pairwiseobject *)type->tp_alloc(type, 0);
    if (po == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    po->it = it;
    po->old = NULL;
    po->result = PyTuple_Pack(2, Py_None, Py_None);
    if (po->result == NULL) {
        Py_DECREF(po);
        return NULL;
    }
    return (PyObject *)po;
}

static void
pairwise_dealloc(PyObject *op)
{
    pairwiseobject *po = pairwiseobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(po);
    PyObject_GC_UnTrack(po);
    Py_XDECREF(po->it);
    Py_XDECREF(po->old);
    Py_XDECREF(po->result);
    tp->tp_free(po);
    Py_DECREF(tp);
}

static int
pairwise_traverse(PyObject *op, visitproc visit, void *arg)
{
    pairwiseobject *po = pairwiseobject_CAST(op);
    Py_VISIT(Py_TYPE(po));
    Py_VISIT(po->it);
    Py_VISIT(po->old);
    Py_VISIT(po->result);
    return 0;
}

static PyObject *
pairwise_next(PyObject *op)
{
    pairwiseobject *po = pairwiseobject_CAST(op);
    PyObject *it = po->it;
    PyObject *old = po->old;
    PyObject *new, *result;

    if (it == NULL) {
        return NULL;
    }
    if (old == NULL) {
        old = (*Py_TYPE(it)->tp_iternext)(it);
        Py_XSETREF(po->old, old);
        if (old == NULL) {
            Py_CLEAR(po->it);
            return NULL;
        }
        it = po->it;
        if (it == NULL) {
            Py_CLEAR(po->old);
            return NULL;
        }
    }
    Py_INCREF(old);
    new = (*Py_TYPE(it)->tp_iternext)(it);
    if (new == NULL) {
        Py_CLEAR(po->it);
        Py_CLEAR(po->old);
        Py_DECREF(old);
        return NULL;
    }

    result = po->result;
    if (_PyObject_IsUniquelyReferenced(result)) {
        Py_INCREF(result);
        PyObject *last_old = PyTuple_GET_ITEM(result, 0);
        PyObject *last_new = PyTuple_GET_ITEM(result, 1);
        PyTuple_SET_ITEM(result, 0, Py_NewRef(old));
        PyTuple_SET_ITEM(result, 1, Py_NewRef(new));
        Py_DECREF(last_old);
        Py_DECREF(last_new);
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        _PyTuple_Recycle(result);
    }
    else {
        result = PyTuple_New(2);
        if (result != NULL) {
            PyTuple_SET_ITEM(result, 0, Py_NewRef(old));
            PyTuple_SET_ITEM(result, 1, Py_NewRef(new));
        }
    }

    Py_XSETREF(po->old, new);
    Py_DECREF(old);
    return result;
}

static PyType_Slot pairwise_slots[] = {
    {Py_tp_dealloc, pairwise_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)pairwise_new__doc__},
    {Py_tp_traverse, pairwise_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, pairwise_next},
    {Py_tp_alloc, PyType_GenericAlloc},
    {Py_tp_new, pairwise_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec pairwise_spec = {
    .name = "itertools.pairwise",
    .basicsize = sizeof(pairwiseobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = pairwise_slots,
};


/* groupby object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *it;
    PyObject *keyfunc;
    PyObject *tgtkey;
    PyObject *currkey;
    PyObject *currvalue;
    const void *currgrouper;  /* borrowed reference */
    itertools_state *state;
} groupbyobject;

#define groupbyobject_CAST(op)  ((groupbyobject *)(op))

static PyObject *_grouper_create(groupbyobject *, PyObject *);

/*[clinic input]
@classmethod
itertools.groupby.__new__

    iterable as it: object
        Elements to divide into groups according to the key function.
    key as keyfunc: object = None
        A function for computing the group category for each element.
        If the key function is not specified or is None, the element itself
        is used for grouping.

make an iterator that returns consecutive keys and groups from the iterable
[clinic start generated code]*/

static PyObject *
itertools_groupby_impl(PyTypeObject *type, PyObject *it, PyObject *keyfunc)
/*[clinic end generated code: output=cbb1ae3a90fd4141 input=6b3d123e87ff65a1]*/
{
    groupbyobject *gbo;

    gbo = (groupbyobject *)type->tp_alloc(type, 0);
    if (gbo == NULL)
        return NULL;
    gbo->tgtkey = NULL;
    gbo->currkey = NULL;
    gbo->currvalue = NULL;
    gbo->keyfunc = Py_NewRef(keyfunc);
    gbo->it = PyObject_GetIter(it);
    if (gbo->it == NULL) {
        Py_DECREF(gbo);
        return NULL;
    }
    gbo->state = find_state_by_type(type);
    return (PyObject *)gbo;
}

static void
groupby_dealloc(PyObject *op)
{
    groupbyobject *gbo = groupbyobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(gbo);
    PyObject_GC_UnTrack(gbo);
    Py_XDECREF(gbo->it);
    Py_XDECREF(gbo->keyfunc);
    Py_XDECREF(gbo->tgtkey);
    Py_XDECREF(gbo->currkey);
    Py_XDECREF(gbo->currvalue);
    tp->tp_free(gbo);
    Py_DECREF(tp);
}

static int
groupby_traverse(PyObject *op, visitproc visit, void *arg)
{
    groupbyobject *gbo = groupbyobject_CAST(op);
    Py_VISIT(Py_TYPE(gbo));
    Py_VISIT(gbo->it);
    Py_VISIT(gbo->keyfunc);
    Py_VISIT(gbo->tgtkey);
    Py_VISIT(gbo->currkey);
    Py_VISIT(gbo->currvalue);
    return 0;
}

Py_LOCAL_INLINE(int)
groupby_step(groupbyobject *gbo)
{
    PyObject *newvalue, *newkey, *oldvalue;

    newvalue = PyIter_Next(gbo->it);
    if (newvalue == NULL)
        return -1;

    if (gbo->keyfunc == Py_None) {
        newkey = Py_NewRef(newvalue);
    } else {
        newkey = PyObject_CallOneArg(gbo->keyfunc, newvalue);
        if (newkey == NULL) {
            Py_DECREF(newvalue);
            return -1;
        }
    }

    oldvalue = gbo->currvalue;
    gbo->currvalue = newvalue;
    Py_XSETREF(gbo->currkey, newkey);
    Py_XDECREF(oldvalue);
    return 0;
}

static PyObject *
groupby_next(PyObject *op)
{
    PyObject *r, *grouper;
    groupbyobject *gbo = groupbyobject_CAST(op);

    gbo->currgrouper = NULL;
    /* skip to next iteration group */
    for (;;) {
        if (gbo->currkey == NULL)
            /* pass */;
        else if (gbo->tgtkey == NULL)
            break;
        else {
            /* A user-defined __eq__ can re-enter groupby and advance the iterator,
               mutating gbo->tgtkey / gbo->currkey while we are comparing them.
               Take local snapshots and hold strong references so INCREF/DECREF
               apply to the same objects even under re-entrancy. */
            PyObject *tgtkey = gbo->tgtkey;
            PyObject *currkey = gbo->currkey;

            Py_INCREF(tgtkey);
            Py_INCREF(currkey);
            int rcmp = PyObject_RichCompareBool(tgtkey, currkey, Py_EQ);
            Py_DECREF(tgtkey);
            Py_DECREF(currkey);

            if (rcmp == -1)
                return NULL;
            else if (rcmp == 0)
                break;
        }

        if (groupby_step(gbo) < 0)
            return NULL;
    }
    Py_INCREF(gbo->currkey);
    Py_XSETREF(gbo->tgtkey, gbo->currkey);

    grouper = _grouper_create(gbo, gbo->tgtkey);
    if (grouper == NULL)
        return NULL;

    r = PyTuple_Pack(2, gbo->currkey, grouper);
    Py_DECREF(grouper);
    return r;
}

static PyType_Slot groupby_slots[] = {
    {Py_tp_dealloc, groupby_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_groupby__doc__},
    {Py_tp_traverse, groupby_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, groupby_next},
    {Py_tp_new, itertools_groupby},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec groupby_spec = {
    .name = "itertools.groupby",
    .basicsize= sizeof(groupbyobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = groupby_slots,
};

/* _grouper object (internal) ************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *parent;
    PyObject *tgtkey;
} _grouperobject;

#define _grouperobject_CAST(op) ((_grouperobject *)(op))

/*[clinic input]
@classmethod
itertools._grouper.__new__

    parent: object(subclass_of='clinic_state_by_cls()->groupby_type')
    tgtkey: object
    /
[clinic start generated code]*/

static PyObject *
itertools__grouper_impl(PyTypeObject *type, PyObject *parent,
                        PyObject *tgtkey)
/*[clinic end generated code: output=462efb1cdebb5914 input=afe05eb477118f12]*/
{
    return _grouper_create(groupbyobject_CAST(parent), tgtkey);
}

static PyObject *
_grouper_create(groupbyobject *parent, PyObject *tgtkey)
{
    itertools_state *state = parent->state;
    _grouperobject *igo = PyObject_GC_New(_grouperobject, state->_grouper_type);
    if (igo == NULL)
        return NULL;
    igo->parent = Py_NewRef(parent);
    igo->tgtkey = Py_NewRef(tgtkey);
    parent->currgrouper = igo;  /* borrowed reference */

    PyObject_GC_Track(igo);
    return (PyObject *)igo;
}

static void
_grouper_dealloc(PyObject *op)
{
    _grouperobject *igo = _grouperobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(igo);
    PyObject_GC_UnTrack(igo);
    Py_DECREF(igo->parent);
    Py_DECREF(igo->tgtkey);
    PyObject_GC_Del(igo);
    Py_DECREF(tp);
}

static int
_grouper_traverse(PyObject *op, visitproc visit, void *arg)
{
    _grouperobject *igo = _grouperobject_CAST(op);
    Py_VISIT(Py_TYPE(igo));
    Py_VISIT(igo->parent);
    Py_VISIT(igo->tgtkey);
    return 0;
}

static PyObject *
_grouper_next(PyObject *op)
{
    _grouperobject *igo = _grouperobject_CAST(op);
    groupbyobject *gbo = groupbyobject_CAST(igo->parent);
    PyObject *r;
    int rcmp;

    if (gbo->currgrouper != igo)
        return NULL;
    if (gbo->currvalue == NULL) {
        if (groupby_step(gbo) < 0)
            return NULL;
    }

    assert(gbo->currkey != NULL);
    /* A user-defined __eq__ can re-enter the grouper and advance the iterator,
       mutating gbo->currkey while we are comparing them.
       Take local snapshots and hold strong references so INCREF/DECREF
       apply to the same objects even under re-entrancy. */
    PyObject *tgtkey = Py_NewRef(igo->tgtkey);
    PyObject *currkey = Py_NewRef(gbo->currkey);
    rcmp = PyObject_RichCompareBool(tgtkey, currkey, Py_EQ);
    Py_DECREF(tgtkey);
    Py_DECREF(currkey);

    if (rcmp <= 0)
        /* got any error or current group is end */
        return NULL;

    r = gbo->currvalue;
    gbo->currvalue = NULL;
    Py_CLEAR(gbo->currkey);

    return r;
}

static PyType_Slot _grouper_slots[] = {
    {Py_tp_dealloc, _grouper_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_traverse, _grouper_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, _grouper_next},
    {Py_tp_new, itertools__grouper},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec _grouper_spec = {
    .name = "itertools._grouper",
    .basicsize = sizeof(_grouperobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = _grouper_slots,
};


/* tee object and with supporting function and objects ***********************/

/* The teedataobject pre-allocates space for LINKCELLS number of objects.
   To help the object fit neatly inside cache lines (space for 16 to 32
   pointers), the value should be a multiple of 16 minus  space for
   the other structure members including PyHEAD overhead.  The larger the
   value, the less memory overhead per object and the less time spent
   allocating/deallocating new links.  The smaller the number, the less
   wasted space and the more rapid freeing of older data.
*/
#define LINKCELLS 57

typedef struct {
    PyObject_HEAD
    PyObject *it;
    int numread;                /* 0 <= numread <= LINKCELLS */
    int running;
    PyObject *nextlink;
    PyObject *(values[LINKCELLS]);
} teedataobject;

#define teedataobject_CAST(op)  ((teedataobject *)(op))

typedef struct {
    PyObject_HEAD
    teedataobject *dataobj;
    int index;                  /* 0 <= index <= LINKCELLS */
    PyObject *weakreflist;
    itertools_state *state;
} teeobject;

#define teeobject_CAST(op)  ((teeobject *)(op))

static PyObject *
teedataobject_newinternal(itertools_state *state, PyObject *it)
{
    teedataobject *tdo;

    tdo = PyObject_GC_New(teedataobject, state->teedataobject_type);
    if (tdo == NULL)
        return NULL;

    tdo->running = 0;
    tdo->numread = 0;
    tdo->nextlink = NULL;
    tdo->it = Py_NewRef(it);
    PyObject_GC_Track(tdo);
    return (PyObject *)tdo;
}

static PyObject *
teedataobject_jumplink(itertools_state *state, teedataobject *tdo)
{
    PyObject *link;
    Py_BEGIN_CRITICAL_SECTION(tdo);
    if (tdo->nextlink == NULL)
        tdo->nextlink = teedataobject_newinternal(state, tdo->it);
    link = Py_XNewRef(tdo->nextlink);
    Py_END_CRITICAL_SECTION();
    return link;
}

static PyObject *
teedataobject_getitem_lock_held(teedataobject *tdo, int i)
{
    PyObject *value;

    assert(i < LINKCELLS);
    if (i < tdo->numread)
        value = tdo->values[i];
    else {
        /* this is the lead iterator, so fetch more data */
        assert(i == tdo->numread);
        if (tdo->running) {
            PyErr_SetString(PyExc_RuntimeError,
                            "cannot re-enter the tee iterator");
            return NULL;
        }
        tdo->running = 1;
        value = PyIter_Next(tdo->it);
        tdo->running = 0;
        if (value == NULL)
            return NULL;
        tdo->numread++;
        tdo->values[i] = value;
    }
    return Py_NewRef(value);
}

static PyObject *
teedataobject_getitem(teedataobject *tdo, int i)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(tdo);
    result = teedataobject_getitem_lock_held(tdo, i);
    Py_END_CRITICAL_SECTION();
    return result;
}

static int
teedataobject_traverse(PyObject *op, visitproc visit, void * arg)
{
    int i;
    teedataobject *tdo = teedataobject_CAST(op);

    Py_VISIT(Py_TYPE(tdo));
    Py_VISIT(tdo->it);
    for (i = 0; i < tdo->numread; i++)
        Py_VISIT(tdo->values[i]);
    Py_VISIT(tdo->nextlink);
    return 0;
}

static void
teedataobject_safe_decref(PyObject *obj)
{
    while (obj && _PyObject_IsUniquelyReferenced(obj)) {
        teedataobject *tmp = teedataobject_CAST(obj);
        PyObject *nextlink;
        Py_BEGIN_CRITICAL_SECTION(obj);
        nextlink = tmp->nextlink;
        tmp->nextlink = NULL;
        Py_END_CRITICAL_SECTION();
        Py_SETREF(obj, nextlink);
    }
    Py_XDECREF(obj);
}

static int
teedataobject_clear(PyObject *op)
{
    int i;
    PyObject *tmp;
    teedataobject *tdo = teedataobject_CAST(op);

    Py_BEGIN_CRITICAL_SECTION(op);
    Py_CLEAR(tdo->it);
    for (i=0 ; i<tdo->numread ; i++)
        Py_CLEAR(tdo->values[i]);
    tmp = tdo->nextlink;
    tdo->nextlink = NULL;
    Py_END_CRITICAL_SECTION();
    teedataobject_safe_decref(tmp);
    return 0;
}

static void
teedataobject_dealloc(PyObject *op)
{
    PyTypeObject *tp = Py_TYPE(op);
    PyObject_GC_UnTrack(op);
    (void)teedataobject_clear(op);
    PyObject_GC_Del(op);
    Py_DECREF(tp);
}

/*[clinic input]
@classmethod
itertools.teedataobject.__new__
    iterable as it: object
    values: object(subclass_of='&PyList_Type')
    next: object
    /
Data container common to multiple tee objects.
[clinic start generated code]*/

static PyObject *
itertools_teedataobject_impl(PyTypeObject *type, PyObject *it,
                             PyObject *values, PyObject *next)
/*[clinic end generated code: output=3343ceb07e08df5e input=be60f2fabd2b72ba]*/
{
    teedataobject *tdo;
    Py_ssize_t i, len;

    itertools_state *state = get_module_state_by_cls(type);
    assert(type == state->teedataobject_type);

    tdo = (teedataobject *)teedataobject_newinternal(state, it);
    if (!tdo)
        return NULL;

    len = PyList_GET_SIZE(values);
    if (len > LINKCELLS)
        goto err;
    for (i=0; i<len; i++) {
        tdo->values[i] = PyList_GET_ITEM(values, i);
        Py_INCREF(tdo->values[i]);
    }
    /* len <= LINKCELLS < INT_MAX */
    tdo->numread = Py_SAFE_DOWNCAST(len, Py_ssize_t, int);

    if (len == LINKCELLS) {
        if (next != Py_None) {
            if (!Py_IS_TYPE(next, state->teedataobject_type))
                goto err;
            assert(tdo->nextlink == NULL);
            tdo->nextlink = Py_NewRef(next);
        }
    } else {
        if (next != Py_None)
            goto err; /* shouldn't have a next if we are not full */
    }
    return (PyObject*)tdo;

err:
    Py_XDECREF(tdo);
    PyErr_SetString(PyExc_ValueError, "Invalid arguments");
    return NULL;
}

static PyType_Slot teedataobject_slots[] = {
    {Py_tp_dealloc, teedataobject_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_teedataobject__doc__},
    {Py_tp_traverse, teedataobject_traverse},
    {Py_tp_clear, teedataobject_clear},
    {Py_tp_new, itertools_teedataobject},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec teedataobject_spec = {
    .name = "itertools._tee_dataobject",
    .basicsize = sizeof(teedataobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = teedataobject_slots,
};


static PyObject *
tee_next(PyObject *op)
{
    teeobject *to = teeobject_CAST(op);
    PyObject *value;

#ifndef Py_GIL_DISABLED
    /* The GIL already serializes access, so keep the simple path without the
       snapshot and revalidation that the free-threaded build needs. */
    if (to->index >= LINKCELLS) {
        PyObject *link = teedataobject_jumplink(to->state, to->dataobj);
        if (link == NULL) {
            return NULL;
        }
        Py_SETREF(to->dataobj, (teedataobject *)link);
        to->index = 0;
    }
    value = teedataobject_getitem(to->dataobj, to->index);
    if (value == NULL) {
        return NULL;
    }
    to->index++;
    return value;
#else
    for (;;) {
        teedataobject *dataobj;
        int index;

        /* Snapshot the branch position (strong ref to the shared data object)
           under the tee lock; the data object is locked separately, not nested,
           then the advance is revalidated. */
        Py_BEGIN_CRITICAL_SECTION(op);
        dataobj = (teedataobject *)Py_NewRef((PyObject *)to->dataobj);
        index = to->index;
        Py_END_CRITICAL_SECTION();

        if (index < LINKCELLS) {
            value = teedataobject_getitem(dataobj, index);
            if (value != NULL) {
                Py_BEGIN_CRITICAL_SECTION(op);
                if (to->dataobj == dataobj && to->index == index) {
                    to->index = index + 1;
                }
                Py_END_CRITICAL_SECTION();
            }
            Py_DECREF(dataobj);
            return value;
        }

        PyObject *link = teedataobject_jumplink(to->state, dataobj);
        if (link == NULL) {
            Py_DECREF(dataobj);
            return NULL;
        }
        Py_BEGIN_CRITICAL_SECTION(op);
        if (to->dataobj == dataobj) {
            Py_SETREF(to->dataobj, (teedataobject *)link);
            to->index = 0;
            link = NULL;
        }
        Py_END_CRITICAL_SECTION();
        Py_XDECREF(link);
        Py_DECREF(dataobj);
    }
#endif
}

static int
tee_traverse(PyObject *op, visitproc visit, void *arg)
{
    teeobject *to = teeobject_CAST(op);
    Py_VISIT(Py_TYPE(to));
    Py_VISIT((PyObject *)to->dataobj);
    return 0;
}

static teeobject *
tee_copy_impl(teeobject *to)
{
    teeobject *newto = PyObject_GC_New(teeobject, Py_TYPE(to));
    if (newto == NULL) {
        return NULL;
    }
    Py_BEGIN_CRITICAL_SECTION(to);
    newto->dataobj = (teedataobject *)Py_NewRef(to->dataobj);
    newto->index = to->index;
    Py_END_CRITICAL_SECTION();
    newto->weakreflist = NULL;
    newto->state = to->state;
    PyObject_GC_Track(newto);
    return newto;
}

static inline PyObject *
tee_copy(PyObject *op, PyObject *Py_UNUSED(ignored))
{
    teeobject *to = teeobject_CAST(op);
    return (PyObject *)tee_copy_impl(to);
}

PyDoc_STRVAR(teecopy_doc, "Returns an independent iterator.");

static PyObject *
tee_fromiterable(itertools_state *state, PyObject *iterable)
{
    teeobject *to;
    PyObject *it;

    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;
    if (PyObject_TypeCheck(it, state->tee_type)) {
        to = tee_copy_impl((teeobject *)it);  // 'it' can be fast casted
        goto done;
    }

    PyObject *dataobj = teedataobject_newinternal(state, it);
    if (!dataobj) {
        to = NULL;
        goto done;
    }
    to = PyObject_GC_New(teeobject, state->tee_type);
    if (to == NULL) {
        Py_DECREF(dataobj);
        goto done;
    }
    to->dataobj = (teedataobject *)dataobj;
    to->index = 0;
    to->weakreflist = NULL;
    to->state = state;
    PyObject_GC_Track(to);
done:
    Py_DECREF(it);
    return (PyObject *)to;
}

/*[clinic input]
@classmethod
itertools._tee.__new__
    iterable: object
    /
Iterator wrapped to make it copyable.
[clinic start generated code]*/

static PyObject *
itertools__tee_impl(PyTypeObject *type, PyObject *iterable)
/*[clinic end generated code: output=b02d3fd26c810c3f input=adc0779d2afe37a2]*/
{
    itertools_state *state = get_module_state_by_cls(type);
    return tee_fromiterable(state, iterable);
}

static int
tee_clear(PyObject *op)
{
    teeobject *to = teeobject_CAST(op);
    if (to->weakreflist != NULL)
        PyObject_ClearWeakRefs(op);
    Py_CLEAR(to->dataobj);
    return 0;
}

static void
tee_dealloc(PyObject *op)
{
    PyTypeObject *tp = Py_TYPE(op);
    PyObject_GC_UnTrack(op);
    (void)tee_clear(op);
    PyObject_GC_Del(op);
    Py_DECREF(tp);
}

static PyMethodDef tee_methods[] = {
    {"__copy__", tee_copy, METH_NOARGS, teecopy_doc},
    {NULL,              NULL}           /* sentinel */
};

static PyMemberDef tee_members[] = {
    {"__weaklistoffset__", Py_T_PYSSIZET, offsetof(teeobject, weakreflist), Py_READONLY},
    {NULL},
};

static PyType_Slot tee_slots[] = {
    {Py_tp_dealloc, tee_dealloc},
    {Py_tp_doc, (void *)itertools__tee__doc__},
    {Py_tp_traverse, tee_traverse},
    {Py_tp_clear, tee_clear},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, tee_next},
    {Py_tp_methods, tee_methods},
    {Py_tp_members, tee_members},
    {Py_tp_new, itertools__tee},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec tee_spec = {
    .name = "itertools._tee",
    .basicsize = sizeof(teeobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = tee_slots,
};

/*[clinic input]
itertools.tee
    iterable: object
    n: Py_ssize_t = 2
    /
Returns a tuple of n independent iterators.
[clinic start generated code]*/

static PyObject *
itertools_tee_impl(PyObject *module, PyObject *iterable, Py_ssize_t n)
/*[clinic end generated code: output=1c64519cd859c2f0 input=c99a1472c425d66d]*/
{
    Py_ssize_t i;
    PyObject *it, *to, *result;

    if (n < 0) {
        PyErr_SetString(PyExc_ValueError, "n must be >= 0");
        return NULL;
    }
    result = PyTuple_New(n);
    if (result == NULL)
        return NULL;
    if (n == 0)
        return result;
    it = PyObject_GetIter(iterable);
    if (it == NULL) {
        Py_DECREF(result);
        return NULL;
    }

    itertools_state *state = get_module_state(module);
    to = tee_fromiterable(state, it);
    Py_DECREF(it);
    if (to == NULL) {
        Py_DECREF(result);
        return NULL;
    }

    PyTuple_SET_ITEM(result, 0, to);
    for (i = 1; i < n; i++) {
        to = tee_copy(to, NULL);
        if (to == NULL) {
            Py_DECREF(result);
            return NULL;
        }
        PyTuple_SET_ITEM(result, i, to);
    }
    return result;
}


/* cycle object **************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *it;
    PyObject *saved;
    Py_ssize_t index;
} cycleobject;

#define cycleobject_CAST(op)    ((cycleobject *)(op))

/*[clinic input]
@classmethod
itertools.cycle.__new__
    iterable: object
    /
Return elements from the iterable until it is exhausted. Then repeat the sequence indefinitely.
[clinic start generated code]*/

static PyObject *
itertools_cycle_impl(PyTypeObject *type, PyObject *iterable)
/*[clinic end generated code: output=f60e5ec17a45b35c input=9d1d84bcf66e908b]*/
{
    PyObject *it;
    PyObject *saved;
    cycleobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;

    saved = PyList_New(0);
    if (saved == NULL) {
        Py_DECREF(it);
        return NULL;
    }

    /* create cycleobject structure */
    lz = (cycleobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        Py_DECREF(saved);
        return NULL;
    }
    lz->it = it;
    lz->saved = saved;
    lz->index = -1;

    return (PyObject *)lz;
}

static void
cycle_dealloc(PyObject *op)
{
    cycleobject *lz = cycleobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->it);
    Py_XDECREF(lz->saved);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
cycle_traverse(PyObject *op, visitproc visit, void *arg)
{
    cycleobject *lz = cycleobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    Py_VISIT(lz->saved);
    return 0;
}

static PyObject *
cycle_next(PyObject *op)
{
    cycleobject *lz = cycleobject_CAST(op);
    PyObject *item;

    Py_ssize_t index = FT_ATOMIC_LOAD_SSIZE_RELAXED(lz->index);

    if (index < 0) {
        item = PyIter_Next(lz->it);
        if (item != NULL) {
            if (PyList_Append(lz->saved, item)) {
                Py_DECREF(item);
                return NULL;
            }
            return item;
        }
        /* Note:  StopIteration is already cleared by PyIter_Next() */
        if (PyErr_Occurred())
            return NULL;
        index = 0;
        FT_ATOMIC_STORE_SSIZE_RELAXED(lz->index, 0);
#ifndef Py_GIL_DISABLED
        Py_CLEAR(lz->it);
#endif
    }
    if (PyList_GET_SIZE(lz->saved) == 0)
        return NULL;
    item = PyList_GetItemRef(lz->saved, index);
    assert(item);
    index++;
    if (index >= PyList_GET_SIZE(lz->saved)) {
        index = 0;
    }
    FT_ATOMIC_STORE_SSIZE_RELAXED(lz->index, index);
    return item;
}

static PyType_Slot cycle_slots[] = {
    {Py_tp_dealloc, cycle_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_cycle__doc__},
    {Py_tp_traverse, cycle_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, cycle_next},
    {Py_tp_new, itertools_cycle},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec cycle_spec = {
    .name = "itertools.cycle",
    .basicsize = sizeof(cycleobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = cycle_slots,
};


/* dropwhile object **********************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *func;
    PyObject *it;
    long start;
} dropwhileobject;

#define dropwhileobject_CAST(op)    ((dropwhileobject *)(op))

/*[clinic input]
@classmethod
itertools.dropwhile.__new__
    predicate as func: object
    iterable as seq: object
    /
Drop items from the iterable while predicate(item) is true.

Afterwards, return every element until the iterable is exhausted.
[clinic start generated code]*/

static PyObject *
itertools_dropwhile_impl(PyTypeObject *type, PyObject *func, PyObject *seq)
/*[clinic end generated code: output=92f9d0d89af149e4 input=d39737147c9f0a26]*/
{
    PyObject *it;
    dropwhileobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(seq);
    if (it == NULL)
        return NULL;

    /* create dropwhileobject structure */
    lz = (dropwhileobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    lz->func = Py_NewRef(func);
    lz->it = it;
    lz->start = 0;

    return (PyObject *)lz;
}

static void
dropwhile_dealloc(PyObject *op)
{
    dropwhileobject *lz = dropwhileobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->func);
    Py_XDECREF(lz->it);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
dropwhile_traverse(PyObject *op, visitproc visit, void *arg)
{
    dropwhileobject *lz = dropwhileobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    Py_VISIT(lz->func);
    return 0;
}

static PyObject *
dropwhile_next(PyObject *op)
{
    dropwhileobject *lz = dropwhileobject_CAST(op);
    PyObject *item, *good;
    PyObject *it = lz->it;
    long ok;
    PyObject *(*iternext)(PyObject *);

    iternext = *Py_TYPE(it)->tp_iternext;
    for (;;) {
        item = iternext(it);
        if (item == NULL)
            return NULL;
        if (lz->start == 1)
            return item;

        good = PyObject_CallOneArg(lz->func, item);
        if (good == NULL) {
            Py_DECREF(item);
            return NULL;
        }
        ok = PyObject_IsTrue(good);
        Py_DECREF(good);
        if (ok == 0) {
            lz->start = 1;
            return item;
        }
        Py_DECREF(item);
        if (ok < 0)
            return NULL;
    }
}

static PyType_Slot dropwhile_slots[] = {
    {Py_tp_dealloc, dropwhile_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_dropwhile__doc__},
    {Py_tp_traverse, dropwhile_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, dropwhile_next},
    {Py_tp_new, itertools_dropwhile},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec dropwhile_spec = {
    .name = "itertools.dropwhile",
    .basicsize = sizeof(dropwhileobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = dropwhile_slots,
};


/* takewhile object **********************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *func;
    PyObject *it;
    long stop;
} takewhileobject;

#define takewhileobject_CAST(op)    ((takewhileobject *)(op))

/*[clinic input]
@classmethod
itertools.takewhile.__new__
    predicate as func: object
    iterable as seq: object
    /
Return successive entries from an iterable as long as the predicate evaluates to true for each entry.
[clinic start generated code]*/

static PyObject *
itertools_takewhile_impl(PyTypeObject *type, PyObject *func, PyObject *seq)
/*[clinic end generated code: output=bb179ea7864e2ef6 input=ba5255f7519aa119]*/
{
    PyObject *it;
    takewhileobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(seq);
    if (it == NULL)
        return NULL;

    /* create takewhileobject structure */
    lz = (takewhileobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    lz->func = Py_NewRef(func);
    lz->it = it;
    lz->stop = 0;

    return (PyObject *)lz;
}

static void
takewhile_dealloc(PyObject *op)
{
    takewhileobject *lz = takewhileobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->func);
    Py_XDECREF(lz->it);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
takewhile_traverse(PyObject *op, visitproc visit, void *arg)
{
    takewhileobject *lz = takewhileobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    Py_VISIT(lz->func);
    return 0;
}

static PyObject *
takewhile_next(PyObject *op)
{
    takewhileobject *lz = takewhileobject_CAST(op);
    PyObject *item, *good;
    PyObject *it = lz->it;
    long ok;

    if (lz->stop == 1)
        return NULL;

    item = (*Py_TYPE(it)->tp_iternext)(it);
    if (item == NULL)
        return NULL;

    good = PyObject_CallOneArg(lz->func, item);
    if (good == NULL) {
        Py_DECREF(item);
        return NULL;
    }
    ok = PyObject_IsTrue(good);
    Py_DECREF(good);
    if (ok > 0)
        return item;
    Py_DECREF(item);
    if (ok == 0)
        lz->stop = 1;
    return NULL;
}

static PyType_Slot takewhile_slots[] = {
    {Py_tp_dealloc, takewhile_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_takewhile__doc__},
    {Py_tp_traverse, takewhile_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, takewhile_next},
    {Py_tp_new, itertools_takewhile},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec takewhile_spec = {
    .name = "itertools.takewhile",
    .basicsize = sizeof(takewhileobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = takewhile_slots,
};


/* islice object *************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *it;
    Py_ssize_t next;
    Py_ssize_t stop;
    Py_ssize_t step;
    Py_ssize_t cnt;
} isliceobject;

#define isliceobject_CAST(op)   ((isliceobject *)(op))

static PyObject *
islice_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *seq;
    Py_ssize_t start=0, stop=-1, step=1;
    PyObject *it, *a1=NULL, *a2=NULL, *a3=NULL;
    Py_ssize_t numargs;
    isliceobject *lz;

    itertools_state *st = find_state_by_type(type);
    PyTypeObject *islice_type = st->islice_type;
    if ((type == islice_type || type->tp_init == islice_type->tp_init) &&
        !_PyArg_NoKeywords("islice", kwds))
        return NULL;

    if (!PyArg_UnpackTuple(args, "islice", 2, 4, &seq, &a1, &a2, &a3))
        return NULL;

    numargs = PyTuple_Size(args);
    if (numargs == 2) {
        if (a1 != Py_None) {
            stop = PyNumber_AsSsize_t(a1, PyExc_OverflowError);
            if (stop == -1) {
                if (PyErr_Occurred())
                    PyErr_Clear();
                PyErr_SetString(PyExc_ValueError,
                   "Stop argument for islice() must be None or "
                   "an integer: 0 <= x <= sys.maxsize.");
                return NULL;
            }
        }
    } else {
        if (a1 != Py_None)
            start = PyNumber_AsSsize_t(a1, PyExc_OverflowError);
        if (start == -1 && PyErr_Occurred())
            PyErr_Clear();
        if (a2 != Py_None) {
            stop = PyNumber_AsSsize_t(a2, PyExc_OverflowError);
            if (stop == -1) {
                if (PyErr_Occurred())
                    PyErr_Clear();
                PyErr_SetString(PyExc_ValueError,
                   "Stop argument for islice() must be None or "
                   "an integer: 0 <= x <= sys.maxsize.");
                return NULL;
            }
        }
    }
    if (start<0 || stop<-1) {
        PyErr_SetString(PyExc_ValueError,
           "Indices for islice() must be None or "
           "an integer: 0 <= x <= sys.maxsize.");
        return NULL;
    }

    if (a3 != NULL) {
        if (a3 != Py_None)
            step = PyNumber_AsSsize_t(a3, PyExc_OverflowError);
        if (step == -1 && PyErr_Occurred())
            PyErr_Clear();
    }
    if (step<1) {
        PyErr_SetString(PyExc_ValueError,
           "Step for islice() must be a positive integer or None.");
        return NULL;
    }

    /* Get iterator. */
    it = PyObject_GetIter(seq);
    if (it == NULL)
        return NULL;

    /* create isliceobject structure */
    lz = (isliceobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    lz->it = it;
    lz->next = start;
    lz->stop = stop;
    lz->step = step;
    lz->cnt = 0L;

    return (PyObject *)lz;
}

static void
islice_dealloc(PyObject *op)
{
    isliceobject *lz = isliceobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->it);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
islice_traverse(PyObject *op, visitproc visit, void *arg)
{
    isliceobject *lz = isliceobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    return 0;
}

static PyObject *
islice_next(PyObject *op)
{
    isliceobject *lz = isliceobject_CAST(op);
    PyObject *item;
    PyObject *it = lz->it;
    Py_ssize_t stop = lz->stop;
    Py_ssize_t oldnext;
    PyObject *(*iternext)(PyObject *);

    if (it == NULL)
        return NULL;

    iternext = *Py_TYPE(it)->tp_iternext;
    while (lz->cnt < lz->next) {
        item = iternext(it);
        if (item == NULL)
            goto empty;
        Py_DECREF(item);
        lz->cnt++;
    }
    if (stop != -1 && lz->cnt >= stop)
        goto empty;
    item = iternext(it);
    if (item == NULL)
        goto empty;
    lz->cnt++;
    oldnext = lz->next;
    /* The (size_t) cast below avoids the danger of undefined
       behaviour from signed integer overflow. */
    lz->next += (size_t)lz->step;
    if (lz->next < oldnext || (stop != -1 && lz->next > stop))
        lz->next = stop;
    return item;

empty:
    Py_CLEAR(lz->it);
    return NULL;
}

PyDoc_STRVAR(islice_doc,
"islice(iterable, stop) --> islice object\n\
islice(iterable, start, stop[, step]) --> islice object\n\
\n\
Return an iterator whose next() method returns selected values from an\n\
iterable.  If start is specified, will skip all preceding elements;\n\
otherwise, start defaults to zero.  Step defaults to one.  If\n\
specified as another value, step determines how many values are\n\
skipped between successive calls.  Works like a slice() on a list\n\
but returns an iterator.");

static PyType_Slot islice_slots[] = {
    {Py_tp_dealloc, islice_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)islice_doc},
    {Py_tp_traverse, islice_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, islice_next},
    {Py_tp_new, islice_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec islice_spec = {
    .name = "itertools.islice",
    .basicsize = sizeof(isliceobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = islice_slots,
};


/* starmap object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *func;
    PyObject *it;
} starmapobject;

#define starmapobject_CAST(op)  ((starmapobject *)(op))

/*[clinic input]
@classmethod
itertools.starmap.__new__
    function as func: object
    iterable as seq: object
    /
Return an iterator whose values are returned from the function evaluated with an argument tuple taken from the given sequence.
[clinic start generated code]*/

static PyObject *
itertools_starmap_impl(PyTypeObject *type, PyObject *func, PyObject *seq)
/*[clinic end generated code: output=79eeb81d452c6e8d input=844766df6a0d4dad]*/
{
    PyObject *it;
    starmapobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(seq);
    if (it == NULL)
        return NULL;

    /* create starmapobject structure */
    lz = (starmapobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    lz->func = Py_NewRef(func);
    lz->it = it;

    return (PyObject *)lz;
}

static void
starmap_dealloc(PyObject *op)
{
    starmapobject *lz = starmapobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->func);
    Py_XDECREF(lz->it);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
starmap_traverse(PyObject *op, visitproc visit, void *arg)
{
    starmapobject *lz = starmapobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    Py_VISIT(lz->func);
    return 0;
}

static PyObject *
starmap_next(PyObject *op)
{
    starmapobject *lz = starmapobject_CAST(op);
    PyObject *args;
    PyObject *result;
    PyObject *it = lz->it;

    args = (*Py_TYPE(it)->tp_iternext)(it);
    if (args == NULL)
        return NULL;
    if (!PyTuple_CheckExact(args)) {
        PyObject *newargs = PySequence_Tuple(args);
        Py_DECREF(args);
        if (newargs == NULL)
            return NULL;
        args = newargs;
    }
    result = PyObject_Call(lz->func, args, NULL);
    Py_DECREF(args);
    return result;
}

static PyType_Slot starmap_slots[] = {
    {Py_tp_dealloc, starmap_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_starmap__doc__},
    {Py_tp_traverse, starmap_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, starmap_next},
    {Py_tp_new, itertools_starmap},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec starmap_spec = {
    .name = "itertools.starmap",
    .basicsize = sizeof(starmapobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = starmap_slots,
};


/* chain object **************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *source;                   /* Iterator over input iterables */
    PyObject *active;                   /* Currently running input iterator */
} chainobject;

#define chainobject_CAST(op)    ((chainobject *)(op))

static PyObject *
chain_new_internal(PyTypeObject *type, PyObject *source)
{
    chainobject *lz;

    lz = (chainobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(source);
        return NULL;
    }

    lz->source = source;
    lz->active = NULL;
    return (PyObject *)lz;
}

static PyObject *
chain_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    PyObject *source;

    itertools_state *state = find_state_by_type(type);
    PyTypeObject *chain_type = state->chain_type;
    if ((type == chain_type || type->tp_init == chain_type->tp_init) &&
        !_PyArg_NoKeywords("chain", kwds))
        return NULL;

    source = PyObject_GetIter(args);
    if (source == NULL)
        return NULL;

    return chain_new_internal(type, source);
}

/*[clinic input]
@classmethod
itertools.chain.from_iterable
    iterable as arg: object
    /
Alternative chain() constructor taking a single iterable argument that evaluates lazily.
[clinic start generated code]*/

static PyObject *
itertools_chain_from_iterable_impl(PyTypeObject *type, PyObject *arg)
/*[clinic end generated code: output=3d7ea7d46b9e43f5 input=72c39e3a2ca3be85]*/
{
    PyObject *source;

    source = PyObject_GetIter(arg);
    if (source == NULL)
        return NULL;

    return chain_new_internal(type, source);
}

static void
chain_dealloc(PyObject *op)
{
    chainobject *lz = chainobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->active);
    Py_XDECREF(lz->source);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
chain_traverse(PyObject *op, visitproc visit, void *arg)
{
    chainobject *lz = chainobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->source);
    Py_VISIT(lz->active);
    return 0;
}

static inline PyObject *
chain_next_lock_held(PyObject *op)
{
    chainobject *lz = chainobject_CAST(op);
    PyObject *item;

    /* lz->source is the iterator of iterables. If it's NULL, we've already
     * consumed them all. lz->active is the current iterator. If it's NULL,
     * we should grab a new one from lz->source. */
    while (lz->source != NULL) {
        if (lz->active == NULL) {
            PyObject *iterable = PyIter_Next(lz->source);
            if (iterable == NULL) {
                Py_CLEAR(lz->source);
                return NULL;            /* no more input sources */
            }
            lz->active = PyObject_GetIter(iterable);
            Py_DECREF(iterable);
            if (lz->active == NULL) {
                Py_CLEAR(lz->source);
                return NULL;            /* input not iterable */
            }
        }
        item = (*Py_TYPE(lz->active)->tp_iternext)(lz->active);
        if (item != NULL)
            return item;
        if (PyErr_Occurred()) {
            if (PyErr_ExceptionMatches(PyExc_StopIteration))
                PyErr_Clear();
            else
                return NULL;            /* input raised an exception */
        }
        /* lz->active is consumed, try with the next iterable. */
        Py_CLEAR(lz->active);
    }
    /* Everything had been consumed already. */
    return NULL;
}

static PyObject *
chain_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = chain_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

PyDoc_STRVAR(chain_doc,
"chain(*iterables)\n\
--\n\
\n\
Return a chain object whose .__next__() method returns elements from the\n\
first iterable until it is exhausted, then elements from the next\n\
iterable, until all of the iterables are exhausted.");

PyDoc_STRVAR(chain_class_getitem_doc,
"chain is generic over the type of its contents.\n\
This is the union of the types of the input iterable contents.");

static PyMethodDef chain_methods[] = {
    ITERTOOLS_CHAIN_FROM_ITERABLE_METHODDEF
    {"__class_getitem__",    Py_GenericAlias,
    METH_O|METH_CLASS,       chain_class_getitem_doc},
    {NULL,              NULL}           /* sentinel */
};

static PyType_Slot chain_slots[] = {
    {Py_tp_dealloc, chain_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)chain_doc},
    {Py_tp_traverse, chain_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, chain_next},
    {Py_tp_methods, chain_methods},
    {Py_tp_new, chain_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec chain_spec = {
    .name = "itertools.chain",
    .basicsize = sizeof(chainobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = chain_slots,
};


/* product object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *pools;        /* tuple of pool tuples */
    Py_ssize_t *indices;    /* one index per pool */
    PyObject *result;       /* most recently returned result tuple */
    int stopped;            /* set to 1 when the iterator is exhausted */
} productobject;

#define productobject_CAST(op)  ((productobject *)(op))

static PyObject *
product_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    productobject *lz;
    Py_ssize_t nargs, npools, repeat=1;
    PyObject *pools = NULL;
    Py_ssize_t *indices = NULL;
    Py_ssize_t i;

    if (kwds != NULL) {
        char *kwlist[] = {"repeat", 0};
        PyObject *tmpargs = PyTuple_New(0);
        if (tmpargs == NULL)
            return NULL;
        if (!PyArg_ParseTupleAndKeywords(tmpargs, kwds, "|n:product",
                                         kwlist, &repeat)) {
            Py_DECREF(tmpargs);
            return NULL;
        }
        Py_DECREF(tmpargs);
        if (repeat < 0) {
            PyErr_SetString(PyExc_ValueError,
                            "repeat argument cannot be negative");
            return NULL;
        }
    }

    assert(PyTuple_CheckExact(args));
    if (repeat == 0) {
        nargs = 0;
    } else {
        nargs = PyTuple_GET_SIZE(args);
        if ((size_t)nargs > PY_SSIZE_T_MAX/sizeof(Py_ssize_t)/repeat) {
            PyErr_SetString(PyExc_OverflowError, "repeat argument too large");
            return NULL;
        }
    }
    npools = nargs * repeat;

    indices = PyMem_New(Py_ssize_t, npools);
    if (indices == NULL) {
        PyErr_NoMemory();
        goto error;
    }

    pools = PyTuple_New(npools);
    if (pools == NULL)
        goto error;

    for (i=0; i < nargs ; ++i) {
        PyObject *item = PyTuple_GET_ITEM(args, i);
        PyObject *pool = PySequence_Tuple(item);
        if (pool == NULL)
            goto error;
        PyTuple_SET_ITEM(pools, i, pool);
        indices[i] = 0;
    }
    for ( ; i < npools; ++i) {
        PyObject *pool = PyTuple_GET_ITEM(pools, i - nargs);
        Py_INCREF(pool);
        PyTuple_SET_ITEM(pools, i, pool);
        indices[i] = 0;
    }

    /* create productobject structure */
    lz = (productobject *)type->tp_alloc(type, 0);
    if (lz == NULL)
        goto error;

    lz->pools = pools;
    lz->indices = indices;
    lz->result = NULL;
    lz->stopped = 0;

    return (PyObject *)lz;

error:
    if (indices != NULL)
        PyMem_Free(indices);
    Py_XDECREF(pools);
    return NULL;
}

static void
product_dealloc(PyObject *op)
{
    productobject *lz = productobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->pools);
    Py_XDECREF(lz->result);
    PyMem_Free(lz->indices);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static PyObject *
product_sizeof(PyObject *op, PyObject *Py_UNUSED(ignored))
{
    productobject *lz = productobject_CAST(op);
    size_t res = _PyObject_SIZE(Py_TYPE(lz));
    res += (size_t)PyTuple_GET_SIZE(lz->pools) * sizeof(Py_ssize_t);
    return PyLong_FromSize_t(res);
}

PyDoc_STRVAR(sizeof_doc, "Returns size in memory, in bytes.");

static int
product_traverse(PyObject *op, visitproc visit, void *arg)
{
    productobject *lz = productobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->pools);
    Py_VISIT(lz->result);
    return 0;
}

static PyObject *
product_next_lock_held(PyObject *op)
{
    productobject *lz = productobject_CAST(op);
    PyObject *pool;
    PyObject *elem;
    PyObject *oldelem;
    PyObject *pools = lz->pools;
    PyObject *result = lz->result;
    Py_ssize_t npools = PyTuple_GET_SIZE(pools);
    Py_ssize_t i;

    if (lz->stopped)
        return NULL;

    if (result == NULL) {
        /* On the first pass, return an initial tuple filled with the
           first element from each pool. */
        result = PyTuple_New(npools);
        if (result == NULL)
            goto empty;
        lz->result = result;
        for (i=0; i < npools; i++) {
            pool = PyTuple_GET_ITEM(pools, i);
            if (PyTuple_GET_SIZE(pool) == 0)
                goto empty;
            elem = PyTuple_GET_ITEM(pool, 0);
            Py_INCREF(elem);
            PyTuple_SET_ITEM(result, i, elem);
        }
    } else {
        Py_ssize_t *indices = lz->indices;

        /* Copy the previous result tuple or re-use it if available */
        if (!_PyObject_IsUniquelyReferenced(result)) {
            PyObject *old_result = result;
            result = _PyTuple_FromArray(_PyTuple_ITEMS(old_result), npools);
            if (result == NULL)
                goto empty;
            lz->result = result;
            Py_DECREF(old_result);
        }
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        else {
            _PyTuple_Recycle(result);
        }
        /* Now, we've got the only copy so we can update it in-place */
        assert (npools==0 || Py_REFCNT(result) == 1);

        /* Update the pool indices right-to-left.  Only advance to the
           next pool when the previous one rolls-over */
        for (i=npools-1 ; i >= 0 ; i--) {
            pool = PyTuple_GET_ITEM(pools, i);
            indices[i]++;
            if (indices[i] == PyTuple_GET_SIZE(pool)) {
                /* Roll-over and advance to next pool */
                indices[i] = 0;
                elem = PyTuple_GET_ITEM(pool, 0);
                Py_INCREF(elem);
                oldelem = PyTuple_GET_ITEM(result, i);
                PyTuple_SET_ITEM(result, i, elem);
                Py_DECREF(oldelem);
            } else {
                /* No rollover. Just increment and stop here. */
                elem = PyTuple_GET_ITEM(pool, indices[i]);
                Py_INCREF(elem);
                oldelem = PyTuple_GET_ITEM(result, i);
                PyTuple_SET_ITEM(result, i, elem);
                Py_DECREF(oldelem);
                break;
            }
        }

        /* If i is negative, then the indices have all rolled-over
           and we're done. */
        if (i < 0)
            goto empty;
    }

    return Py_NewRef(result);

empty:
    lz->stopped = 1;
    return NULL;
}

static PyObject *
product_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = product_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

static PyMethodDef product_methods[] = {
    {"__sizeof__", product_sizeof, METH_NOARGS, sizeof_doc},
    {NULL,              NULL}   /* sentinel */
};

PyDoc_STRVAR(product_doc,
"product(*iterables, repeat=1)\n\
--\n\
\n\
Cartesian product of input iterables.  Equivalent to nested for-loops.\n\n\
For example, product(A, B) returns the same as:  ((x,y) for x in A for y in B).\n\
The leftmost iterators are in the outermost for-loop, so the output tuples\n\
cycle in a manner similar to an odometer (with the rightmost element changing\n\
on every iteration).\n\n\
To compute the product of an iterable with itself, specify the number\n\
of repetitions with the optional repeat keyword argument. For example,\n\
product(A, repeat=4) means the same as product(A, A, A, A).\n\n\
product('ab', range(3)) --> ('a',0) ('a',1) ('a',2) ('b',0) ('b',1) ('b',2)\n\
product((0,1), (0,1), (0,1)) --> (0,0,0) (0,0,1) (0,1,0) (0,1,1) (1,0,0) ...");

static PyType_Slot product_slots[] = {
    {Py_tp_dealloc, product_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)product_doc},
    {Py_tp_traverse, product_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, product_next},
    {Py_tp_methods, product_methods},
    {Py_tp_new, product_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec product_spec = {
    .name = "itertools.product",
    .basicsize = sizeof(productobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = product_slots,
};


/* combinations object *******************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *pool;         /* input converted to a tuple */
    Py_ssize_t *indices;    /* one index per result element */
    PyObject *result;       /* most recently returned result tuple */
    Py_ssize_t r;           /* size of result tuple */
    int stopped;            /* set to 1 when the iterator is exhausted */
} combinationsobject;

#define combinationsobject_CAST(op) ((combinationsobject *)(op))

/*[clinic input]
@classmethod
itertools.combinations.__new__
    iterable: object
    r: Py_ssize_t
Return successive r-length combinations of elements in the iterable.

combinations(range(4), 3) --> (0,1,2), (0,1,3), (0,2,3), (1,2,3)
[clinic start generated code]*/

static PyObject *
itertools_combinations_impl(PyTypeObject *type, PyObject *iterable,
                            Py_ssize_t r)
/*[clinic end generated code: output=87a689b39c40039c input=06bede09e3da20f8]*/
{
    combinationsobject *co;
    Py_ssize_t n;
    PyObject *pool = NULL;
    Py_ssize_t *indices = NULL;
    Py_ssize_t i;

    pool = PySequence_Tuple(iterable);
    if (pool == NULL)
        goto error;
    n = PyTuple_GET_SIZE(pool);
    if (r < 0) {
        PyErr_SetString(PyExc_ValueError, "r must be non-negative");
        goto error;
    }

    indices = PyMem_New(Py_ssize_t, r);
    if (indices == NULL) {
        PyErr_NoMemory();
        goto error;
    }

    for (i=0 ; i<r ; i++)
        indices[i] = i;

    /* create combinationsobject structure */
    co = (combinationsobject *)type->tp_alloc(type, 0);
    if (co == NULL)
        goto error;

    co->pool = pool;
    co->indices = indices;
    co->result = NULL;
    co->r = r;
    co->stopped = r > n ? 1 : 0;

    return (PyObject *)co;

error:
    if (indices != NULL)
        PyMem_Free(indices);
    Py_XDECREF(pool);
    return NULL;
}

static void
combinations_dealloc(PyObject *op)
{
    combinationsobject *co = combinationsobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(co);
    PyObject_GC_UnTrack(co);
    Py_XDECREF(co->pool);
    Py_XDECREF(co->result);
    PyMem_Free(co->indices);
    tp->tp_free(co);
    Py_DECREF(tp);
}

static PyObject *
combinations_sizeof(PyObject *op, PyObject *Py_UNUSED(args))
{
    combinationsobject *co = combinationsobject_CAST(op);
    size_t res = _PyObject_SIZE(Py_TYPE(co));
    res += (size_t)co->r * sizeof(Py_ssize_t);
    return PyLong_FromSize_t(res);
}

static int
combinations_traverse(PyObject *op, visitproc visit, void *arg)
{
    combinationsobject *co = combinationsobject_CAST(op);
    Py_VISIT(Py_TYPE(co));
    Py_VISIT(co->pool);
    Py_VISIT(co->result);
    return 0;
}

static PyObject *
combinations_next_lock_held(PyObject *op)
{
    combinationsobject *co = combinationsobject_CAST(op);
    PyObject *elem;
    PyObject *oldelem;
    PyObject *pool = co->pool;
    Py_ssize_t *indices = co->indices;
    PyObject *result = co->result;
    Py_ssize_t n = PyTuple_GET_SIZE(pool);
    Py_ssize_t r = co->r;
    Py_ssize_t i, j, index;

    if (co->stopped)
        return NULL;

    if (result == NULL) {
        /* On the first pass, initialize result tuple using the indices */
        result = PyTuple_New(r);
        if (result == NULL)
            goto empty;
        co->result = result;
        for (i=0; i<r ; i++) {
            index = indices[i];
            elem = PyTuple_GET_ITEM(pool, index);
            Py_INCREF(elem);
            PyTuple_SET_ITEM(result, i, elem);
        }
    } else {
        /* Copy the previous result tuple or re-use it if available */
        if (!_PyObject_IsUniquelyReferenced(result)) {
            PyObject *old_result = result;
            result = _PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
            if (result == NULL)
                goto empty;
            co->result = result;
            Py_DECREF(old_result);
        }
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        else {
            _PyTuple_Recycle(result);
        }
        /* Now, we've got the only copy so we can update it in-place
         * CPython's empty tuple is a singleton and cached in
         * PyTuple's freelist.
         */
        assert(r == 0 || Py_REFCNT(result) == 1);

        /* Scan indices right-to-left until finding one that is not
           at its maximum (i + n - r). */
        for (i=r-1 ; i >= 0 && indices[i] == i+n-r ; i--)
            ;

        /* If i is negative, then the indices are all at
           their maximum value and we're done. */
        if (i < 0)
            goto empty;

        /* Increment the current index which we know is not at its
           maximum.  Then move back to the right setting each index
           to its lowest possible value (one higher than the index
           to its left -- this maintains the sort order invariant). */
        indices[i]++;
        for (j=i+1 ; j<r ; j++)
            indices[j] = indices[j-1] + 1;

        /* Update the result tuple for the new indices
           starting with i, the leftmost index that changed */
        for ( ; i<r ; i++) {
            index = indices[i];
            elem = PyTuple_GET_ITEM(pool, index);
            Py_INCREF(elem);
            oldelem = PyTuple_GET_ITEM(result, i);
            PyTuple_SET_ITEM(result, i, elem);
            Py_DECREF(oldelem);
        }
    }

    return Py_NewRef(result);

empty:
    co->stopped = 1;
    return NULL;
}

static PyObject *
combinations_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = combinations_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

static PyMethodDef combinations_methods[] = {
    {"__sizeof__", combinations_sizeof, METH_NOARGS, sizeof_doc},
    {NULL,              NULL}   /* sentinel */
};

static PyType_Slot combinations_slots[] = {
    {Py_tp_dealloc, combinations_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_combinations__doc__},
    {Py_tp_traverse, combinations_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, combinations_next},
    {Py_tp_methods, combinations_methods},
    {Py_tp_new, itertools_combinations},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec combinations_spec = {
    .name = "itertools.combinations",
    .basicsize = sizeof(combinationsobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = combinations_slots,
};


/* combinations with replacement object **************************************/

/* Equivalent to:

        def combinations_with_replacement(iterable, r):
            "combinations_with_replacement('ABC', 2) --> AA AB AC BB BC CC"
            # number items returned:  (n+r-1)! / r! / (n-1)!
            pool = tuple(iterable)
            n = len(pool)
            indices = [0] * r
            yield tuple(pool[i] for i in indices)
            while 1:
                for i in reversed(range(r)):
                    if indices[i] != n - 1:
                        break
                else:
                    return
                indices[i:] = [indices[i] + 1] * (r - i)
                yield tuple(pool[i] for i in indices)

        def combinations_with_replacement2(iterable, r):
            'Alternate version that filters from product()'
            pool = tuple(iterable)
            n = len(pool)
            for indices in product(range(n), repeat=r):
                if sorted(indices) == list(indices):
                    yield tuple(pool[i] for i in indices)
*/
typedef struct {
    PyObject_HEAD
    PyObject *pool;         /* input converted to a tuple */
    Py_ssize_t *indices;    /* one index per result element */
    PyObject *result;       /* most recently returned result tuple */
    Py_ssize_t r;           /* size of result tuple */
    int stopped;            /* set to 1 when the cwr iterator is exhausted */
} cwrobject;

#define cwrobject_CAST(op)  ((cwrobject *)(op))

/*[clinic input]
@classmethod
itertools.combinations_with_replacement.__new__
    iterable: object
    r: Py_ssize_t
Return successive r-length combinations of elements in the iterable allowing individual elements to have successive repeats.

combinations_with_replacement('ABC', 2) --> ('A','A'), ('A','B'), ('A','C'), ('B','B'), ('B','C'), ('C','C')
[clinic start generated code]*/

static PyObject *
itertools_combinations_with_replacement_impl(PyTypeObject *type,
                                             PyObject *iterable,
                                             Py_ssize_t r)
/*[clinic end generated code: output=48b26856d4e659ca input=1dc58e82a0878fdc]*/
{
    cwrobject *co;
    Py_ssize_t n;
    PyObject *pool = NULL;
    Py_ssize_t *indices = NULL;
    Py_ssize_t i;

    pool = PySequence_Tuple(iterable);
    if (pool == NULL)
        goto error;
    n = PyTuple_GET_SIZE(pool);
    if (r < 0) {
        PyErr_SetString(PyExc_ValueError, "r must be non-negative");
        goto error;
    }

    indices = PyMem_New(Py_ssize_t, r);
    if (indices == NULL) {
        PyErr_NoMemory();
        goto error;
    }

    for (i=0 ; i<r ; i++)
        indices[i] = 0;

    /* create cwrobject structure */
    co = (cwrobject *)type->tp_alloc(type, 0);
    if (co == NULL)
        goto error;

    co->pool = pool;
    co->indices = indices;
    co->result = NULL;
    co->r = r;
    co->stopped = !n && r;

    return (PyObject *)co;

error:
    if (indices != NULL)
        PyMem_Free(indices);
    Py_XDECREF(pool);
    return NULL;
}

static void
cwr_dealloc(PyObject *op)
{
    cwrobject *co = cwrobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(co);
    PyObject_GC_UnTrack(co);
    Py_XDECREF(co->pool);
    Py_XDECREF(co->result);
    PyMem_Free(co->indices);
    tp->tp_free(co);
    Py_DECREF(tp);
}

static PyObject *
cwr_sizeof(PyObject *op, PyObject *Py_UNUSED(args))
{
    cwrobject *co = cwrobject_CAST(op);
    size_t res = _PyObject_SIZE(Py_TYPE(co));
    res += (size_t)co->r * sizeof(Py_ssize_t);
    return PyLong_FromSize_t(res);
}

static int
cwr_traverse(PyObject *op, visitproc visit, void *arg)
{
    cwrobject *co = cwrobject_CAST(op);
    Py_VISIT(Py_TYPE(co));
    Py_VISIT(co->pool);
    Py_VISIT(co->result);
    return 0;
}

static PyObject *
cwr_next_lock_held(PyObject *op)
{
    cwrobject *co = cwrobject_CAST(op);
    PyObject *elem;
    PyObject *oldelem;
    PyObject *pool = co->pool;
    Py_ssize_t *indices = co->indices;
    PyObject *result = co->result;
    Py_ssize_t n = PyTuple_GET_SIZE(pool);
    Py_ssize_t r = co->r;
    Py_ssize_t i, index;

    if (co->stopped)
        return NULL;

    if (result == NULL) {
        /* On the first pass, initialize result tuple with pool[0] */
        result = PyTuple_New(r);
        if (result == NULL)
            goto empty;
        co->result = result;
        if (n > 0) {
            elem = PyTuple_GET_ITEM(pool, 0);
            for (i=0; i<r ; i++) {
                assert(indices[i] == 0);
                Py_INCREF(elem);
                PyTuple_SET_ITEM(result, i, elem);
            }
        }
    } else {
        /* Copy the previous result tuple or re-use it if available */
        if (!_PyObject_IsUniquelyReferenced(result)) {
            PyObject *old_result = result;
            result = _PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
            if (result == NULL)
                goto empty;
            co->result = result;
            Py_DECREF(old_result);
        }
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        else {
            _PyTuple_Recycle(result);
        }
        /* Now, we've got the only copy so we can update it in-place CPython's
           empty tuple is a singleton and cached in PyTuple's freelist. */
        assert(r == 0 || Py_REFCNT(result) == 1);

       /* Scan indices right-to-left until finding one that is not
        * at its maximum (n-1). */
        for (i=r-1 ; i >= 0 && indices[i] == n-1; i--)
            ;

        /* If i is negative, then the indices are all at
           their maximum value and we're done. */
        if (i < 0)
            goto empty;

        /* Increment the current index which we know is not at its
           maximum.  Then set all to the right to the same value. */
        index = indices[i] + 1;
        assert(index < n);
        elem = PyTuple_GET_ITEM(pool, index);
        for ( ; i<r ; i++) {
            indices[i] = index;
            Py_INCREF(elem);
            oldelem = PyTuple_GET_ITEM(result, i);
            PyTuple_SET_ITEM(result, i, elem);
            Py_DECREF(oldelem);
        }
    }

    return Py_NewRef(result);

empty:
    co->stopped = 1;
    return NULL;
}

static PyObject *
cwr_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = cwr_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

static PyMethodDef cwr_methods[] = {
    {"__sizeof__", cwr_sizeof, METH_NOARGS, sizeof_doc},
    {NULL,              NULL}   /* sentinel */
};

static PyType_Slot cwr_slots[] = {
    {Py_tp_dealloc, cwr_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_combinations_with_replacement__doc__},
    {Py_tp_traverse, cwr_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, cwr_next},
    {Py_tp_methods, cwr_methods},
    {Py_tp_new, itertools_combinations_with_replacement},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec cwr_spec = {
    .name = "itertools.combinations_with_replacement",
    .basicsize = sizeof(cwrobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = cwr_slots,
};


/* permutations object ********************************************************

def permutations(iterable, r=None):
    # permutations('ABCD', 2) --> AB AC AD BA BC BD CA CB CD DA DB DC
    # permutations(range(3)) --> 012 021 102 120 201 210
    pool = tuple(iterable)
    n = len(pool)
    r = n if r is None else r
    if r > n:
        return
    indices = list(range(n))
    cycles = list(range(n, n-r, -1))
    yield tuple(pool[i] for i in indices[:r])
    while n:
        for i in reversed(range(r)):
            cycles[i] -= 1
            if cycles[i] == 0:
                indices[i:] = indices[i+1:] + indices[i:i+1]
                cycles[i] = n - i
            else:
                j = cycles[i]
                indices[i], indices[-j] = indices[-j], indices[i]
                yield tuple(pool[i] for i in indices[:r])
                break
        else:
            return
*/

typedef struct {
    PyObject_HEAD
    PyObject *pool;         /* input converted to a tuple */
    Py_ssize_t *indices;    /* one index per element in the pool */
    Py_ssize_t *cycles;     /* one rollover counter per element in the result */
    PyObject *result;       /* most recently returned result tuple */
    Py_ssize_t r;           /* size of result tuple */
    int stopped;            /* set to 1 when the iterator is exhausted */
} permutationsobject;

#define permutationsobject_CAST(op) ((permutationsobject *)(op))

/*[clinic input]
@classmethod
itertools.permutations.__new__
    iterable: object
    r as robj: object = None
Return successive r-length permutations of elements in the iterable.

permutations(range(3), 2) --> (0,1), (0,2), (1,0), (1,2), (2,0), (2,1)
[clinic start generated code]*/

static PyObject *
itertools_permutations_impl(PyTypeObject *type, PyObject *iterable,
                            PyObject *robj)
/*[clinic end generated code: output=296a72fa76d620ea input=57d0170a4ac0ec7a]*/
{
    permutationsobject *po;
    Py_ssize_t n;
    Py_ssize_t r;
    PyObject *pool = NULL;
    Py_ssize_t *indices = NULL;
    Py_ssize_t *cycles = NULL;
    Py_ssize_t i;

    pool = PySequence_Tuple(iterable);
    if (pool == NULL)
        goto error;
    n = PyTuple_GET_SIZE(pool);

    r = n;
    if (robj != Py_None) {
        if (!PyLong_Check(robj)) {
            PyErr_SetString(PyExc_TypeError, "Expected int as r");
            goto error;
        }
        r = PyLong_AsSsize_t(robj);
        if (r == -1 && PyErr_Occurred())
            goto error;
    }
    if (r < 0) {
        PyErr_SetString(PyExc_ValueError, "r must be non-negative");
        goto error;
    }

    indices = PyMem_New(Py_ssize_t, n);
    cycles = PyMem_New(Py_ssize_t, r);
    if (indices == NULL || cycles == NULL) {
        PyErr_NoMemory();
        goto error;
    }

    for (i=0 ; i<n ; i++)
        indices[i] = i;
    for (i=0 ; i<r ; i++)
        cycles[i] = n - i;

    /* create permutationsobject structure */
    po = (permutationsobject *)type->tp_alloc(type, 0);
    if (po == NULL)
        goto error;

    po->pool = pool;
    po->indices = indices;
    po->cycles = cycles;
    po->result = NULL;
    po->r = r;
    po->stopped = r > n ? 1 : 0;

    return (PyObject *)po;

error:
    if (indices != NULL)
        PyMem_Free(indices);
    if (cycles != NULL)
        PyMem_Free(cycles);
    Py_XDECREF(pool);
    return NULL;
}

static void
permutations_dealloc(PyObject *op)
{
    permutationsobject *po = permutationsobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(po);
    PyObject_GC_UnTrack(po);
    Py_XDECREF(po->pool);
    Py_XDECREF(po->result);
    PyMem_Free(po->indices);
    PyMem_Free(po->cycles);
    tp->tp_free(po);
    Py_DECREF(tp);
}

static PyObject *
permutations_sizeof(PyObject *op, PyObject *Py_UNUSED(args))
{
    permutationsobject *po = permutationsobject_CAST(op);
    size_t res = _PyObject_SIZE(Py_TYPE(po));
    res += (size_t)PyTuple_GET_SIZE(po->pool) * sizeof(Py_ssize_t);
    res += (size_t)po->r * sizeof(Py_ssize_t);
    return PyLong_FromSize_t(res);
}

static int
permutations_traverse(PyObject *op, visitproc visit, void *arg)
{
    permutationsobject *po = permutationsobject_CAST(op);
    Py_VISIT(Py_TYPE(po));
    Py_VISIT(po->pool);
    Py_VISIT(po->result);
    return 0;
}

static PyObject *
permutations_next_lock_held(PyObject *op)
{
    permutationsobject *po = permutationsobject_CAST(op);
    PyObject *elem;
    PyObject *oldelem;
    PyObject *pool = po->pool;
    Py_ssize_t *indices = po->indices;
    Py_ssize_t *cycles = po->cycles;
    PyObject *result = po->result;
    Py_ssize_t n = PyTuple_GET_SIZE(pool);
    Py_ssize_t r = po->r;
    Py_ssize_t i, j, k, index;

    if (po->stopped)
        return NULL;

    if (result == NULL) {
        /* On the first pass, initialize result tuple using the indices */
        result = PyTuple_New(r);
        if (result == NULL)
            goto empty;
        po->result = result;
        for (i=0; i<r ; i++) {
            index = indices[i];
            elem = PyTuple_GET_ITEM(pool, index);
            Py_INCREF(elem);
            PyTuple_SET_ITEM(result, i, elem);
        }
    } else {
        if (n == 0)
            goto empty;

        /* Copy the previous result tuple or re-use it if available */
        if (!_PyObject_IsUniquelyReferenced(result)) {
            PyObject *old_result = result;
            result = _PyTuple_FromArray(_PyTuple_ITEMS(old_result), r);
            if (result == NULL)
                goto empty;
            po->result = result;
            Py_DECREF(old_result);
        }
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        else {
            _PyTuple_Recycle(result);
        }
        /* Now, we've got the only copy so we can update it in-place */
        assert(r == 0 || Py_REFCNT(result) == 1);

        /* Decrement rightmost cycle, moving leftward upon zero rollover */
        for (i=r-1 ; i>=0 ; i--) {
            cycles[i] -= 1;
            if (cycles[i] == 0) {
                /* rotatation: indices[i:] = indices[i+1:] + indices[i:i+1] */
                index = indices[i];
                for (j=i ; j<n-1 ; j++)
                    indices[j] = indices[j+1];
                indices[n-1] = index;
                cycles[i] = n - i;
            } else {
                j = cycles[i];
                index = indices[i];
                indices[i] = indices[n-j];
                indices[n-j] = index;

                for (k=i; k<r ; k++) {
                    /* start with i, the leftmost element that changed */
                    /* yield tuple(pool[k] for k in indices[:r]) */
                    index = indices[k];
                    elem = PyTuple_GET_ITEM(pool, index);
                    Py_INCREF(elem);
                    oldelem = PyTuple_GET_ITEM(result, k);
                    PyTuple_SET_ITEM(result, k, elem);
                    Py_DECREF(oldelem);
                }
                break;
            }
        }
        /* If i is negative, then the cycles have all
           rolled-over and we're done. */
        if (i < 0)
            goto empty;
    }
    return Py_NewRef(result);

empty:
    po->stopped = 1;
    return NULL;
}

static PyObject *
permutations_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = permutations_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

static PyMethodDef permuations_methods[] = {
    {"__sizeof__", permutations_sizeof, METH_NOARGS, sizeof_doc},
    {NULL,              NULL}   /* sentinel */
};

static PyType_Slot permutations_slots[] = {
    {Py_tp_dealloc, permutations_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_permutations__doc__},
    {Py_tp_traverse, permutations_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, permutations_next},
    {Py_tp_methods, permuations_methods},
    {Py_tp_new, itertools_permutations},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec permutations_spec = {
    .name = "itertools.permutations",
    .basicsize = sizeof(permutationsobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = permutations_slots,
};


/* accumulate object ********************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *total;
    PyObject *it;
    PyObject *binop;
    PyObject *initial;
    itertools_state *state;
} accumulateobject;

#define accumulateobject_CAST(op)   ((accumulateobject *)(op))

/*[clinic input]
@classmethod
itertools.accumulate.__new__
    iterable: object
    func as binop: object = None
    *
    initial: object = None
Return series of accumulated sums (or other binary function results).
[clinic start generated code]*/

static PyObject *
itertools_accumulate_impl(PyTypeObject *type, PyObject *iterable,
                          PyObject *binop, PyObject *initial)
/*[clinic end generated code: output=66da2650627128f8 input=c4ce20ac59bf7ffd]*/
{
    PyObject *it;
    accumulateobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;

    /* create accumulateobject structure */
    lz = (accumulateobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }

    if (binop != Py_None) {
        lz->binop = Py_XNewRef(binop);
    }
    lz->total = NULL;
    lz->it = it;
    lz->initial = Py_XNewRef(initial);
    lz->state = find_state_by_type(type);
    return (PyObject *)lz;
}

static void
accumulate_dealloc(PyObject *op)
{
    accumulateobject *lz = accumulateobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->binop);
    Py_XDECREF(lz->total);
    Py_XDECREF(lz->it);
    Py_XDECREF(lz->initial);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
accumulate_traverse(PyObject *op, visitproc visit, void *arg)
{
    accumulateobject *lz = accumulateobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->binop);
    Py_VISIT(lz->it);
    Py_VISIT(lz->total);
    Py_VISIT(lz->initial);
    return 0;
}

static PyObject *
accumulate_next_lock_held(PyObject *op)
{
    accumulateobject *lz = accumulateobject_CAST(op);
    PyObject *val, *newtotal;

    if (lz->initial != Py_None) {
        lz->total = lz->initial;
        lz->initial = Py_NewRef(Py_None);
        return Py_NewRef(lz->total);
    }
    val = (*Py_TYPE(lz->it)->tp_iternext)(lz->it);
    if (val == NULL)
        return NULL;

    if (lz->total == NULL) {
        lz->total = Py_NewRef(val);
        return lz->total;
    }

    if (lz->binop == NULL)
        newtotal = PyNumber_Add(lz->total, val);
    else
        newtotal = PyObject_CallFunctionObjArgs(lz->binop, lz->total, val, NULL);
    Py_DECREF(val);
    if (newtotal == NULL)
        return NULL;

    Py_INCREF(newtotal);
    Py_SETREF(lz->total, newtotal);
    return newtotal;
}

static PyObject *
accumulate_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = accumulate_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

static PyType_Slot accumulate_slots[] = {
    {Py_tp_dealloc, accumulate_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_accumulate__doc__},
    {Py_tp_traverse, accumulate_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, accumulate_next},
    {Py_tp_new, itertools_accumulate},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec accumulate_spec = {
    .name = "itertools.accumulate",
    .basicsize = sizeof(accumulateobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = accumulate_slots,
};


/* compress object ************************************************************/

/* Equivalent to:

    def compress(data, selectors):
        "compress('ABCDEF', [1,0,1,0,1,1]) --> A C E F"
        return (d for d, s in zip(data, selectors) if s)
*/

typedef struct {
    PyObject_HEAD
    PyObject *data;
    PyObject *selectors;
} compressobject;

#define compressobject_CAST(op) ((compressobject *)(op))

/*[clinic input]
@classmethod
itertools.compress.__new__
    data as seq1: object
    selectors as seq2: object
Return data elements corresponding to true selector elements.

Forms a shorter iterator from selected data elements using the selectors
to choose the data elements.
[clinic start generated code]*/

static PyObject *
itertools_compress_impl(PyTypeObject *type, PyObject *seq1, PyObject *seq2)
/*[clinic end generated code: output=7e67157212ed09e0 input=32ca4347dbc46749]*/
{
    PyObject *data=NULL, *selectors=NULL;
    compressobject *lz;

    data = PyObject_GetIter(seq1);
    if (data == NULL)
        goto fail;
    selectors = PyObject_GetIter(seq2);
    if (selectors == NULL)
        goto fail;

    /* create compressobject structure */
    lz = (compressobject *)type->tp_alloc(type, 0);
    if (lz == NULL)
        goto fail;
    lz->data = data;
    lz->selectors = selectors;
    return (PyObject *)lz;

fail:
    Py_XDECREF(data);
    Py_XDECREF(selectors);
    return NULL;
}

static void
compress_dealloc(PyObject *op)
{
    compressobject *lz = compressobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->data);
    Py_XDECREF(lz->selectors);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
compress_traverse(PyObject *op, visitproc visit, void *arg)
{
    compressobject *lz = compressobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->data);
    Py_VISIT(lz->selectors);
    return 0;
}

static PyObject *
compress_next(PyObject *op)
{
    compressobject *lz = compressobject_CAST(op);
    PyObject *data = lz->data, *selectors = lz->selectors;
    PyObject *datum, *selector;
    PyObject *(*datanext)(PyObject *) = *Py_TYPE(data)->tp_iternext;
    PyObject *(*selectornext)(PyObject *) = *Py_TYPE(selectors)->tp_iternext;
    int ok;

    while (1) {
        /* Steps:  get datum, get selector, evaluate selector.
           Order is important (to match the pure python version
           in terms of which input gets a chance to raise an
           exception first).
        */

        datum = datanext(data);
        if (datum == NULL)
            return NULL;

        selector = selectornext(selectors);
        if (selector == NULL) {
            Py_DECREF(datum);
            return NULL;
        }

        ok = PyObject_IsTrue(selector);
        Py_DECREF(selector);
        if (ok > 0)
            return datum;
        Py_DECREF(datum);
        if (ok < 0)
            return NULL;
    }
}

static PyType_Slot compress_slots[] = {
    {Py_tp_dealloc, compress_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_compress__doc__},
    {Py_tp_traverse, compress_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, compress_next},
    {Py_tp_new, itertools_compress},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec compress_spec = {
    .name = "itertools.compress",
    .basicsize = sizeof(compressobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = compress_slots,
};


/* filterfalse object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *func;
    PyObject *it;
} filterfalseobject;

#define filterfalseobject_CAST(op)  ((filterfalseobject *)(op))

/*[clinic input]
@classmethod
itertools.filterfalse.__new__
    function as func: object
    iterable as seq: object
    /
Return those items of iterable for which function(item) is false.

If function is None, return the items that are false.
[clinic start generated code]*/

static PyObject *
itertools_filterfalse_impl(PyTypeObject *type, PyObject *func, PyObject *seq)
/*[clinic end generated code: output=55f87eab9fc0484e input=2d684a2c66f99cde]*/
{
    PyObject *it;
    filterfalseobject *lz;

    /* Get iterator. */
    it = PyObject_GetIter(seq);
    if (it == NULL)
        return NULL;

    /* create filterfalseobject structure */
    lz = (filterfalseobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(it);
        return NULL;
    }
    lz->func = Py_NewRef(func);
    lz->it = it;

    return (PyObject *)lz;
}

static void
filterfalse_dealloc(PyObject *op)
{
    filterfalseobject *lz = filterfalseobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->func);
    Py_XDECREF(lz->it);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
filterfalse_traverse(PyObject *op, visitproc visit, void *arg)
{
    filterfalseobject *lz = filterfalseobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->it);
    Py_VISIT(lz->func);
    return 0;
}

static PyObject *
filterfalse_next(PyObject *op)
{
    filterfalseobject *lz = filterfalseobject_CAST(op);
    PyObject *item;
    PyObject *it = lz->it;
    long ok;
    PyObject *(*iternext)(PyObject *);

    iternext = *Py_TYPE(it)->tp_iternext;
    for (;;) {
        item = iternext(it);
        if (item == NULL)
            return NULL;

        if (lz->func == Py_None || lz->func == (PyObject *)&PyBool_Type) {
            ok = PyObject_IsTrue(item);
        } else {
            PyObject *good;
            good = PyObject_CallOneArg(lz->func, item);
            if (good == NULL) {
                Py_DECREF(item);
                return NULL;
            }
            ok = PyObject_IsTrue(good);
            Py_DECREF(good);
        }
        if (ok == 0)
            return item;
        Py_DECREF(item);
        if (ok < 0)
            return NULL;
    }
}

static PyType_Slot filterfalse_slots[] = {
    {Py_tp_dealloc, filterfalse_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_filterfalse__doc__},
    {Py_tp_traverse, filterfalse_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, filterfalse_next},
    {Py_tp_new, itertools_filterfalse},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec filterfalse_spec = {
    .name = "itertools.filterfalse",
    .basicsize = sizeof(filterfalseobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = filterfalse_slots,
};


/* count object ************************************************************/

typedef struct {
    PyObject_HEAD
    Py_ssize_t cnt;
    PyObject *long_cnt;
    PyObject *long_step;
} countobject;

#define countobject_CAST(op)    ((countobject *)(op))

/* Counting logic and invariants:

fast_mode:  when cnt an integer < PY_SSIZE_T_MAX and no step is specified.

    assert(long_cnt == NULL && long_step==PyLong(1));
    Advances with:  cnt += 1
    When count hits PY_SSIZE_T_MAX, switch to slow_mode.

slow_mode:  when cnt == PY_SSIZE_T_MAX, step is not int(1), or cnt is a float.

    assert(cnt == PY_SSIZE_T_MAX && long_cnt != NULL && long_step != NULL);
    All counting is done with python objects (no overflows or underflows).
    Advances with:  long_cnt += long_step
    Step may be zero -- effectively a slow version of repeat(cnt).
    Either long_cnt or long_step may be a float, Fraction, or Decimal.
*/

/*[clinic input]
@classmethod
itertools.count.__new__
    start as long_cnt: object(c_default="NULL") = 0
    step as long_step: object(c_default="NULL") = 1
Return a count object whose .__next__() method returns consecutive values.

Equivalent to:
    def count(firstval=0, step=1):
        x = firstval
        while 1:
            yield x
            x += step
[clinic start generated code]*/

static PyObject *
itertools_count_impl(PyTypeObject *type, PyObject *long_cnt,
                     PyObject *long_step)
/*[clinic end generated code: output=09a9250aebd00b1c input=d7a85eec18bfcd94]*/
{
    countobject *lz;
    int fast_mode;
    Py_ssize_t cnt = 0;
    long step;

    if ((long_cnt != NULL && !PyNumber_Check(long_cnt)) ||
        (long_step != NULL && !PyNumber_Check(long_step))) {
                    PyErr_SetString(PyExc_TypeError, "a number is required");
                    return NULL;
    }

    fast_mode = (long_cnt == NULL || PyLong_Check(long_cnt)) &&
                (long_step == NULL || PyLong_Check(long_step));

    /* If not specified, start defaults to 0 */
    if (long_cnt != NULL) {
        if (fast_mode) {
            assert(PyLong_Check(long_cnt));
            cnt = PyLong_AsSsize_t(long_cnt);
            if (cnt == -1 && PyErr_Occurred()) {
                PyErr_Clear();
                fast_mode = 0;
            }
        }
    } else {
        cnt = 0;
        long_cnt = _PyLong_GetZero();
    }
    Py_INCREF(long_cnt);

    /* If not specified, step defaults to 1 */
    if (long_step == NULL) {
        long_step = _PyLong_GetOne();
    }
    Py_INCREF(long_step);

    assert(long_cnt != NULL && long_step != NULL);

    /* Fast mode only works when the step is 1 */
    if (fast_mode) {
        assert(PyLong_Check(long_step));
        step = PyLong_AsLong(long_step);
        if (step != 1) {
            fast_mode = 0;
            if (step == -1 && PyErr_Occurred())
                PyErr_Clear();
        }
    }

    if (fast_mode)
        Py_CLEAR(long_cnt);
    else
        cnt = PY_SSIZE_T_MAX;

    assert((long_cnt == NULL && fast_mode) ||
           (cnt == PY_SSIZE_T_MAX && long_cnt != NULL && !fast_mode));
    assert(!fast_mode ||
           (PyLong_Check(long_step) && PyLong_AS_LONG(long_step) == 1));

    /* create countobject structure */
    lz = (countobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_XDECREF(long_cnt);
        Py_DECREF(long_step);
        return NULL;
    }
    lz->cnt = cnt;
    lz->long_cnt = long_cnt;
    lz->long_step = long_step;

    return (PyObject *)lz;
}

static void
count_dealloc(PyObject *op)
{
    countobject *lz = countobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->long_cnt);
    Py_XDECREF(lz->long_step);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
count_traverse(PyObject *op, visitproc visit, void *arg)
{
    countobject *lz = countobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->long_cnt);
    Py_VISIT(lz->long_step);
    return 0;
}

static PyObject *
count_nextlong(countobject *lz)
{
    PyObject *long_cnt;
    PyObject *stepped_up;

    long_cnt = lz->long_cnt;
    if (long_cnt == NULL) {
        /* Switch to slow_mode */
        long_cnt = PyLong_FromSsize_t(PY_SSIZE_T_MAX);
        if (long_cnt == NULL)
            return NULL;
    }
    assert(lz->cnt == PY_SSIZE_T_MAX && long_cnt != NULL);

    stepped_up = PyNumber_Add(long_cnt, lz->long_step);
    if (stepped_up == NULL)
        return NULL;
    lz->long_cnt = stepped_up;
    return long_cnt;
}

static PyObject *
count_next(PyObject *op)
{
    countobject *lz = countobject_CAST(op);
#ifndef Py_GIL_DISABLED
    if (lz->cnt == PY_SSIZE_T_MAX)
        return count_nextlong(lz);
    return PyLong_FromSsize_t(lz->cnt++);
#else
    // free-threading version
    // fast mode uses compare-exchange loop
    // slow mode uses a critical section
    PyObject *returned;
    Py_ssize_t cnt;

    cnt = _Py_atomic_load_ssize_relaxed(&lz->cnt);
    for (;;) {
        if (cnt == PY_SSIZE_T_MAX) {
            Py_BEGIN_CRITICAL_SECTION(lz);
            returned = count_nextlong(lz);
            Py_END_CRITICAL_SECTION();
            return returned;
        }
        if (_Py_atomic_compare_exchange_ssize(&lz->cnt, &cnt, cnt + 1)) {
            return PyLong_FromSsize_t(cnt);
        }
    }
#endif
}

static PyObject *
count_repr(PyObject *op)
{
    countobject *lz = countobject_CAST(op);
    if (lz->long_cnt == NULL) {
        Py_ssize_t cnt = FT_ATOMIC_LOAD_SSIZE_RELAXED(lz->cnt);
        return PyUnicode_FromFormat("%s(%zd)",
                                    _PyType_Name(Py_TYPE(lz)), cnt);
    }

    if (PyLong_Check(lz->long_step)) {
        long step = PyLong_AsLong(lz->long_step);
        if (step == -1 && PyErr_Occurred()) {
            PyErr_Clear();
        }
        if (step == 1) {
            /* Don't display step when it is an integer equal to 1 */
            return PyUnicode_FromFormat("%s(%R)",
                                        _PyType_Name(Py_TYPE(lz)),
                                        lz->long_cnt);
        }
    }
    return PyUnicode_FromFormat("%s(%R, %R)",
                                _PyType_Name(Py_TYPE(lz)),
                                lz->long_cnt, lz->long_step);
}

static PyType_Slot count_slots[] = {
    {Py_tp_dealloc, count_dealloc},
    {Py_tp_repr, count_repr},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)itertools_count__doc__},
    {Py_tp_traverse, count_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, count_next},
    {Py_tp_new, itertools_count},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec count_spec = {
    .name = "itertools.count",
    .basicsize = sizeof(countobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = count_slots,
};


/* repeat object ************************************************************/

typedef struct {
    PyObject_HEAD
    PyObject *element;
    Py_ssize_t cnt;
} repeatobject;

#define repeatobject_CAST(op)   ((repeatobject *)(op))

static PyObject *
repeat_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    repeatobject *ro;
    PyObject *element;
    Py_ssize_t cnt = -1, n_args;
    static char *kwargs[] = {"object", "times", NULL};

    n_args = PyTuple_GET_SIZE(args);
    if (kwds != NULL)
        n_args += PyDict_GET_SIZE(kwds);
    if (!PyArg_ParseTupleAndKeywords(args, kwds, "O|n:repeat", kwargs,
                                     &element, &cnt))
        return NULL;
    /* Does user supply times argument? */
    if (n_args == 2 && cnt < 0)
        cnt = 0;

    ro = (repeatobject *)type->tp_alloc(type, 0);
    if (ro == NULL)
        return NULL;
    ro->element = Py_NewRef(element);
    ro->cnt = cnt;
    return (PyObject *)ro;
}

static void
repeat_dealloc(PyObject *op)
{
    repeatobject *ro = repeatobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(ro);
    PyObject_GC_UnTrack(ro);
    Py_XDECREF(ro->element);
    tp->tp_free(ro);
    Py_DECREF(tp);
}

static int
repeat_traverse(PyObject *op, visitproc visit, void *arg)
{
    repeatobject *ro = repeatobject_CAST(op);
    Py_VISIT(Py_TYPE(ro));
    Py_VISIT(ro->element);
    return 0;
}

static PyObject *
repeat_next(PyObject *op)
{
    repeatobject *ro = repeatobject_CAST(op);
    Py_ssize_t cnt = FT_ATOMIC_LOAD_SSIZE_RELAXED(ro->cnt);
    if (cnt == 0) {
        return NULL;
    }
    if (cnt > 0) {
        cnt--;
        FT_ATOMIC_STORE_SSIZE_RELAXED(ro->cnt, cnt);
    }
    return Py_NewRef(ro->element);
}

static PyObject *
repeat_repr(PyObject *op)
{
    repeatobject *ro = repeatobject_CAST(op);
    if (ro->cnt == -1)
        return PyUnicode_FromFormat("%s(%R)",
                                    _PyType_Name(Py_TYPE(ro)), ro->element);
    else
        return PyUnicode_FromFormat("%s(%R, %zd)",
                                    _PyType_Name(Py_TYPE(ro)), ro->element,
                                    ro->cnt);
}

static PyObject *
repeat_len(PyObject *op, PyObject *Py_UNUSED(args))
{
    repeatobject *ro = repeatobject_CAST(op);
    if (ro->cnt == -1) {
        PyErr_SetString(PyExc_TypeError, "len() of unsized object");
        return NULL;
    }
    return PyLong_FromSize_t(ro->cnt);
}

PyDoc_STRVAR(length_hint_doc, "Private method returning an estimate of len(list(it)).");

static PyMethodDef repeat_methods[] = {
    {"__length_hint__", repeat_len, METH_NOARGS, length_hint_doc},
    {NULL,              NULL}           /* sentinel */
};

PyDoc_STRVAR(repeat_doc,
"repeat(object [,times]) -> create an iterator which returns the object\n\
for the specified number of times.  If not specified, returns the object\n\
endlessly.");

static PyType_Slot repeat_slots[] = {
    {Py_tp_dealloc, repeat_dealloc},
    {Py_tp_repr, repeat_repr},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)repeat_doc},
    {Py_tp_traverse, repeat_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, repeat_next},
    {Py_tp_methods, repeat_methods},
    {Py_tp_new, repeat_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec repeat_spec = {
    .name = "itertools.repeat",
    .basicsize = sizeof(repeatobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = repeat_slots,
};


/* ziplongest object *********************************************************/

typedef struct {
    PyObject_HEAD
    Py_ssize_t tuplesize;
    Py_ssize_t numactive;
    PyObject *ittuple;                  /* tuple of iterators */
    PyObject *result;
    PyObject *fillvalue;
} ziplongestobject;

#define ziplongestobject_CAST(op)   ((ziplongestobject *)(op))

static PyObject *
zip_longest_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    ziplongestobject *lz;
    Py_ssize_t i;
    PyObject *ittuple;  /* tuple of iterators */
    PyObject *result;
    PyObject *fillvalue = Py_None;
    Py_ssize_t tuplesize;

    if (kwds != NULL && PyDict_CheckExact(kwds) && PyDict_GET_SIZE(kwds) > 0) {
        fillvalue = NULL;
        if (PyDict_GET_SIZE(kwds) == 1) {
            fillvalue = PyDict_GetItemWithError(kwds, &_Py_ID(fillvalue));
        }
        if (fillvalue == NULL) {
            if (!PyErr_Occurred()) {
                PyErr_SetString(PyExc_TypeError,
                    "zip_longest() got an unexpected keyword argument");
            }
            return NULL;
        }
    }

    /* args must be a tuple */
    assert(PyTuple_Check(args));
    tuplesize = PyTuple_GET_SIZE(args);

    /* obtain iterators */
    ittuple = PyTuple_New(tuplesize);
    if (ittuple == NULL)
        return NULL;
    for (i=0; i < tuplesize; i++) {
        PyObject *item = PyTuple_GET_ITEM(args, i);
        PyObject *it = PyObject_GetIter(item);
        if (it == NULL) {
            Py_DECREF(ittuple);
            return NULL;
        }
        PyTuple_SET_ITEM(ittuple, i, it);
    }

    /* create a result holder */
    result = PyTuple_New(tuplesize);
    if (result == NULL) {
        Py_DECREF(ittuple);
        return NULL;
    }
    for (i=0 ; i < tuplesize ; i++) {
        Py_INCREF(Py_None);
        PyTuple_SET_ITEM(result, i, Py_None);
    }

    /* create ziplongestobject structure */
    lz = (ziplongestobject *)type->tp_alloc(type, 0);
    if (lz == NULL) {
        Py_DECREF(ittuple);
        Py_DECREF(result);
        return NULL;
    }
    lz->ittuple = ittuple;
    lz->tuplesize = tuplesize;
    lz->numactive = tuplesize;
    lz->result = result;
    lz->fillvalue = Py_NewRef(fillvalue);
    return (PyObject *)lz;
}

static void
zip_longest_dealloc(PyObject *op)
{
    ziplongestobject *lz = ziplongestobject_CAST(op);
    PyTypeObject *tp = Py_TYPE(lz);
    PyObject_GC_UnTrack(lz);
    Py_XDECREF(lz->ittuple);
    Py_XDECREF(lz->result);
    Py_XDECREF(lz->fillvalue);
    tp->tp_free(lz);
    Py_DECREF(tp);
}

static int
zip_longest_traverse(PyObject *op, visitproc visit, void *arg)
{
    ziplongestobject *lz = ziplongestobject_CAST(op);
    Py_VISIT(Py_TYPE(lz));
    Py_VISIT(lz->ittuple);
    Py_VISIT(lz->result);
    Py_VISIT(lz->fillvalue);
    return 0;
}

static PyObject *
zip_longest_next_lock_held(PyObject *op)
{
    ziplongestobject *lz = ziplongestobject_CAST(op);
    Py_ssize_t i;
    Py_ssize_t tuplesize = lz->tuplesize;
    PyObject *result = lz->result;
    PyObject *it;
    PyObject *item;
    PyObject *olditem;

    if (tuplesize == 0)
        return NULL;
    if (lz->numactive == 0)
        return NULL;
    if (_PyObject_IsUniquelyReferenced(result)) {
        Py_INCREF(result);
        for (i=0 ; i < tuplesize ; i++) {
            it = PyTuple_GET_ITEM(lz->ittuple, i);
            if (it == NULL) {
                item = Py_NewRef(lz->fillvalue);
            } else {
                item = PyIter_Next(it);
                if (item == NULL) {
                    lz->numactive -= 1;
                    if (lz->numactive == 0 || PyErr_Occurred()) {
                        lz->numactive = 0;
                        Py_DECREF(result);
                        return NULL;
                    } else {
                        item = Py_NewRef(lz->fillvalue);
                        PyTuple_SET_ITEM(lz->ittuple, i, NULL);
                        Py_DECREF(it);
                    }
                }
            }
            olditem = PyTuple_GET_ITEM(result, i);
            PyTuple_SET_ITEM(result, i, item);
            Py_DECREF(olditem);
        }
        // bpo-42536: The GC may have untracked this result tuple. Since we're
        // recycling it, make sure it's tracked again:
        _PyTuple_Recycle(result);
    } else {
        result = PyTuple_New(tuplesize);
        if (result == NULL)
            return NULL;
        for (i=0 ; i < tuplesize ; i++) {
            it = PyTuple_GET_ITEM(lz->ittuple, i);
            if (it == NULL) {
                item = Py_NewRef(lz->fillvalue);
            } else {
                item = PyIter_Next(it);
                if (item == NULL) {
                    lz->numactive -= 1;
                    if (lz->numactive == 0 || PyErr_Occurred()) {
                        lz->numactive = 0;
                        Py_DECREF(result);
                        return NULL;
                    } else {
                        item = Py_NewRef(lz->fillvalue);
                        PyTuple_SET_ITEM(lz->ittuple, i, NULL);
                        Py_DECREF(it);
                    }
                }
            }
            PyTuple_SET_ITEM(result, i, item);
        }
    }
    return result;
}

static PyObject *
zip_longest_next(PyObject *op)
{
    PyObject *result;
    Py_BEGIN_CRITICAL_SECTION(op);
    result = zip_longest_next_lock_held(op);
    Py_END_CRITICAL_SECTION()
    return result;
}

PyDoc_STRVAR(zip_longest_doc,
"zip_longest(*iterables, fillvalue=None)\n\
--\n\
\n\
Return a zip_longest object whose .__next__() method returns a tuple where\n\
the i-th element comes from the i-th iterable argument.  The .__next__()\n\
method continues until the longest iterable in the argument sequence\n\
is exhausted and then it raises StopIteration.  When the shorter iterables\n\
are exhausted, the fillvalue is substituted in their place.  The fillvalue\n\
defaults to None or can be specified by a keyword argument.\n\
");

static PyType_Slot ziplongest_slots[] = {
    {Py_tp_dealloc, zip_longest_dealloc},
    {Py_tp_getattro, PyObject_GenericGetAttr},
    {Py_tp_doc, (void *)zip_longest_doc},
    {Py_tp_traverse, zip_longest_traverse},
    {Py_tp_iter, PyObject_SelfIter},
    {Py_tp_iternext, zip_longest_next},
    {Py_tp_new, zip_longest_new},
    {Py_tp_free, PyObject_GC_Del},
    {0, NULL},
};

static PyType_Spec ziplongest_spec = {
    .name = "itertools.zip_longest",
    .basicsize = sizeof(ziplongestobject),
    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE |
              Py_TPFLAGS_IMMUTABLETYPE),
    .slots = ziplongest_slots,
};


/* module level code ********************************************************/

PyDoc_STRVAR(module_doc,
"Functional tools for creating and using iterators.\n\
\n\
Infinite iterators:\n\
count(start=0, step=1) --> start, start+step, start+2*step, ...\n\
cycle(p) --> p0, p1, ... plast, p0, p1, ...\n\
repeat(elem [,n]) --> elem, elem, elem, ... endlessly or up to n times\n\
\n\
Iterators terminating on the shortest input sequence:\n\
accumulate(p[, func]) --> p0, p0+p1, p0+p1+p2\n\
batched(p, n) --> [p0, p1, ..., p_n-1], [p_n, p_n+1, ..., p_2n-1], ...\n\
chain(p, q, ...) --> p0, p1, ... plast, q0, q1, ...\n\
chain.from_iterable([p, q, ...]) --> p0, p1, ... plast, q0, q1, ...\n\
compress(data, selectors) --> (d[0] if s[0]), (d[1] if s[1]), ...\n\
dropwhile(predicate, seq) --> seq[n], seq[n+1], starting when predicate fails\n\
groupby(iterable[, keyfunc]) --> sub-iterators grouped by value of keyfunc(v)\n\
filterfalse(predicate, seq) --> elements of seq where predicate(elem) is False\n\
islice(seq, [start,] stop [, step]) --> elements from\n\
       seq[start:stop:step]\n\
pairwise(s) --> (s[0],s[1]), (s[1],s[2]), (s[2], s[3]), ...\n\
starmap(fun, seq) --> fun(*seq[0]), fun(*seq[1]), ...\n\
tee(it, n=2) --> (it1, it2 , ... itn) splits one iterator into n\n\
takewhile(predicate, seq) --> seq[0], seq[1], until predicate fails\n\
zip_longest(p, q, ...) --> (p[0], q[0]), (p[1], q[1]), ...\n\
\n\
Combinatoric generators:\n\
product(p, q, ... [repeat=1]) --> cartesian product\n\
permutations(p[, r])\n\
combinations(p, r)\n\
combinations_with_replacement(p, r)\n\
");

static int
itertoolsmodule_traverse(PyObject *mod, visitproc visit, void *arg)
{
    itertools_state *state = get_module_state(mod);
    Py_VISIT(state->accumulate_type);
    Py_VISIT(state->batched_type);
    Py_VISIT(state->chain_type);
    Py_VISIT(state->combinations_type);
    Py_VISIT(state->compress_type);
    Py_VISIT(state->count_type);
    Py_VISIT(state->cwr_type);
    Py_VISIT(state->cycle_type);
    Py_VISIT(state->dropwhile_type);
    Py_VISIT(state->filterfalse_type);
    Py_VISIT(state->groupby_type);
    Py_VISIT(state->_grouper_type);
    Py_VISIT(state->islice_type);
    Py_VISIT(state->pairwise_type);
    Py_VISIT(state->permutations_type);
    Py_VISIT(state->product_type);
    Py_VISIT(state->repeat_type);
    Py_VISIT(state->starmap_type);
    Py_VISIT(state->takewhile_type);
    Py_VISIT(state->tee_type);
    Py_VISIT(state->teedataobject_type);
    Py_VISIT(state->ziplongest_type);
    return 0;
}

static int
itertoolsmodule_clear(PyObject *mod)
{
    itertools_state *state = get_module_state(mod);
    Py_CLEAR(state->accumulate_type);
    Py_CLEAR(state->batched_type);
    Py_CLEAR(state->chain_type);
    Py_CLEAR(state->combinations_type);
    Py_CLEAR(state->compress_type);
    Py_CLEAR(state->count_type);
    Py_CLEAR(state->cwr_type);
    Py_CLEAR(state->cycle_type);
    Py_CLEAR(state->dropwhile_type);
    Py_CLEAR(state->filterfalse_type);
    Py_CLEAR(state->groupby_type);
    Py_CLEAR(state->_grouper_type);
    Py_CLEAR(state->islice_type);
    Py_CLEAR(state->pairwise_type);
    Py_CLEAR(state->permutations_type);
    Py_CLEAR(state->product_type);
    Py_CLEAR(state->repeat_type);
    Py_CLEAR(state->starmap_type);
    Py_CLEAR(state->takewhile_type);
    Py_CLEAR(state->tee_type);
    Py_CLEAR(state->teedataobject_type);
    Py_CLEAR(state->ziplongest_type);
    return 0;
}

static void
itertoolsmodule_free(void *mod)
{
    (void)itertoolsmodule_clear((PyObject *)mod);
}

#define ADD_TYPE(module, type, spec)                                     \
do {                                                                     \
    type = (PyTypeObject *)PyType_FromModuleAndSpec(module, spec, NULL); \
    if (type == NULL) {                                                  \
        return -1;                                                       \
    }                                                                    \
    if (PyModule_AddType(module, type) < 0) {                            \
        return -1;                                                       \
    }                                                                    \
} while (0)

static int
itertoolsmodule_exec(PyObject *mod)
{
    itertools_state *state = get_module_state(mod);
    ADD_TYPE(mod, state->accumulate_type, &accumulate_spec);
    ADD_TYPE(mod, state->batched_type, &batched_spec);
    ADD_TYPE(mod, state->chain_type, &chain_spec);
    ADD_TYPE(mod, state->combinations_type, &combinations_spec);
    ADD_TYPE(mod, state->compress_type, &compress_spec);
    ADD_TYPE(mod, state->count_type, &count_spec);
    ADD_TYPE(mod, state->cwr_type, &cwr_spec);
    ADD_TYPE(mod, state->cycle_type, &cycle_spec);
    ADD_TYPE(mod, state->dropwhile_type, &dropwhile_spec);
    ADD_TYPE(mod, state->filterfalse_type, &filterfalse_spec);
    ADD_TYPE(mod, state->groupby_type, &groupby_spec);
    ADD_TYPE(mod, state->_grouper_type, &_grouper_spec);
    ADD_TYPE(mod, state->islice_type, &islice_spec);
    ADD_TYPE(mod, state->pairwise_type, &pairwise_spec);
    ADD_TYPE(mod, state->permutations_type, &permutations_spec);
    ADD_TYPE(mod, state->product_type, &product_spec);
    ADD_TYPE(mod, state->repeat_type, &repeat_spec);
    ADD_TYPE(mod, state->starmap_type, &starmap_spec);
    ADD_TYPE(mod, state->takewhile_type, &takewhile_spec);
    ADD_TYPE(mod, state->tee_type, &tee_spec);
    ADD_TYPE(mod, state->teedataobject_type, &teedataobject_spec);
    ADD_TYPE(mod, state->ziplongest_type, &ziplongest_spec);

    Py_SET_TYPE(state->teedataobject_type, &PyType_Type);
    return 0;
}

static struct PyModuleDef_Slot itertoolsmodule_slots[] = {
    {Py_mod_exec, itertoolsmodule_exec},
    {Py_mod_multiple_interpreters, Py_MOD_PER_INTERPRETER_GIL_SUPPORTED},
    {Py_mod_gil, Py_MOD_GIL_NOT_USED},
    {0, NULL}
};

static PyMethodDef module_methods[] = {
    ITERTOOLS_TEE_METHODDEF
    {NULL, NULL} /* sentinel */
};


static struct PyModuleDef itertoolsmodule = {
    .m_base = PyModuleDef_HEAD_INIT,
    .m_name = "itertools",
    .m_doc = module_doc,
    .m_size = sizeof(itertools_state),
    .m_methods = module_methods,
    .m_slots = itertoolsmodule_slots,
    .m_traverse = itertoolsmodule_traverse,
    .m_clear = itertoolsmodule_clear,
    .m_free = itertoolsmodule_free,
};

PyMODINIT_FUNC
PyInit_itertools(void)
{
    return PyModuleDef_Init(&itertoolsmodule);
}