cpython/Modules/_collectionsmodule.c

#include "Python.h"
#include "structmember.h"

/* collections module implementation of a deque() datatype
   Written and maintained by Raymond D. Hettinger <python@rcn.com>
   Copyright (c) 2004 Python Software Foundation.
   All rights reserved.
*/

/* The block length may be set to any number over 1.  Larger numbers
 * reduce the number of calls to the memory allocator but take more
 * memory.  Ideally, BLOCKLEN should be set with an eye to the
 * length of a cache line.
 */

#define BLOCKLEN 62
#define CENTER ((BLOCKLEN - 1) / 2)

/* A `dequeobject` is composed of a doubly-linked list of `block` nodes.
 * This list is not circular (the leftmost block has leftlink==NULL,
 * and the rightmost block has rightlink==NULL).  A deque d's first
 * element is at d.leftblock[leftindex] and its last element is at
 * d.rightblock[rightindex]; note that, unlike as for Python slice
 * indices, these indices are inclusive on both ends.  By being inclusive
 * on both ends, algorithms for left and right operations become
 * symmetrical which simplifies the design.
 *
 * The list of blocks is never empty, so d.leftblock and d.rightblock
 * are never equal to NULL.
 *
 * The indices, d.leftindex and d.rightindex are always in the range
 *     0 <= index < BLOCKLEN.
 * Their exact relationship is:
 *     (d.leftindex + d.len - 1) % BLOCKLEN == d.rightindex.
 *
 * Empty deques have d.len == 0; d.leftblock==d.rightblock;
 * d.leftindex == CENTER+1; and d.rightindex == CENTER.
 * Checking for d.len == 0 is the intended way to see whether d is empty.
 *
 * Whenever d.leftblock == d.rightblock,
 *     d.leftindex + d.len - 1 == d.rightindex.
 *
 * However, when d.leftblock != d.rightblock, d.leftindex and d.rightindex
 * become indices into distinct blocks and either may be larger than the
 * other.
 */

typedef struct BLOCK {
    struct BLOCK *leftlink;
    struct BLOCK *rightlink;
    PyObject *data[BLOCKLEN];
} block;

#define MAXFREEBLOCKS 10
static Py_ssize_t numfreeblocks = 0;
static block *freeblocks[MAXFREEBLOCKS];

static block *
newblock(block *leftlink, block *rightlink, Py_ssize_t len) {
    block *b;
    /* To prevent len from overflowing PY_SSIZE_T_MAX on 64-bit machines, we
     * refuse to allocate new blocks if the current len is dangerously
     * close.  There is some extra margin to prevent spurious arithmetic
     * overflows at various places.  The following check ensures that
     * the blocks allocated to the deque, in the worst case, can only
     * have PY_SSIZE_T_MAX-2 entries in total.
     */
    if (len >= PY_SSIZE_T_MAX - 2*BLOCKLEN) {
        PyErr_SetString(PyExc_OverflowError,
                        "cannot add more blocks to the deque");
        return NULL;
    }
    if (numfreeblocks) {
        numfreeblocks -= 1;
        b = freeblocks[numfreeblocks];
    } else {
        b = PyMem_Malloc(sizeof(block));
        if (b == NULL) {
            PyErr_NoMemory();
            return NULL;
        }
    }
    b->leftlink = leftlink;
    b->rightlink = rightlink;
    return b;
}

static void
freeblock(block *b)
{
    if (numfreeblocks < MAXFREEBLOCKS) {
        freeblocks[numfreeblocks] = b;
        numfreeblocks++;
    } else {
        PyMem_Free(b);
    }
}

typedef struct {
    PyObject_HEAD
    block *leftblock;
    block *rightblock;
    Py_ssize_t leftindex;       /* in range(BLOCKLEN) */
    Py_ssize_t rightindex;      /* in range(BLOCKLEN) */
    Py_ssize_t len;
    Py_ssize_t maxlen;
    long state;         /* incremented whenever the indices move */
    PyObject *weakreflist; /* List of weak references */
} dequeobject;

/* The deque's size limit is d.maxlen.  The limit can be zero or positive.
 * If there is no limit, then d.maxlen == -1.
 *
 * After an item is added to a deque, we check to see if the size has grown past
 * the limit. If it has, we get the size back down to the limit by popping an
 * item off of the opposite end.  The methods that can trigger this are append(),
 * appendleft(), extend(), and extendleft().
 */

#define TRIM(d, popfunction)                                    \
    if (d->maxlen != -1 && d->len > d->maxlen) {                \
        PyObject *rv = popfunction(d, NULL);                \
        assert(rv != NULL  &&  d->len <= d->maxlen);        \
        Py_DECREF(rv);                                      \
    }

static PyTypeObject deque_type;

static PyObject *
deque_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    dequeobject *deque;
    block *b;

    /* create dequeobject structure */
    deque = (dequeobject *)type->tp_alloc(type, 0);
    if (deque == NULL)
        return NULL;

    b = newblock(NULL, NULL, 0);
    if (b == NULL) {
        Py_DECREF(deque);
        return NULL;
    }

    assert(BLOCKLEN >= 2);
    deque->leftblock = b;
    deque->rightblock = b;
    deque->leftindex = CENTER + 1;
    deque->rightindex = CENTER;
    deque->len = 0;
    deque->state = 0;
    deque->weakreflist = NULL;
    deque->maxlen = -1;

    return (PyObject *)deque;
}

static PyObject *
deque_pop(dequeobject *deque, PyObject *unused)
{
    PyObject *item;
    block *prevblock;

    if (deque->len == 0) {
        PyErr_SetString(PyExc_IndexError, "pop from an empty deque");
        return NULL;
    }
    item = deque->rightblock->data[deque->rightindex];
    deque->rightindex--;
    deque->len--;
    deque->state++;

    if (deque->rightindex == -1) {
        if (deque->len == 0) {
            assert(deque->leftblock == deque->rightblock);
            assert(deque->leftindex == deque->rightindex+1);
            /* re-center instead of freeing a block */
            deque->leftindex = CENTER + 1;
            deque->rightindex = CENTER;
        } else {
            prevblock = deque->rightblock->leftlink;
            assert(deque->leftblock != deque->rightblock);
            freeblock(deque->rightblock);
            prevblock->rightlink = NULL;
            deque->rightblock = prevblock;
            deque->rightindex = BLOCKLEN - 1;
        }
    }
    return item;
}

PyDoc_STRVAR(pop_doc, "Remove and return the rightmost element.");

static PyObject *
deque_popleft(dequeobject *deque, PyObject *unused)
{
    PyObject *item;
    block *prevblock;

    if (deque->len == 0) {
        PyErr_SetString(PyExc_IndexError, "pop from an empty deque");
        return NULL;
    }
    assert(deque->leftblock != NULL);
    item = deque->leftblock->data[deque->leftindex];
    deque->leftindex++;
    deque->len--;
    deque->state++;

    if (deque->leftindex == BLOCKLEN) {
        if (deque->len == 0) {
            assert(deque->leftblock == deque->rightblock);
            assert(deque->leftindex == deque->rightindex+1);
            /* re-center instead of freeing a block */
            deque->leftindex = CENTER + 1;
            deque->rightindex = CENTER;
        } else {
            assert(deque->leftblock != deque->rightblock);
            prevblock = deque->leftblock->rightlink;
            freeblock(deque->leftblock);
            assert(prevblock != NULL);
            prevblock->leftlink = NULL;
            deque->leftblock = prevblock;
            deque->leftindex = 0;
        }
    }
    return item;
}

PyDoc_STRVAR(popleft_doc, "Remove and return the leftmost element.");

static PyObject *
deque_append(dequeobject *deque, PyObject *item)
{
    deque->state++;
    if (deque->rightindex == BLOCKLEN-1) {
        block *b = newblock(deque->rightblock, NULL, deque->len);
        if (b == NULL)
            return NULL;
        assert(deque->rightblock->rightlink == NULL);
        deque->rightblock->rightlink = b;
        deque->rightblock = b;
        deque->rightindex = -1;
    }
    Py_INCREF(item);
    deque->len++;
    deque->rightindex++;
    deque->rightblock->data[deque->rightindex] = item;
    TRIM(deque, deque_popleft);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(append_doc, "Add an element to the right side of the deque.");

static PyObject *
deque_appendleft(dequeobject *deque, PyObject *item)
{
    deque->state++;
    if (deque->leftindex == 0) {
        block *b = newblock(NULL, deque->leftblock, deque->len);
        if (b == NULL)
            return NULL;
        assert(deque->leftblock->leftlink == NULL);
        deque->leftblock->leftlink = b;
        deque->leftblock = b;
        deque->leftindex = BLOCKLEN;
    }
    Py_INCREF(item);
    deque->len++;
    deque->leftindex--;
    deque->leftblock->data[deque->leftindex] = item;
    TRIM(deque, deque_pop);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(appendleft_doc, "Add an element to the left side of the deque.");


/* Run an iterator to exhaustion.  Shortcut for
   the extend/extendleft methods when maxlen == 0. */
static PyObject*
consume_iterator(PyObject *it)
{
    PyObject *item;

    while ((item = PyIter_Next(it)) != NULL) {
        Py_DECREF(item);
    }
    Py_DECREF(it);
    if (PyErr_Occurred())
        return NULL;
    Py_RETURN_NONE;
}

static PyObject *
deque_extend(dequeobject *deque, PyObject *iterable)
{
    PyObject *it, *item;

    /* Handle case where id(deque) == id(iterable) */
    if ((PyObject *)deque == iterable) {
        PyObject *result;
        PyObject *s = PySequence_List(iterable);
        if (s == NULL)
            return NULL;
        result = deque_extend(deque, s);
        Py_DECREF(s);
        return result;
    }

    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;

    if (deque->maxlen == 0)
        return consume_iterator(it);

    while ((item = PyIter_Next(it)) != NULL) {
        deque->state++;
        if (deque->rightindex == BLOCKLEN-1) {
            block *b = newblock(deque->rightblock, NULL,
                                deque->len);
            if (b == NULL) {
                Py_DECREF(item);
                Py_DECREF(it);
                return NULL;
            }
            assert(deque->rightblock->rightlink == NULL);
            deque->rightblock->rightlink = b;
            deque->rightblock = b;
            deque->rightindex = -1;
        }
        deque->len++;
        deque->rightindex++;
        deque->rightblock->data[deque->rightindex] = item;
        TRIM(deque, deque_popleft);
    }
    Py_DECREF(it);
    if (PyErr_Occurred())
        return NULL;
    Py_RETURN_NONE;
}

PyDoc_STRVAR(extend_doc,
"Extend the right side of the deque with elements from the iterable");

static PyObject *
deque_extendleft(dequeobject *deque, PyObject *iterable)
{
    PyObject *it, *item;

    /* Handle case where id(deque) == id(iterable) */
    if ((PyObject *)deque == iterable) {
        PyObject *result;
        PyObject *s = PySequence_List(iterable);
        if (s == NULL)
            return NULL;
        result = deque_extendleft(deque, s);
        Py_DECREF(s);
        return result;
    }

    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;

    if (deque->maxlen == 0)
        return consume_iterator(it);

    while ((item = PyIter_Next(it)) != NULL) {
        deque->state++;
        if (deque->leftindex == 0) {
            block *b = newblock(NULL, deque->leftblock,
                                deque->len);
            if (b == NULL) {
                Py_DECREF(item);
                Py_DECREF(it);
                return NULL;
            }
            assert(deque->leftblock->leftlink == NULL);
            deque->leftblock->leftlink = b;
            deque->leftblock = b;
            deque->leftindex = BLOCKLEN;
        }
        deque->len++;
        deque->leftindex--;
        deque->leftblock->data[deque->leftindex] = item;
        TRIM(deque, deque_pop);
    }
    Py_DECREF(it);
    if (PyErr_Occurred())
        return NULL;
    Py_RETURN_NONE;
}

PyDoc_STRVAR(extendleft_doc,
"Extend the left side of the deque with elements from the iterable");

static PyObject *
deque_inplace_concat(dequeobject *deque, PyObject *other)
{
    PyObject *result;

    result = deque_extend(deque, other);
    if (result == NULL)
        return result;
    Py_DECREF(result);
    Py_INCREF(deque);
    return (PyObject *)deque;
}

static int
_deque_rotate(dequeobject *deque, Py_ssize_t n)
{
    Py_ssize_t i, len=deque->len, halflen=(len+1)>>1;
    PyObject *item, *rv;

    if (len == 0)
        return 0;
    if (n > halflen || n < -halflen) {
        n %= len;
        if (n > halflen)
            n -= len;
        else if (n < -halflen)
            n += len;
    }

    for (i=0 ; i<n ; i++) {
        item = deque_pop(deque, NULL);
        assert (item != NULL);
        rv = deque_appendleft(deque, item);
        Py_DECREF(item);
        if (rv == NULL)
            return -1;
        Py_DECREF(rv);
    }
    for (i=0 ; i>n ; i--) {
        item = deque_popleft(deque, NULL);
        assert (item != NULL);
        rv = deque_append(deque, item);
        Py_DECREF(item);
        if (rv == NULL)
            return -1;
        Py_DECREF(rv);
    }
    return 0;
}

static PyObject *
deque_rotate(dequeobject *deque, PyObject *args)
{
    Py_ssize_t n=1;

    if (!PyArg_ParseTuple(args, "|n:rotate", &n))
        return NULL;
    if (_deque_rotate(deque, n) == 0)
        Py_RETURN_NONE;
    return NULL;
}

PyDoc_STRVAR(rotate_doc,
"Rotate the deque n steps to the right (default n=1).  If n is negative, rotates left.");

static PyObject *
deque_reverse(dequeobject *deque, PyObject *unused)
{
    block *leftblock = deque->leftblock;
    block *rightblock = deque->rightblock;
    Py_ssize_t leftindex = deque->leftindex;
    Py_ssize_t rightindex = deque->rightindex;
    Py_ssize_t n = (deque->len)/2;
    Py_ssize_t i;
    PyObject *tmp;

    for (i=0 ; i<n ; i++) {
        /* Validate that pointers haven't met in the middle */
        assert(leftblock != rightblock || leftindex < rightindex);

        /* Swap */
        tmp = leftblock->data[leftindex];
        leftblock->data[leftindex] = rightblock->data[rightindex];
        rightblock->data[rightindex] = tmp;

        /* Advance left block/index pair */
        leftindex++;
        if (leftindex == BLOCKLEN) {
            if (leftblock->rightlink == NULL)
                break;
            leftblock = leftblock->rightlink;
            leftindex = 0;
        }

        /* Step backwards with the right block/index pair */
        rightindex--;
        if (rightindex == -1) {
            if (rightblock->leftlink == NULL)
                break;
            rightblock = rightblock->leftlink;
            rightindex = BLOCKLEN - 1;
        }
    }
    Py_RETURN_NONE;
}

PyDoc_STRVAR(reverse_doc,
"D.reverse() -- reverse *IN PLACE*");

static PyObject *
deque_count(dequeobject *deque, PyObject *v)
{
    block *leftblock = deque->leftblock;
    Py_ssize_t leftindex = deque->leftindex;
    Py_ssize_t n = deque->len;
    Py_ssize_t i;
    Py_ssize_t count = 0;
    PyObject *item;
    long start_state = deque->state;
    int cmp;

    for (i=0 ; i<n ; i++) {
        item = leftblock->data[leftindex];
        cmp = PyObject_RichCompareBool(item, v, Py_EQ);
        if (cmp > 0)
            count++;
        else if (cmp < 0)
            return NULL;

        if (start_state != deque->state) {
            PyErr_SetString(PyExc_RuntimeError,
                            "deque mutated during iteration");
            return NULL;
        }

        /* Advance left block/index pair */
        leftindex++;
        if (leftindex == BLOCKLEN) {
            if (leftblock->rightlink == NULL)  /* can occur when i==n-1 */
                break;
            leftblock = leftblock->rightlink;
            leftindex = 0;
        }
    }
    return PyLong_FromSsize_t(count);
}

PyDoc_STRVAR(count_doc,
"D.count(value) -> integer -- return number of occurrences of value");

static Py_ssize_t
deque_len(dequeobject *deque)
{
    return deque->len;
}

static PyObject *
deque_remove(dequeobject *deque, PyObject *value)
{
    Py_ssize_t i, n=deque->len;

    for (i=0 ; i<n ; i++) {
        PyObject *item = deque->leftblock->data[deque->leftindex];
        int cmp = PyObject_RichCompareBool(item, value, Py_EQ);

        if (deque->len != n) {
            PyErr_SetString(PyExc_IndexError,
                "deque mutated during remove().");
            return NULL;
        }
        if (cmp > 0) {
            PyObject *tgt = deque_popleft(deque, NULL);
            assert (tgt != NULL);
            Py_DECREF(tgt);
            if (_deque_rotate(deque, i) == -1)
                return NULL;
            Py_RETURN_NONE;
        }
        else if (cmp < 0) {
            _deque_rotate(deque, i);
            return NULL;
        }
        _deque_rotate(deque, -1);
    }
    PyErr_SetString(PyExc_ValueError, "deque.remove(x): x not in deque");
    return NULL;
}

PyDoc_STRVAR(remove_doc,
"D.remove(value) -- remove first occurrence of value.");

static int
deque_clear(dequeobject *deque)
{
    PyObject *item;

    while (deque->len) {
        item = deque_pop(deque, NULL);
        assert (item != NULL);
        Py_DECREF(item);
    }
    assert(deque->leftblock == deque->rightblock &&
           deque->leftindex - 1 == deque->rightindex &&
           deque->len == 0);
    return 0;
}

static PyObject *
deque_item(dequeobject *deque, Py_ssize_t i)
{
    block *b;
    PyObject *item;
    Py_ssize_t n, index=i;

    if (i < 0 || i >= deque->len) {
        PyErr_SetString(PyExc_IndexError,
                        "deque index out of range");
        return NULL;
    }

    if (i == 0) {
        i = deque->leftindex;
        b = deque->leftblock;
    } else if (i == deque->len - 1) {
        i = deque->rightindex;
        b = deque->rightblock;
    } else {
        i += deque->leftindex;
        n = i / BLOCKLEN;
        i %= BLOCKLEN;
        if (index < (deque->len >> 1)) {
            b = deque->leftblock;
            while (n--)
                b = b->rightlink;
        } else {
            n = (deque->leftindex + deque->len - 1) / BLOCKLEN - n;
            b = deque->rightblock;
            while (n--)
                b = b->leftlink;
        }
    }
    item = b->data[i];
    Py_INCREF(item);
    return item;
}

/* delitem() implemented in terms of rotate for simplicity and reasonable
   performance near the end points.  If for some reason this method becomes
   popular, it is not hard to re-implement this using direct data movement
   (similar to code in list slice assignment) and achieve a two or threefold
   performance boost.
*/

static int
deque_del_item(dequeobject *deque, Py_ssize_t i)
{
    PyObject *item;

    assert (i >= 0 && i < deque->len);
    if (_deque_rotate(deque, -i) == -1)
        return -1;

    item = deque_popleft(deque, NULL);
    assert (item != NULL);
    Py_DECREF(item);

    return _deque_rotate(deque, i);
}

static int
deque_ass_item(dequeobject *deque, Py_ssize_t i, PyObject *v)
{
    PyObject *old_value;
    block *b;
    Py_ssize_t n, len=deque->len, halflen=(len+1)>>1, index=i;

    if (i < 0 || i >= len) {
        PyErr_SetString(PyExc_IndexError,
                        "deque index out of range");
        return -1;
    }
    if (v == NULL)
        return deque_del_item(deque, i);

    i += deque->leftindex;
    n = i / BLOCKLEN;
    i %= BLOCKLEN;
    if (index <= halflen) {
        b = deque->leftblock;
        while (n--)
            b = b->rightlink;
    } else {
        n = (deque->leftindex + len - 1) / BLOCKLEN - n;
        b = deque->rightblock;
        while (n--)
            b = b->leftlink;
    }
    Py_INCREF(v);
    old_value = b->data[i];
    b->data[i] = v;
    Py_DECREF(old_value);
    return 0;
}

static PyObject *
deque_clearmethod(dequeobject *deque)
{
    int rv;

    rv = deque_clear(deque);
    assert (rv != -1);
    Py_RETURN_NONE;
}

PyDoc_STRVAR(clear_doc, "Remove all elements from the deque.");

static void
deque_dealloc(dequeobject *deque)
{
    PyObject_GC_UnTrack(deque);
    if (deque->weakreflist != NULL)
        PyObject_ClearWeakRefs((PyObject *) deque);
    if (deque->leftblock != NULL) {
        deque_clear(deque);
        assert(deque->leftblock != NULL);
        freeblock(deque->leftblock);
    }
    deque->leftblock = NULL;
    deque->rightblock = NULL;
    Py_TYPE(deque)->tp_free(deque);
}

static int
deque_traverse(dequeobject *deque, visitproc visit, void *arg)
{
    block *b;
    PyObject *item;
    Py_ssize_t index;
    Py_ssize_t indexlo = deque->leftindex;

    for (b = deque->leftblock; b != NULL; b = b->rightlink) {
        const Py_ssize_t indexhi = b == deque->rightblock ?
                                 deque->rightindex :
                     BLOCKLEN - 1;

        for (index = indexlo; index <= indexhi; ++index) {
            item = b->data[index];
            Py_VISIT(item);
        }
        indexlo = 0;
    }
    return 0;
}

static PyObject *
deque_copy(PyObject *deque)
{
    if (((dequeobject *)deque)->maxlen == -1)
        return PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "O", deque, NULL);
    else
        return PyObject_CallFunction((PyObject *)(Py_TYPE(deque)), "Oi",
            deque, ((dequeobject *)deque)->maxlen, NULL);
}

PyDoc_STRVAR(copy_doc, "Return a shallow copy of a deque.");

static PyObject *
deque_reduce(dequeobject *deque)
{
    PyObject *dict, *result, *aslist;

    dict = PyObject_GetAttrString((PyObject *)deque, "__dict__");
    if (dict == NULL)
        PyErr_Clear();
    aslist = PySequence_List((PyObject *)deque);
    if (aslist == NULL) {
        Py_XDECREF(dict);
        return NULL;
    }
    if (dict == NULL) {
        if (deque->maxlen == -1)
            result = Py_BuildValue("O(O)", Py_TYPE(deque), aslist);
        else
            result = Py_BuildValue("O(On)", Py_TYPE(deque), aslist, deque->maxlen);
    } else {
        if (deque->maxlen == -1)
            result = Py_BuildValue("O(OO)O", Py_TYPE(deque), aslist, Py_None, dict);
        else
            result = Py_BuildValue("O(On)O", Py_TYPE(deque), aslist, deque->maxlen, dict);
    }
    Py_XDECREF(dict);
    Py_DECREF(aslist);
    return result;
}

PyDoc_STRVAR(reduce_doc, "Return state information for pickling.");

static PyObject *
deque_repr(PyObject *deque)
{
    PyObject *aslist, *result;
    int i;

    i = Py_ReprEnter(deque);
    if (i != 0) {
        if (i < 0)
            return NULL;
        return PyUnicode_FromString("[...]");
    }

    aslist = PySequence_List(deque);
    if (aslist == NULL) {
        Py_ReprLeave(deque);
        return NULL;
    }
    if (((dequeobject *)deque)->maxlen != -1)

        result = PyUnicode_FromFormat("deque(%R, maxlen=%zd)",
                                      aslist, ((dequeobject *)deque)->maxlen);
    else
        result = PyUnicode_FromFormat("deque(%R)", aslist);
    Py_DECREF(aslist);
    Py_ReprLeave(deque);
    return result;
}

static PyObject *
deque_richcompare(PyObject *v, PyObject *w, int op)
{
    PyObject *it1=NULL, *it2=NULL, *x, *y;
    Py_ssize_t vs, ws;
    int b, cmp=-1;

    if (!PyObject_TypeCheck(v, &deque_type) ||
        !PyObject_TypeCheck(w, &deque_type)) {
        Py_RETURN_NOTIMPLEMENTED;
    }

    /* Shortcuts */
    vs = ((dequeobject *)v)->len;
    ws = ((dequeobject *)w)->len;
    if (op == Py_EQ) {
        if (v == w)
            Py_RETURN_TRUE;
        if (vs != ws)
            Py_RETURN_FALSE;
    }
    if (op == Py_NE) {
        if (v == w)
            Py_RETURN_FALSE;
        if (vs != ws)
            Py_RETURN_TRUE;
    }

    /* Search for the first index where items are different */
    it1 = PyObject_GetIter(v);
    if (it1 == NULL)
        goto done;
    it2 = PyObject_GetIter(w);
    if (it2 == NULL)
        goto done;
    for (;;) {
        x = PyIter_Next(it1);
        if (x == NULL && PyErr_Occurred())
            goto done;
        y = PyIter_Next(it2);
        if (x == NULL || y == NULL)
            break;
        b = PyObject_RichCompareBool(x, y, Py_EQ);
        if (b == 0) {
            cmp = PyObject_RichCompareBool(x, y, op);
            Py_DECREF(x);
            Py_DECREF(y);
            goto done;
        }
        Py_DECREF(x);
        Py_DECREF(y);
        if (b == -1)
            goto done;
    }
    /* We reached the end of one deque or both */
    Py_XDECREF(x);
    Py_XDECREF(y);
    if (PyErr_Occurred())
        goto done;
    switch (op) {
    case Py_LT: cmp = y != NULL; break;  /* if w was longer */
    case Py_LE: cmp = x == NULL; break;  /* if v was not longer */
    case Py_EQ: cmp = x == y;    break;  /* if we reached the end of both */
    case Py_NE: cmp = x != y;    break;  /* if one deque continues */
    case Py_GT: cmp = x != NULL; break;  /* if v was longer */
    case Py_GE: cmp = y == NULL; break;  /* if w was not longer */
    }

done:
    Py_XDECREF(it1);
    Py_XDECREF(it2);
    if (cmp == 1)
        Py_RETURN_TRUE;
    if (cmp == 0)
        Py_RETURN_FALSE;
    return NULL;
}

static int
deque_init(dequeobject *deque, PyObject *args, PyObject *kwdargs)
{
    PyObject *iterable = NULL;
    PyObject *maxlenobj = NULL;
    Py_ssize_t maxlen = -1;
    char *kwlist[] = {"iterable", "maxlen", 0};

    if (!PyArg_ParseTupleAndKeywords(args, kwdargs, "|OO:deque", kwlist, &iterable, &maxlenobj))
        return -1;
    if (maxlenobj != NULL && maxlenobj != Py_None) {
        maxlen = PyLong_AsSsize_t(maxlenobj);
        if (maxlen == -1 && PyErr_Occurred())
            return -1;
        if (maxlen < 0) {
            PyErr_SetString(PyExc_ValueError, "maxlen must be non-negative");
            return -1;
        }
    }
    deque->maxlen = maxlen;
    deque_clear(deque);
    if (iterable != NULL) {
        PyObject *rv = deque_extend(deque, iterable);
        if (rv == NULL)
            return -1;
        Py_DECREF(rv);
    }
    return 0;
}

static PyObject *
deque_get_maxlen(dequeobject *deque)
{
    if (deque->maxlen == -1)
        Py_RETURN_NONE;
    return PyLong_FromSsize_t(deque->maxlen);
}

static PyGetSetDef deque_getset[] = {
    {"maxlen", (getter)deque_get_maxlen, (setter)NULL,
     "maximum size of a deque or None if unbounded"},
    {0}
};

static PySequenceMethods deque_as_sequence = {
    (lenfunc)deque_len,                 /* sq_length */
    0,                                  /* sq_concat */
    0,                                  /* sq_repeat */
    (ssizeargfunc)deque_item,           /* sq_item */
    0,                                  /* sq_slice */
    (ssizeobjargproc)deque_ass_item,            /* sq_ass_item */
    0,                                  /* sq_ass_slice */
    0,                                  /* sq_contains */
    (binaryfunc)deque_inplace_concat,           /* sq_inplace_concat */
    0,                                  /* sq_inplace_repeat */

};

/* deque object ********************************************************/

static PyObject *deque_iter(dequeobject *deque);
static PyObject *deque_reviter(dequeobject *deque);
PyDoc_STRVAR(reversed_doc,
    "D.__reversed__() -- return a reverse iterator over the deque");

static PyMethodDef deque_methods[] = {
    {"append",                  (PyCFunction)deque_append,
        METH_O,                  append_doc},
    {"appendleft",              (PyCFunction)deque_appendleft,
        METH_O,                  appendleft_doc},
    {"clear",                   (PyCFunction)deque_clearmethod,
        METH_NOARGS,             clear_doc},
    {"__copy__",                (PyCFunction)deque_copy,
        METH_NOARGS,             copy_doc},
    {"count",                   (PyCFunction)deque_count,
        METH_O,                         count_doc},
    {"extend",                  (PyCFunction)deque_extend,
        METH_O,                  extend_doc},
    {"extendleft",              (PyCFunction)deque_extendleft,
        METH_O,                  extendleft_doc},
    {"pop",                     (PyCFunction)deque_pop,
        METH_NOARGS,             pop_doc},
    {"popleft",                 (PyCFunction)deque_popleft,
        METH_NOARGS,             popleft_doc},
    {"__reduce__",      (PyCFunction)deque_reduce,
        METH_NOARGS,             reduce_doc},
    {"remove",                  (PyCFunction)deque_remove,
        METH_O,                  remove_doc},
    {"__reversed__",            (PyCFunction)deque_reviter,
        METH_NOARGS,             reversed_doc},
    {"reverse",                 (PyCFunction)deque_reverse,
        METH_NOARGS,             reverse_doc},
    {"rotate",                  (PyCFunction)deque_rotate,
        METH_VARARGS,           rotate_doc},
    {NULL,              NULL}   /* sentinel */
};

PyDoc_STRVAR(deque_doc,
"deque(iterable[, maxlen]) --> deque object\n\
\n\
Build an ordered collection with optimized access from its endpoints.");

static PyTypeObject deque_type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "collections.deque",                /* tp_name */
    sizeof(dequeobject),                /* tp_basicsize */
    0,                                  /* tp_itemsize */
    /* methods */
    (destructor)deque_dealloc,          /* tp_dealloc */
    0,                                  /* tp_print */
    0,                                  /* tp_getattr */
    0,                                  /* tp_setattr */
    0,                                  /* tp_reserved */
    deque_repr,                         /* tp_repr */
    0,                                  /* tp_as_number */
    &deque_as_sequence,                 /* tp_as_sequence */
    0,                                  /* tp_as_mapping */
    PyObject_HashNotImplemented,        /* tp_hash */
    0,                                  /* tp_call */
    0,                                  /* tp_str */
    PyObject_GenericGetAttr,            /* tp_getattro */
    0,                                  /* tp_setattro */
    0,                                  /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
                                        /* tp_flags */
    deque_doc,                          /* tp_doc */
    (traverseproc)deque_traverse,       /* tp_traverse */
    (inquiry)deque_clear,               /* tp_clear */
    (richcmpfunc)deque_richcompare,     /* tp_richcompare */
    offsetof(dequeobject, weakreflist), /* tp_weaklistoffset*/
    (getiterfunc)deque_iter,            /* tp_iter */
    0,                                  /* tp_iternext */
    deque_methods,                      /* tp_methods */
    0,                                  /* tp_members */
    deque_getset,                       /* tp_getset */
    0,                                  /* tp_base */
    0,                                  /* tp_dict */
    0,                                  /* tp_descr_get */
    0,                                  /* tp_descr_set */
    0,                                  /* tp_dictoffset */
    (initproc)deque_init,               /* tp_init */
    PyType_GenericAlloc,                /* tp_alloc */
    deque_new,                          /* tp_new */
    PyObject_GC_Del,                    /* tp_free */
};

/*********************** Deque Iterator **************************/

typedef struct {
    PyObject_HEAD
    Py_ssize_t index;
    block *b;
    dequeobject *deque;
    long state;         /* state when the iterator is created */
    Py_ssize_t counter;    /* number of items remaining for iteration */
} dequeiterobject;

static PyTypeObject dequeiter_type;

static PyObject *
deque_iter(dequeobject *deque)
{
    dequeiterobject *it;

    it = PyObject_GC_New(dequeiterobject, &dequeiter_type);
    if (it == NULL)
        return NULL;
    it->b = deque->leftblock;
    it->index = deque->leftindex;
    Py_INCREF(deque);
    it->deque = deque;
    it->state = deque->state;
    it->counter = deque->len;
    PyObject_GC_Track(it);
    return (PyObject *)it;
}

static int
dequeiter_traverse(dequeiterobject *dio, visitproc visit, void *arg)
{
    Py_VISIT(dio->deque);
    return 0;
}

static void
dequeiter_dealloc(dequeiterobject *dio)
{
    Py_XDECREF(dio->deque);
    PyObject_GC_Del(dio);
}

static PyObject *
dequeiter_next(dequeiterobject *it)
{
    PyObject *item;

    if (it->deque->state != it->state) {
        it->counter = 0;
        PyErr_SetString(PyExc_RuntimeError,
                        "deque mutated during iteration");
        return NULL;
    }
    if (it->counter == 0)
        return NULL;
    assert (!(it->b == it->deque->rightblock &&
              it->index > it->deque->rightindex));

    item = it->b->data[it->index];
    it->index++;
    it->counter--;
    if (it->index == BLOCKLEN && it->counter > 0) {
        assert (it->b->rightlink != NULL);
        it->b = it->b->rightlink;
        it->index = 0;
    }
    Py_INCREF(item);
    return item;
}

static PyObject *
dequeiter_len(dequeiterobject *it)
{
    return PyLong_FromSsize_t(it->counter);
}

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

static PyMethodDef dequeiter_methods[] = {
    {"__length_hint__", (PyCFunction)dequeiter_len, METH_NOARGS, length_hint_doc},
    {NULL,              NULL}           /* sentinel */
};

static PyTypeObject dequeiter_type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "deque_iterator",                           /* tp_name */
    sizeof(dequeiterobject),                    /* tp_basicsize */
    0,                                          /* tp_itemsize */
    /* methods */
    (destructor)dequeiter_dealloc,              /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    0,                                          /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)dequeiter_traverse,           /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    PyObject_SelfIter,                          /* tp_iter */
    (iternextfunc)dequeiter_next,               /* tp_iternext */
    dequeiter_methods,                          /* tp_methods */
    0,
};

/*********************** Deque Reverse Iterator **************************/

static PyTypeObject dequereviter_type;

static PyObject *
deque_reviter(dequeobject *deque)
{
    dequeiterobject *it;

    it = PyObject_GC_New(dequeiterobject, &dequereviter_type);
    if (it == NULL)
        return NULL;
    it->b = deque->rightblock;
    it->index = deque->rightindex;
    Py_INCREF(deque);
    it->deque = deque;
    it->state = deque->state;
    it->counter = deque->len;
    PyObject_GC_Track(it);
    return (PyObject *)it;
}

static PyObject *
dequereviter_next(dequeiterobject *it)
{
    PyObject *item;
    if (it->counter == 0)
        return NULL;

    if (it->deque->state != it->state) {
        it->counter = 0;
        PyErr_SetString(PyExc_RuntimeError,
                        "deque mutated during iteration");
        return NULL;
    }
    assert (!(it->b == it->deque->leftblock &&
              it->index < it->deque->leftindex));

    item = it->b->data[it->index];
    it->index--;
    it->counter--;
    if (it->index == -1 && it->counter > 0) {
        assert (it->b->leftlink != NULL);
        it->b = it->b->leftlink;
        it->index = BLOCKLEN - 1;
    }
    Py_INCREF(item);
    return item;
}

static PyTypeObject dequereviter_type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "deque_reverse_iterator",                   /* tp_name */
    sizeof(dequeiterobject),                    /* tp_basicsize */
    0,                                          /* tp_itemsize */
    /* methods */
    (destructor)dequeiter_dealloc,              /* tp_dealloc */
    0,                                          /* tp_print */
    0,                                          /* tp_getattr */
    0,                                          /* tp_setattr */
    0,                                          /* tp_reserved */
    0,                                          /* tp_repr */
    0,                                          /* tp_as_number */
    0,                                          /* tp_as_sequence */
    0,                                          /* tp_as_mapping */
    0,                                          /* tp_hash */
    0,                                          /* tp_call */
    0,                                          /* tp_str */
    PyObject_GenericGetAttr,                    /* tp_getattro */
    0,                                          /* tp_setattro */
    0,                                          /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,/* tp_flags */
    0,                                          /* tp_doc */
    (traverseproc)dequeiter_traverse,           /* tp_traverse */
    0,                                          /* tp_clear */
    0,                                          /* tp_richcompare */
    0,                                          /* tp_weaklistoffset */
    PyObject_SelfIter,                          /* tp_iter */
    (iternextfunc)dequereviter_next,            /* tp_iternext */
    dequeiter_methods,                          /* tp_methods */
    0,
};

/* defaultdict type *********************************************************/

typedef struct {
    PyDictObject dict;
    PyObject *default_factory;
} defdictobject;

static PyTypeObject defdict_type; /* Forward */

PyDoc_STRVAR(defdict_missing_doc,
"__missing__(key) # Called by __getitem__ for missing key; pseudo-code:\n\
  if self.default_factory is None: raise KeyError((key,))\n\
  self[key] = value = self.default_factory()\n\
  return value\n\
");

static PyObject *
defdict_missing(defdictobject *dd, PyObject *key)
{
    PyObject *factory = dd->default_factory;
    PyObject *value;
    if (factory == NULL || factory == Py_None) {
        /* XXX Call dict.__missing__(key) */
        PyObject *tup;
        tup = PyTuple_Pack(1, key);
        if (!tup) return NULL;
        PyErr_SetObject(PyExc_KeyError, tup);
        Py_DECREF(tup);
        return NULL;
    }
    value = PyEval_CallObject(factory, NULL);
    if (value == NULL)
        return value;
    if (PyObject_SetItem((PyObject *)dd, key, value) < 0) {
        Py_DECREF(value);
        return NULL;
    }
    return value;
}

PyDoc_STRVAR(defdict_copy_doc, "D.copy() -> a shallow copy of D.");

static PyObject *
defdict_copy(defdictobject *dd)
{
    /* This calls the object's class.  That only works for subclasses
       whose class constructor has the same signature.  Subclasses that
       define a different constructor signature must override copy().
    */

    if (dd->default_factory == NULL)
        return PyObject_CallFunctionObjArgs((PyObject*)Py_TYPE(dd), Py_None, dd, NULL);
    return PyObject_CallFunctionObjArgs((PyObject*)Py_TYPE(dd),
                                        dd->default_factory, dd, NULL);
}

static PyObject *
defdict_reduce(defdictobject *dd)
{
    /* __reduce__ must return a 5-tuple as follows:

       - factory function
       - tuple of args for the factory function
       - additional state (here None)
       - sequence iterator (here None)
       - dictionary iterator (yielding successive (key, value) pairs

       This API is used by pickle.py and copy.py.

       For this to be useful with pickle.py, the default_factory
       must be picklable; e.g., None, a built-in, or a global
       function in a module or package.

       Both shallow and deep copying are supported, but for deep
       copying, the default_factory must be deep-copyable; e.g. None,
       or a built-in (functions are not copyable at this time).

       This only works for subclasses as long as their constructor
       signature is compatible; the first argument must be the
       optional default_factory, defaulting to None.
    */
    PyObject *args;
    PyObject *items;
    PyObject *iter;
    PyObject *result;
    if (dd->default_factory == NULL || dd->default_factory == Py_None)
        args = PyTuple_New(0);
    else
        args = PyTuple_Pack(1, dd->default_factory);
    if (args == NULL)
        return NULL;
    items = PyObject_CallMethod((PyObject *)dd, "items", "()");
    if (items == NULL) {
        Py_DECREF(args);
        return NULL;
    }
    iter = PyObject_GetIter(items);
    if (iter == NULL) {
        Py_DECREF(items);
        Py_DECREF(args);
        return NULL;
    }
    result = PyTuple_Pack(5, Py_TYPE(dd), args,
                          Py_None, Py_None, iter);
    Py_DECREF(iter);
    Py_DECREF(items);
    Py_DECREF(args);
    return result;
}

static PyMethodDef defdict_methods[] = {
    {"__missing__", (PyCFunction)defdict_missing, METH_O,
     defdict_missing_doc},
    {"copy", (PyCFunction)defdict_copy, METH_NOARGS,
     defdict_copy_doc},
    {"__copy__", (PyCFunction)defdict_copy, METH_NOARGS,
     defdict_copy_doc},
    {"__reduce__", (PyCFunction)defdict_reduce, METH_NOARGS,
     reduce_doc},
    {NULL}
};

static PyMemberDef defdict_members[] = {
    {"default_factory", T_OBJECT,
     offsetof(defdictobject, default_factory), 0,
     PyDoc_STR("Factory for default value called by __missing__().")},
    {NULL}
};

static void
defdict_dealloc(defdictobject *dd)
{
    Py_CLEAR(dd->default_factory);
    PyDict_Type.tp_dealloc((PyObject *)dd);
}

static PyObject *
defdict_repr(defdictobject *dd)
{
    PyObject *baserepr;
    PyObject *defrepr;
    PyObject *result;
    baserepr = PyDict_Type.tp_repr((PyObject *)dd);
    if (baserepr == NULL)
        return NULL;
    if (dd->default_factory == NULL)
        defrepr = PyUnicode_FromString("None");
    else
    {
        int status = Py_ReprEnter(dd->default_factory);
        if (status != 0) {
            if (status < 0)
                return NULL;
            defrepr = PyUnicode_FromString("...");
        }
        else
            defrepr = PyObject_Repr(dd->default_factory);
        Py_ReprLeave(dd->default_factory);
    }
    if (defrepr == NULL) {
        Py_DECREF(baserepr);
        return NULL;
    }
    result = PyUnicode_FromFormat("defaultdict(%U, %U)",
                                  defrepr, baserepr);
    Py_DECREF(defrepr);
    Py_DECREF(baserepr);
    return result;
}

static int
defdict_traverse(PyObject *self, visitproc visit, void *arg)
{
    Py_VISIT(((defdictobject *)self)->default_factory);
    return PyDict_Type.tp_traverse(self, visit, arg);
}

static int
defdict_tp_clear(defdictobject *dd)
{
    Py_CLEAR(dd->default_factory);
    return PyDict_Type.tp_clear((PyObject *)dd);
}

static int
defdict_init(PyObject *self, PyObject *args, PyObject *kwds)
{
    defdictobject *dd = (defdictobject *)self;
    PyObject *olddefault = dd->default_factory;
    PyObject *newdefault = NULL;
    PyObject *newargs;
    int result;
    if (args == NULL || !PyTuple_Check(args))
        newargs = PyTuple_New(0);
    else {
        Py_ssize_t n = PyTuple_GET_SIZE(args);
        if (n > 0) {
            newdefault = PyTuple_GET_ITEM(args, 0);
            if (!PyCallable_Check(newdefault) && newdefault != Py_None) {
                PyErr_SetString(PyExc_TypeError,
                    "first argument must be callable");
                return -1;
            }
        }
        newargs = PySequence_GetSlice(args, 1, n);
    }
    if (newargs == NULL)
        return -1;
    Py_XINCREF(newdefault);
    dd->default_factory = newdefault;
    result = PyDict_Type.tp_init(self, newargs, kwds);
    Py_DECREF(newargs);
    Py_XDECREF(olddefault);
    return result;
}

PyDoc_STRVAR(defdict_doc,
"defaultdict(default_factory) --> dict with default factory\n\
\n\
The default factory is called without arguments to produce\n\
a new value when a key is not present, in __getitem__ only.\n\
A defaultdict compares equal to a dict with the same items.\n\
");

/* See comment in xxsubtype.c */
#define DEFERRED_ADDRESS(ADDR) 0

static PyTypeObject defdict_type = {
    PyVarObject_HEAD_INIT(DEFERRED_ADDRESS(&PyType_Type), 0)
    "collections.defaultdict",          /* tp_name */
    sizeof(defdictobject),              /* tp_basicsize */
    0,                                  /* tp_itemsize */
    /* methods */
    (destructor)defdict_dealloc,        /* tp_dealloc */
    0,                                  /* tp_print */
    0,                                  /* tp_getattr */
    0,                                  /* tp_setattr */
    0,                                  /* tp_reserved */
    (reprfunc)defdict_repr,             /* tp_repr */
    0,                                  /* tp_as_number */
    0,                                  /* tp_as_sequence */
    0,                                  /* tp_as_mapping */
    0,                                  /* tp_hash */
    0,                                  /* tp_call */
    0,                                  /* tp_str */
    PyObject_GenericGetAttr,            /* tp_getattro */
    0,                                  /* tp_setattro */
    0,                                  /* tp_as_buffer */
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC,
                                    /* tp_flags */
    defdict_doc,                        /* tp_doc */
    defdict_traverse,                   /* tp_traverse */
    (inquiry)defdict_tp_clear,          /* tp_clear */
    0,                                  /* tp_richcompare */
    0,                                  /* tp_weaklistoffset*/
    0,                                  /* tp_iter */
    0,                                  /* tp_iternext */
    defdict_methods,                    /* tp_methods */
    defdict_members,                    /* tp_members */
    0,                                  /* tp_getset */
    DEFERRED_ADDRESS(&PyDict_Type),     /* tp_base */
    0,                                  /* tp_dict */
    0,                                  /* tp_descr_get */
    0,                                  /* tp_descr_set */
    0,                                  /* tp_dictoffset */
    defdict_init,                       /* tp_init */
    PyType_GenericAlloc,                /* tp_alloc */
    0,                                  /* tp_new */
    PyObject_GC_Del,                    /* tp_free */
};

/* helper function for Counter  *********************************************/

PyDoc_STRVAR(_count_elements_doc,
"_count_elements(mapping, iterable) -> None\n\
\n\
Count elements in the iterable, updating the mappping");

static PyObject *
_count_elements(PyObject *self, PyObject *args)
{
    PyObject *it, *iterable, *mapping, *oldval;
    PyObject *newval = NULL;
    PyObject *key = NULL;
    PyObject *one = NULL;

    if (!PyArg_UnpackTuple(args, "_count_elements", 2, 2, &mapping, &iterable))
        return NULL;

    it = PyObject_GetIter(iterable);
    if (it == NULL)
        return NULL;

    one = PyLong_FromLong(1);
    if (one == NULL) {
        Py_DECREF(it);
        return NULL;
    }

    if (PyDict_CheckExact(mapping)) {
        while (1) {
            key = PyIter_Next(it);
            if (key == NULL)
                break;
            oldval = PyDict_GetItem(mapping, key);
            if (oldval == NULL) {
                if (PyDict_SetItem(mapping, key, one) == -1)
                    break;
            } else {
                newval = PyNumber_Add(oldval, one);
                if (newval == NULL)
                    break;
                if (PyDict_SetItem(mapping, key, newval) == -1)
                    break;
                Py_CLEAR(newval);
            }
            Py_DECREF(key);
        }
    } else {
        while (1) {
            key = PyIter_Next(it);
            if (key == NULL)
                break;
            oldval = PyObject_GetItem(mapping, key);
            if (oldval == NULL) {
                if (!PyErr_Occurred() || !PyErr_ExceptionMatches(PyExc_KeyError))
                    break;
                PyErr_Clear();
                Py_INCREF(one);
                newval = one;
            } else {
                newval = PyNumber_Add(oldval, one);
                Py_DECREF(oldval);
                if (newval == NULL)
                    break;
            }
            if (PyObject_SetItem(mapping, key, newval) == -1)
                break;
            Py_CLEAR(newval);
            Py_DECREF(key);
        }
    }

    Py_DECREF(it);
    Py_XDECREF(key);
    Py_XDECREF(newval);
    Py_DECREF(one);
    if (PyErr_Occurred())
        return NULL;
    Py_RETURN_NONE;
}

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

PyDoc_STRVAR(module_doc,
"High performance data structures.\n\
- deque:        ordered collection accessible from endpoints only\n\
- defaultdict:  dict subclass with a default value factory\n\
");

static struct PyMethodDef module_functions[] = {
    {"_count_elements", _count_elements,    METH_VARARGS,   _count_elements_doc},
    {NULL,       NULL}          /* sentinel */
};

static struct PyModuleDef _collectionsmodule = {
    PyModuleDef_HEAD_INIT,
    "_collections",
    module_doc,
    -1,
    module_functions,
    NULL,
    NULL,
    NULL,
    NULL
};

PyMODINIT_FUNC
PyInit__collections(void)
{
    PyObject *m;

    m = PyModule_Create(&_collectionsmodule);
    if (m == NULL)
        return NULL;

    if (PyType_Ready(&deque_type) < 0)
        return NULL;
    Py_INCREF(&deque_type);
    PyModule_AddObject(m, "deque", (PyObject *)&deque_type);

    defdict_type.tp_base = &PyDict_Type;
    if (PyType_Ready(&defdict_type) < 0)
        return NULL;
    Py_INCREF(&defdict_type);
    PyModule_AddObject(m, "defaultdict", (PyObject *)&defdict_type);

    if (PyType_Ready(&dequeiter_type) < 0)
        return NULL;

    if (PyType_Ready(&dequereviter_type) < 0)
        return NULL;

    return m;
}