/* 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;

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

                +----------------------------------------+
                |          data: 62 objects              |
 +----------+   |                                        |   +-----------+
 | leftlink |---|  | ... | Obj1 | Obj2 | Obj3 | ... |    |---| rightlink |
 +----------+   |           30     31     32             |   +-----------+
                +----------------------------------------+

static block *
newblock(block *leftlink, block *rightlink, int len) {
    block *b;
    /* To prevent len from overflowing INT_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 INT_MAX-2 entries in total.
     */
    if (len >= INT_MAX - 2*BLOCKLEN) {
        PyErr_SetString(PyExc_OverflowError,
                "cannot add more blocks to the deque");
        return NULL;
    }
    b = PyMem_Malloc(sizeof(block));
    if (b == NULL) {
        PyErr_NoMemory();
        return NULL;
    }
    b->leftlink = leftlink;
    b->rightlink = rightlink;
    return b;
}

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

    if (type == &deque_type && !_PyArg_NoKeywords("deque()", kwds))
        return NULL;

    /* 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;

    return (PyObject *)deque;
}

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;
    Py_RETURN_NONE;
}