/* 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; }