Doubly-linked lists¶
#include <libcork/ds.h>
This section defines a doubly-linked list data structure. The structure
is “invasive”, since you must place an instance of the
cork_dllist_item
type into the type whose instances will be
stored in the list. The list itself is represented by the
cork_dllist
type.
As an example, we could define the following types for storing groups, as well as the users within each group:
struct group {
const char *group_name;
struct cork_dllist members;
};
struct user {
const char *username;
const char *real_name;
struct cork_dllist_item list;
};
Note that both cork_dllist
and cork_dllist_item
are embedded
directly into our domain-specific types. This means that every list
operation defined in this section is guaranteed to succeed, since no
memory operations will be involved. (The list and any items will have
already been allocated before you try to call the list function.)
Like with any embedded struct
, you can use the
cork_container_of()
macro to obtain a pointer to a struct
user
if you’re given a pointer to a cork_dllist_item
.
-
struct
cork_dllist
¶ A doubly-linked list. The list itself is represented by a sentinel element, representing the empty list.
-
struct
cork_dllist_item
¶ An element of a doubly-linked list. This type will usually be embedded within the type whose instances will be stored in the list.
-
struct cork_dllist_item *
next
¶ -
struct cork_dllist_item *
prev
¶ A pointer to the next (or previous) element in the list. If this element marks the end (or beginning) of the list, then next (or prev) will point to the list’s sentinel value.
-
struct cork_dllist_item *
-
void
cork_dllist_init
(struct cork_dllist *list)¶ -
struct cork_dllist
CORK_DLLIST_INIT
(SYMBOL name)¶ Initializes a doubly-linked list. The list will initially be empty.
The second variant is a static initializer, that lets you initialize a list at compile time, rather than runtime. You must pass in the name of the list for this to work, since we need to be able to extract pointers into the list object.
Querying a list¶
-
size_t
cork_dllist_size
(const struct cork_dllist *list)¶ Returns the number of elements in list.
This operation runs in \(O(n)\) time.
-
bool
cork_dllist_is_empty
(struct cork_dllist *list)¶ Returns whether list is empty.
This operation runs in \(O(1)\) time.
Editing a list¶
-
void
cork_dllist_add_to_head
(struct cork_dllist *list, struct cork_dllist_item *element)¶ -
void
cork_dllist_add_to_tail
(struct cork_dllist *list, struct cork_dllist_item *element)¶ Adds element to list. The
_head
variant adds the new element to the beginning of the list; the_tail
variant adds it to the end.You are responsible for allocating the list element yourself, most likely by allocating the
struct
that you’ve embeddedcork_dllist_item
into.Note
This function assumes that element isn’t already a member of a different list. You’re responsible for calling
cork_dllist_remove()
if this isn’t the case. (If you don’t, the other list will become malformed.)This operation runs in \(O(1)\) time.
-
void
cork_dllist_add_after
(struct cork_dllist_item *pred, struct cork_dllist_item *element)¶ -
void
cork_dllist_add_before
(struct cork_dllist_item *succ, struct cork_dllist_item *element)¶ Adds element to the same list that pred or succ belong to. The
_after
variant ensures that element appears in the list immediately after pred. The_before
variant ensures that element appears in the list immediately before succ.Note
This function assumes that element isn’t already a member of a different list. You’re responsible for calling
cork_dllist_remove()
if this isn’t the case. (If you don’t, the other list will become malformed.)This operation runs in \(O(1)\) time.
-
void
cork_dllist_add_list_to_head
(struct cork_dllist *dest, struct cork_dllist *src)¶ -
void
cork_dllist_add_list_to_tail
(struct cork_dllist *dest, struct cork_dllist *src)¶ Moves all of the elements in src to dest. The
_head
variant moves the elements to the beginning of dest; the_tail
variant moves them to the end. After these functions return, src will be empty.This operation runs in \(O(1)\) time.
-
void
cork_dllist_remove
(struct cork_dllist_item *element)¶ Removes element from the list that it currently belongs to. (Note that you don’t have to pass in a pointer to that list.)
Note
You must not call this function on a list’s sentinel element.
This operation runs in \(O(1)\) time.
Iterating through a list¶
There are two strategies you can use to access all of the elements in a doubly-linked list: visiting and iterating. With visiting, you write a visitor function, which will be applied to each element in the list. (In this case, libcork controls the loop that steps through each element.)
-
int
cork_dllist_visit
(struct cork_dllist *list, void *user_data, cork_dllist_visit_f *func)¶ Apply a function to each element in list. The function is allowed to remove the current element from the list; this will not affect our ability to iterate through the remainder of the list. The function will be given a pointer to the
cork_dllist_item
for each element; you can usecork_container_of()
to get a pointer to the actual element type.If your visitor function ever returns a non-zero value, we will abort the iteration and return that value from
cork_dllist_visit
. If your function always returns0
, then you will visit all of the elements in list, and we’ll return0
fromcork_dllist_visit
.-
int
cork_dllist_visit_f
(void *user_data, struct cork_dllist_item *element)¶ A function that can be applied to each element in a doubly-linked list.
-
int
For instance, you can manually calculate the number of elements in a
list as follows (assuming you didn’t want to use the built-in
cork_dllist_size()
function, of course):
static int
count_elements(void *user_data, struct cork_dllist_item *element)
{
size_t *count = ud;
(*count)++;
return 0;
}
struct cork_dllist *list = /* from somewhere */;
size_t count = 0;
cork_dllist_visit(list, &count, count_elements); /* returns 0 */
/* the number of elements is now in count */
The second strategy is to iterate through the elements yourself.
-
cork_dllist_foreach
(struct cork_dllist *list, struct cork_dllist_item &*curr, struct cork_dllist_item &*next, TYPE element_type, TYPE &*element, FIELD item_field)¶ -
cork_dllist_foreach_void
(struct cork_dllist *list, struct cork_dllist_item &*curr, struct cork_dllist_item &*next)¶ Iterate through each element in list, executing a statement for each one. You must declare two variables of type
struct cork_dllist_item *
, and pass in their names as curr and next. (You’ll usually call the variablescurr
andnext
, too.)For the
_void
variant, your statement can only use thesecork_dllist_item
variables to access the current list element. You can usecork_container_of()
to get a pointer to the actual element type.For the non-
_void
variant, we’ll automatically callcork_container_of()
for you. element_type should be the actual element type, which must contain an embeddedcork_dllist_item()
field. item_field should be the name of this embedded field. You must allocate a pointer to the element type, and pass in its name as element.
For instance, you can use these macros calculate the number of elements as follows:
struct cork_dllist *list = /* from somewhere */;
struct cork_dllist *curr;
struct cork_dllist *next;
size_t count = 0;
cork_dllist_foreach_void(list, curr, next) {
count++;
}
/* the number of elements is now in count */
We’re able to use cork_dllist_foreach_void
since we don’t need to
access the contents of each element to calculate how many of theo there are. If
we wanted to calculuate a sum, however, we’d have to use
cork_dllist_foreach
:
struct element {
unsigned int value;
struct cork_dllist_item item;
};
struct cork_dllist *list = /* from somewhere */;
struct cork_dllist *curr;
struct cork_dllist *next;
struct element *element;
unsigned int sum = 0;
cork_dllist_foreach(list, curr, next, struct element, element, item) {
sum += element->value;
}
/* the sum of the elements is now in sum */
If the foreach
macros don’t provide what you need, you can also iterate
through the list manually.
-
struct cork_dllist_item *
cork_dllist_head
(struct cork_dllist *list)¶ -
struct cork_dllist_item *
cork_dllist_start
(struct cork_dllist *list)¶ Returns the element at the beginning of list. If list is empty, then the
_head
variant will returnNULL
, while the_start
variant will return the list’s sentinel element.
-
struct cork_dllist_item *
cork_dllist_tail
(struct cork_dllist *list)¶ -
struct cork_dllist_item *
cork_dllist_end
(struct cork_dllist *list)¶ Returns the element at the end of list. If list is empty, then the
_tail
variant will returnNULL
, while the_end
variant will return the list’s sentinel element.
-
bool
cork_dllist_is_start
(struct cork_dllist *list, struct cork_dllist_item *element)¶ -
bool
cork_dllist_is_end
(struct cork_dllist *list, struct cork_dllist_item *element)¶ Returns whether element marks the start (or end) of list.
With these functions, manually counting the list elements looks like:
struct cork_dllist *list = /* from somewhere */;
struct cork_dllist_item *curr;
size_t count = 0;
for (curr = cork_dllist_start(list); !cork_dllist_is_end(list, curr);
curr = curr->next) {
count++;
}
/* the number of elements is now in count */
You can also count the elements in reverse order:
struct cork_dllist *list = /* from somewhere */;
struct cork_dllist_item *curr;
size_t count = 0;
for (curr = cork_dllist_end(list); !cork_dllist_is_start(list, curr);
curr = curr->prev) {
count++;
}
/* the number of elements is now in count */