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.

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 embedded cork_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 use cork_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 returns 0, then you will visit all of the elements in list, and we’ll return 0 from cork_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.

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 variables curr and next, too.)

For the _void variant, your statement can only use these cork_dllist_item variables to access the current list element. You can use cork_container_of() to get a pointer to the actual element type.

For the non-_void variant, we’ll automatically call cork_container_of() for you. element_type should be the actual element type, which must contain an embedded cork_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 return NULL, 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 return NULL, 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 */