.. _dllist:

*******************
Doubly-linked lists
*******************

.. highlight:: c

::

  #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
:c:type:`cork_dllist_item` type into the type whose instances will be
stored in the list.  The list itself is represented by the
:c:type:`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
:c:func:`cork_container_of` macro to obtain a pointer to a ``struct
user`` if you're given a pointer to a :c:type:`cork_dllist_item`.


.. type:: struct cork_dllist

   A doubly-linked list.  The list itself is represented by a sentinel
   element, representing the empty list.


.. type:: 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.

   .. member:: 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.


.. function:: 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
---------------

.. function:: size_t cork_dllist_size(const struct cork_dllist \*list)

   Returns the number of elements in *list*.
   
   This operation runs in :math:`O(n)` time.


.. function:: bool cork_dllist_is_empty(struct cork_dllist \*list)

   Returns whether *list* is empty.

   This operation runs in :math:`O(1)` time.


Editing a list
--------------

.. function:: 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 :c:type:`cork_dllist_item`
   into.

   .. note::

      This function assumes that *element* isn't already a member of a different
      list.  You're responsible for calling :c:func:`cork_dllist_remove()` if
      this isn't the case.  (If you don't, the other list will become
      malformed.)

   This operation runs in :math:`O(1)` time.


.. function:: 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 :c:func:`cork_dllist_remove()` if
      this isn't the case.  (If you don't, the other list will become
      malformed.)

   This operation runs in :math:`O(1)` time.


.. function:: 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 :math:`O(1)` time.


.. function:: 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 :math:`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.)

.. function:: 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 :c:type:`cork_dllist_item` for each
   element; you can use :c:func:`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``.

   .. type:: 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
:c:func:`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.

.. macro:: 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
   :c:type:`cork_dllist_item` variables to access the current list element.  You
   can use :c:func:`cork_container_of` to get a pointer to the actual element
   type.

   For the non-``_void`` variant, we'll automatically call
   :c:func:`cork_container_of` for you.  *element_type* should be the actual
   element type, which must contain an embedded :c:func:`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 :c:macro:`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
:c:macro:`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.

.. function:: 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.


.. function:: 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.

.. function:: 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 */
