ilist.h

Go to the documentation of this file.

/*-------------------------------------------------------------------------
 *
 * ilist.h
 *      integrated/inline doubly- and singly-linked lists
 *
 * These list types are useful when there are only a predetermined set of
 * lists that an object could be in.  List links are embedded directly into
 * the objects, and thus no extra memory management overhead is required.
 * (Of course, if only a small proportion of existing objects are in a list,
 * the link fields in the remainder would be wasted space.  But usually,
 * it saves space to not have separately-allocated list nodes.)
 *
 * None of the functions here allocate any memory; they just manipulate
 * externally managed memory.  The APIs for singly and doubly linked lists
 * are identical as far as capabilities of both allow.
 *
 * Each list has a list header, which exists even when the list is empty.
 * An empty singly-linked list has a NULL pointer in its header.
 * There are two kinds of empty doubly linked lists: those that have been
 * initialized to NULL, and those that have been initialized to circularity.
 * (If a dlist is modified and then all its elements are deleted, it will be
 * in the circular state.)  We prefer circular dlists because there are some
 * operations that can be done without branches (and thus faster) on lists
 * that use circular representation.  However, it is often convenient to
 * initialize list headers to zeroes rather than setting them up with an
 * explicit initialization function, so we also allow the other case.
 *
 * EXAMPLES
 *
 * Here's a simple example demonstrating how this can be used.  Let's assume
 * we want to store information about the tables contained in a database.
 *
 * #include "lib/ilist.h"
 *
 * // Define struct for the databases including a list header that will be
 * // used to access the nodes in the table list later on.
 * typedef struct my_database
 * {
 *      char       *datname;
 *      dlist_head  tables;
 *      // ...
 * } my_database;
 *
 * // Define struct for the tables.  Note the list_node element which stores
 * // prev/next list links.  The list_node element need not be first.
 * typedef struct my_table
 * {
 *      char       *tablename;
 *      dlist_node  list_node;
 *      perm_t      permissions;
 *      // ...
 * } my_table;
 *
 * // create a database
 * my_database *db = create_database();
 *
 * // and add a few tables to its table list
 * dlist_push_head(&db->tables, &create_table(db, "a")->list_node);
 * ...
 * dlist_push_head(&db->tables, &create_table(db, "b")->list_node);
 *
 *
 * To iterate over the table list, we allocate an iterator variable and use
 * a specialized looping construct.  Inside a dlist_foreach, the iterator's
 * 'cur' field can be used to access the current element.  iter.cur points to
 * a 'dlist_node', but most of the time what we want is the actual table
 * information; dlist_container() gives us that, like so:
 *
 * dlist_iter   iter;
 * dlist_foreach(iter, &db->tables)
 * {
 *      my_table   *tbl = dlist_container(my_table, list_node, iter.cur);
 *      printf("we have a table: %s in database %s\n",
 *             tbl->tablename, db->datname);
 * }
 *
 *
 * While a simple iteration is useful, we sometimes also want to manipulate
 * the list while iterating.  There is a different iterator element and looping
 * construct for that.  Suppose we want to delete tables that meet a certain
 * criterion:
 *
 * dlist_mutable_iter miter;
 * dlist_foreach_modify(miter, &db->tables)
 * {
 *      my_table   *tbl = dlist_container(my_table, list_node, miter.cur);
 *
 *      if (!tbl->to_be_deleted)
 *          continue;       // don't touch this one
 *
 *      // unlink the current table from the linked list
 *      dlist_delete(miter.cur);
 *      // as these lists never manage memory, we can still access the table
 *      // after it's been unlinked
 *      drop_table(db, tbl);
 * }
 *
 *
 * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 * IDENTIFICATION
 *      src/include/lib/ilist.h
 *-------------------------------------------------------------------------
 */
#ifndef ILIST_H
#define ILIST_H

/*
 * Enable for extra debugging. This is rather expensive, so it's not enabled by
 * default even when USE_ASSERT_CHECKING.
 */
/* #define ILIST_DEBUG */

/*
 * Node of a doubly linked list.
 *
 * Embed this in structs that need to be part of a doubly linked list.
 */
typedef struct dlist_node dlist_node;
struct dlist_node
{
    dlist_node *prev;
    dlist_node *next;
};

/*
 * Head of a doubly linked list.
 *
 * Non-empty lists are internally circularly linked.  Circular lists have the
 * advantage of not needing any branches in the most common list manipulations.
 * An empty list can also be represented as a pair of NULL pointers, making
 * initialization easier.
 */
typedef struct dlist_head
{
    /*
     * head.next either points to the first element of the list; to &head if
     * it's a circular empty list; or to NULL if empty and not circular.
     *
     * head.prev either points to the last element of the list; to &head if
     * it's a circular empty list; or to NULL if empty and not circular.
     */
    dlist_node  head;
} dlist_head;


/*
 * Doubly linked list iterator.
 *
 * Used as state in dlist_foreach() and dlist_reverse_foreach(). To get the
 * current element of the iteration use the 'cur' member.
 *
 * Iterations using this are *not* allowed to change the list while iterating!
 *
 * NB: We use an extra "end" field here to avoid multiple evaluations of
 * arguments in the dlist_foreach() macro.
 */
typedef struct dlist_iter
{
    dlist_node *cur;            /* current element */
    dlist_node *end;            /* last node we'll iterate to */
} dlist_iter;

/*
 * Doubly linked list iterator allowing some modifications while iterating.
 *
 * Used as state in dlist_foreach_modify(). To get the current element of the
 * iteration use the 'cur' member.
 *
 * Iterations using this are only allowed to change the list at the current
 * point of iteration. It is fine to delete the current node, but it is *not*
 * fine to insert or delete adjacent nodes.
 *
 * NB: We need a separate type for mutable iterations so that we can store
 * the 'next' node of the current node in case it gets deleted or modified.
 */
typedef struct dlist_mutable_iter
{
    dlist_node *cur;            /* current element */
    dlist_node *next;           /* next node we'll iterate to */
    dlist_node *end;            /* last node we'll iterate to */
} dlist_mutable_iter;

/*
 * Node of a singly linked list.
 *
 * Embed this in structs that need to be part of a singly linked list.
 */
typedef struct slist_node slist_node;
struct slist_node
{
    slist_node *next;
};

/*
 * Head of a singly linked list.
 *
 * Singly linked lists are not circularly linked, in contrast to doubly linked
 * lists; we just set head.next to NULL if empty.  This doesn't incur any
 * additional branches in the usual manipulations.
 */
typedef struct slist_head
{
    slist_node  head;
} slist_head;

/*
 * Singly linked list iterator.
 *
 * Used as state in slist_foreach(). To get the current element of the
 * iteration use the 'cur' member.
 *
 * It's allowed to modify the list while iterating, with the exception of
 * deleting the iterator's current node; deletion of that node requires
 * care if the iteration is to be continued afterward.  (Doing so and also
 * deleting or inserting adjacent list elements might misbehave; also, if
 * the user frees the current node's storage, continuing the iteration is
 * not safe.)
 *
 * NB: this wouldn't really need to be an extra struct, we could use an
 * slist_node * directly. We prefer a separate type for consistency.
 */
typedef struct slist_iter
{
    slist_node *cur;
} slist_iter;

/*
 * Singly linked list iterator allowing some modifications while iterating.
 *
 * Used as state in slist_foreach_modify(). To get the current element of the
 * iteration use the 'cur' member.
 *
 * The only list modification allowed while iterating is to remove the current
 * node via slist_delete_current() (*not* slist_delete()).  Insertion or
 * deletion of nodes adjacent to the current node would misbehave.
 */
typedef struct slist_mutable_iter
{
    slist_node *cur;            /* current element */
    slist_node *next;           /* next node we'll iterate to */
    slist_node *prev;           /* prev node, for deletions */
} slist_mutable_iter;


/* Static initializers */
#define DLIST_STATIC_INIT(name) {{&(name).head, &(name).head}}
#define SLIST_STATIC_INIT(name) {{NULL}}


/* Prototypes for functions too big to be inline */

/* Caution: this is O(n); consider using slist_delete_current() instead */
extern void slist_delete(slist_head *head, slist_node *node);

#ifdef ILIST_DEBUG
extern void dlist_check(dlist_head *head);
extern void slist_check(slist_head *head);
#else
/*
 * These seemingly useless casts to void are here to keep the compiler quiet
 * about the argument being unused in many functions in a non-debug compile,
 * in which functions the only point of passing the list head pointer is to be
 * able to run these checks.
 */
#define dlist_check(head)   ((void) (head))
#define slist_check(head)   ((void) (head))
#endif                          /* ILIST_DEBUG */

/* doubly linked list implementation */

/*
 * Initialize a doubly linked list.
 * Previous state will be thrown away without any cleanup.
 */
static inline void
dlist_init(dlist_head *head)
{
    head->head.next = head->head.prev = &head->head;
}

/*
 * Is the list empty?
 *
 * An empty list has either its first 'next' pointer set to NULL, or to itself.
 */
static inline bool
dlist_is_empty(dlist_head *head)
{
    dlist_check(head);

    return head->head.next == NULL || head->head.next == &(head->head);
}

/*
 * Insert a node at the beginning of the list.
 */
static inline void
dlist_push_head(dlist_head *head, dlist_node *node)
{
    if (head->head.next == NULL)    /* convert NULL header to circular */
        dlist_init(head);

    node->next = head->head.next;
    node->prev = &head->head;
    node->next->prev = node;
    head->head.next = node;

    dlist_check(head);
}

/*
 * Insert a node at the end of the list.
 */
static inline void
dlist_push_tail(dlist_head *head, dlist_node *node)
{
    if (head->head.next == NULL)    /* convert NULL header to circular */
        dlist_init(head);

    node->next = &head->head;
    node->prev = head->head.prev;
    node->prev->next = node;
    head->head.prev = node;

    dlist_check(head);
}

/*
 * Insert a node after another *in the same list*
 */
static inline void
dlist_insert_after(dlist_node *after, dlist_node *node)
{
    node->prev = after;
    node->next = after->next;
    after->next = node;
    node->next->prev = node;
}

/*
 * Insert a node before another *in the same list*
 */
static inline void
dlist_insert_before(dlist_node *before, dlist_node *node)
{
    node->prev = before->prev;
    node->next = before;
    before->prev = node;
    node->prev->next = node;
}

/*
 * Delete 'node' from its list (it must be in one).
 */
static inline void
dlist_delete(dlist_node *node)
{
    node->prev->next = node->next;
    node->next->prev = node->prev;
}

/*
 * Remove and return the first node from a list (there must be one).
 */
static inline dlist_node *
dlist_pop_head_node(dlist_head *head)
{
    dlist_node *node;

    Assert(!dlist_is_empty(head));
    node = head->head.next;
    dlist_delete(node);
    return node;
}

/*
 * Move element from its current position in the list to the head position in
 * the same list.
 *
 * Undefined behaviour if 'node' is not already part of the list.
 */
static inline void
dlist_move_head(dlist_head *head, dlist_node *node)
{
    /* fast path if it's already at the head */
    if (head->head.next == node)
        return;

    dlist_delete(node);
    dlist_push_head(head, node);

    dlist_check(head);
}

/*
 * Check whether 'node' has a following node.
 * Caution: unreliable if 'node' is not in the list.
 */
static inline bool
dlist_has_next(dlist_head *head, dlist_node *node)
{
    return node->next != &head->head;
}

/*
 * Check whether 'node' has a preceding node.
 * Caution: unreliable if 'node' is not in the list.
 */
static inline bool
dlist_has_prev(dlist_head *head, dlist_node *node)
{
    return node->prev != &head->head;
}

/*
 * Return the next node in the list (there must be one).
 */
static inline dlist_node *
dlist_next_node(dlist_head *head, dlist_node *node)
{
    Assert(dlist_has_next(head, node));
    return node->next;
}

/*
 * Return previous node in the list (there must be one).
 */
static inline dlist_node *
dlist_prev_node(dlist_head *head, dlist_node *node)
{
    Assert(dlist_has_prev(head, node));
    return node->prev;
}

/* internal support function to get address of head element's struct */
static inline void *
dlist_head_element_off(dlist_head *head, size_t off)
{
    Assert(!dlist_is_empty(head));
    return (char *) head->head.next - off;
}

/*
 * Return the first node in the list (there must be one).
 */
static inline dlist_node *
dlist_head_node(dlist_head *head)
{
    return (dlist_node *) dlist_head_element_off(head, 0);
}

/* internal support function to get address of tail element's struct */
static inline void *
dlist_tail_element_off(dlist_head *head, size_t off)
{
    Assert(!dlist_is_empty(head));
    return (char *) head->head.prev - off;
}

/*
 * Return the last node in the list (there must be one).
 */
static inline dlist_node *
dlist_tail_node(dlist_head *head)
{
    return (dlist_node *) dlist_tail_element_off(head, 0);
}

/*
 * Return the containing struct of 'type' where 'membername' is the dlist_node
 * pointed at by 'ptr'.
 *
 * This is used to convert a dlist_node * back to its containing struct.
 */
#define dlist_container(type, membername, ptr)                              \
    (AssertVariableIsOfTypeMacro(ptr, dlist_node *),                        \
     AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node),  \
     ((type *) ((char *) (ptr) - offsetof(type, membername))))

/*
 * Return the address of the first element in the list.
 *
 * The list must not be empty.
 */
#define dlist_head_element(type, membername, lhead)                         \
    (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node),  \
     (type *) dlist_head_element_off(lhead, offsetof(type, membername)))

/*
 * Return the address of the last element in the list.
 *
 * The list must not be empty.
 */
#define dlist_tail_element(type, membername, lhead)                         \
    (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, dlist_node),  \
     ((type *) dlist_tail_element_off(lhead, offsetof(type, membername))))

/*
 * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
 *
 * Access the current element with iter.cur.
 *
 * It is *not* allowed to manipulate the list during iteration.
 */
#define dlist_foreach(iter, lhead)                                          \
    for (AssertVariableIsOfTypeMacro(iter, dlist_iter),                     \
         AssertVariableIsOfTypeMacro(lhead, dlist_head *),                  \
         (iter).end = &(lhead)->head,                                       \
         (iter).cur = (iter).end->next ? (iter).end->next : (iter).end;     \
         (iter).cur != (iter).end;                                          \
         (iter).cur = (iter).cur->next)

/*
 * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
 *
 * Access the current element with iter.cur.
 *
 * Iterations using this are only allowed to change the list at the current
 * point of iteration. It is fine to delete the current node, but it is *not*
 * fine to insert or delete adjacent nodes.
 */
#define dlist_foreach_modify(iter, lhead)                                   \
    for (AssertVariableIsOfTypeMacro(iter, dlist_mutable_iter),             \
         AssertVariableIsOfTypeMacro(lhead, dlist_head *),                  \
         (iter).end = &(lhead)->head,                                       \
         (iter).cur = (iter).end->next ? (iter).end->next : (iter).end,     \
         (iter).next = (iter).cur->next;                                    \
         (iter).cur != (iter).end;                                          \
         (iter).cur = (iter).next, (iter).next = (iter).cur->next)

/*
 * Iterate through the list in reverse order.
 *
 * It is *not* allowed to manipulate the list during iteration.
 */
#define dlist_reverse_foreach(iter, lhead)                                  \
    for (AssertVariableIsOfTypeMacro(iter, dlist_iter),                     \
         AssertVariableIsOfTypeMacro(lhead, dlist_head *),                  \
         (iter).end = &(lhead)->head,                                       \
         (iter).cur = (iter).end->prev ? (iter).end->prev : (iter).end;     \
         (iter).cur != (iter).end;                                          \
         (iter).cur = (iter).cur->prev)


/* singly linked list implementation */

/*
 * Initialize a singly linked list.
 * Previous state will be thrown away without any cleanup.
 */
static inline void
slist_init(slist_head *head)
{
    head->head.next = NULL;
}

/*
 * Is the list empty?
 */
static inline bool
slist_is_empty(slist_head *head)
{
    slist_check(head);

    return head->head.next == NULL;
}

/*
 * Insert a node at the beginning of the list.
 */
static inline void
slist_push_head(slist_head *head, slist_node *node)
{
    node->next = head->head.next;
    head->head.next = node;

    slist_check(head);
}

/*
 * Insert a node after another *in the same list*
 */
static inline void
slist_insert_after(slist_node *after, slist_node *node)
{
    node->next = after->next;
    after->next = node;
}

/*
 * Remove and return the first node from a list (there must be one).
 */
static inline slist_node *
slist_pop_head_node(slist_head *head)
{
    slist_node *node;

    Assert(!slist_is_empty(head));
    node = head->head.next;
    head->head.next = node->next;
    slist_check(head);
    return node;
}

/*
 * Check whether 'node' has a following node.
 */
static inline bool
slist_has_next(slist_head *head, slist_node *node)
{
    slist_check(head);

    return node->next != NULL;
}

/*
 * Return the next node in the list (there must be one).
 */
static inline slist_node *
slist_next_node(slist_head *head, slist_node *node)
{
    Assert(slist_has_next(head, node));
    return node->next;
}

/* internal support function to get address of head element's struct */
static inline void *
slist_head_element_off(slist_head *head, size_t off)
{
    Assert(!slist_is_empty(head));
    return (char *) head->head.next - off;
}

/*
 * Return the first node in the list (there must be one).
 */
static inline slist_node *
slist_head_node(slist_head *head)
{
    return (slist_node *) slist_head_element_off(head, 0);
}

/*
 * Delete the list element the iterator currently points to.
 *
 * Caution: this modifies iter->cur, so don't use that again in the current
 * loop iteration.
 */
static inline void
slist_delete_current(slist_mutable_iter *iter)
{
    /*
     * Update previous element's forward link.  If the iteration is at the
     * first list element, iter->prev will point to the list header's "head"
     * field, so we don't need a special case for that.
     */
    iter->prev->next = iter->next;

    /*
     * Reset cur to prev, so that prev will continue to point to the prior
     * valid list element after slist_foreach_modify() advances to the next.
     */
    iter->cur = iter->prev;
}

/*
 * Return the containing struct of 'type' where 'membername' is the slist_node
 * pointed at by 'ptr'.
 *
 * This is used to convert a slist_node * back to its containing struct.
 */
#define slist_container(type, membername, ptr)                              \
    (AssertVariableIsOfTypeMacro(ptr, slist_node *),                        \
     AssertVariableIsOfTypeMacro(((type *) NULL)->membername, slist_node),  \
     ((type *) ((char *) (ptr) - offsetof(type, membername))))

/*
 * Return the address of the first element in the list.
 *
 * The list must not be empty.
 */
#define slist_head_element(type, membername, lhead)                         \
    (AssertVariableIsOfTypeMacro(((type *) NULL)->membername, slist_node),  \
     (type *) slist_head_element_off(lhead, offsetof(type, membername)))

/*
 * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
 *
 * Access the current element with iter.cur.
 *
 * It's allowed to modify the list while iterating, with the exception of
 * deleting the iterator's current node; deletion of that node requires
 * care if the iteration is to be continued afterward.  (Doing so and also
 * deleting or inserting adjacent list elements might misbehave; also, if
 * the user frees the current node's storage, continuing the iteration is
 * not safe.)
 */
#define slist_foreach(iter, lhead)                                          \
    for (AssertVariableIsOfTypeMacro(iter, slist_iter),                     \
         AssertVariableIsOfTypeMacro(lhead, slist_head *),                  \
         (iter).cur = (lhead)->head.next;                                   \
         (iter).cur != NULL;                                                \
         (iter).cur = (iter).cur->next)

/*
 * Iterate through the list pointed at by 'lhead' storing the state in 'iter'.
 *
 * Access the current element with iter.cur.
 *
 * The only list modification allowed while iterating is to remove the current
 * node via slist_delete_current() (*not* slist_delete()).  Insertion or
 * deletion of nodes adjacent to the current node would misbehave.
 */
#define slist_foreach_modify(iter, lhead)                                   \
    for (AssertVariableIsOfTypeMacro(iter, slist_mutable_iter),             \
         AssertVariableIsOfTypeMacro(lhead, slist_head *),                  \
         (iter).prev = &(lhead)->head,                                      \
         (iter).cur = (iter).prev->next,                                    \
         (iter).next = (iter).cur ? (iter).cur->next : NULL;                \
         (iter).cur != NULL;                                                \
         (iter).prev = (iter).cur,                                          \
         (iter).cur = (iter).next,                                          \
         (iter).next = (iter).next ? (iter).next->next : NULL)

#endif                          /* ILIST_H */